Urban Organic Waste for Urban Farming: Growing Lettuce Using Vermicompost and Thermophilic Compost [original]

agronomy

Article

Urban Organic Waste for Urban Farming: Growing Lettuce

Using Vermicompost and Thermophilic Compost

Corinna Schröder 1,2,*, Franziska Häfner 1, Oliver Christopher Larsen 2and Ariane Krause 1





Citation: Schröder, C.; Häfner, F.;

Larsen, O.C.; Krause, A. Urban

Organic Waste for Urban Farming:

Growing Lettuce Using

Vermicompost and Thermophilic

Compost. Agronomy 2021,11, 1175.

https://doi.org/10.3390/

agronomy11061175

Academic Editor: Arno Rosemarin

Received: 19 April 2021

Accepted: 2 June 2021

Published: 9 June 2021

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Licensee MDPI, Basel, Switzerland.

This article is an open access article

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conditions of the Creative Commons

Attribution (CC BY) license (https://

creativecommons.org/licenses/by/

4.0/).

1Programme area “Next-Generation Horticultural Systems”—HORTSYS, Leibniz Institute of Vegetable and

Ornamental Crops (IGZ) e.V, Theodor-Echtermeyer-Weg 1, 14979 Großbeeren, Germany;

haefner@igzev.de (F.H.); krause@igzev.de (A.K.)

Chair of Circular Economy and Recycling Technology, Technische Universität Berlin, Straße des 17. Juni 135,

10623 Berlin, Germany; oliver[email protected]

*Correspondence: schroeder.corinna@igzev.de; Tel.: +49-(0)33701-78254; Fax: +49-(0)33701-78132

Abstract:

A transformation towards sustainable food production requires improved circular nutrient

management. Urban organic waste contains relevant nutrients and organic matter, yet only 4%

of global urban nitrogen (N) and phosphorus (P) sources are presently recycled. One recycling

approach is the composting of urban wastes for urban horticulture. We characterized compost from

various urban waste fractions and assessed their fertilizer value in a pot trial with lettuce plants.

Seven treatments were investigated: food waste vermicompost with coir and paperboard bedding

material, thermophilic compost from green waste and human feces, two references with mineral

fertilization and a sand control. The lettuce yield and total uptake of P, potassium (K), calcium (Ca),

and magnesium (Mg) were highest in plants grown in coir-based vermicompost. The fecal compost

led to the highest shoot P and K content, but the shoot uptake of Ca and Mg were lower than in

the other treatments. All composts required additional N for lettuce growth. In conclusion, urban

waste-derived vermicompost and fecal compost demonstrate a high delivery rate of plant-available

Ca, Mg, P, and K. Research is needed on macronutrient availability and alternative N sources for

the substitution of synthetic fertilization. These findings support the production of urban waste

composts, furthering efforts in nutrient recycling.

Keywords:

coconut fiber; fecal compost; lettuce; nutrient recovery; organic waste; sustainable food

production; thermophilic compost; phosphorus

1. Introduction

1.1. Background—Circular Food Production for a Sustainable Future

Feeding a growing population in a healthy and sustainable way is one of the most

challenging societal and scientific issues today. On this topic, the EAT-Lancet Commission

on Food, Planet, Health concludes that a ‘great food transformation’ is urgently needed

and demands an ‘agricultural revolution’ regarding food production and consumption

practices [

]. Gerten et al. [

] demonstrated that it is possible to feed ten billion people

within the ‘planetary boundaries’ [

]. However, improved nutrient management and recy-

cling practices, particularly of phosphorus (P) and nitrogen (N), is a key prerequisite for the

required transformation towards more sustainable food production [

]. Circular economies

and material cycling within agri-food systems are necessary to minimize food production’s

environmental impact and to ensure long-term global food supply (e.g., [1,4–7]).

With regard to spatial allocation of food demand and nutrient recycling potentials,

it is expected that, by 2050, nearly 70% of the global population will live in cities [

]. The

potential for nutrient cycling is thus especially high in urbanized environments [

]. In

addition, the recent development of ‘urban farming’ initiatives and businesses, as well as

the popularity of community-based urban gardening (cf. [

]), may lead to an increasing

demand for fertilizers and organic soil amendments in urban areas. Therefore, considering

Agronomy 2021,11, 1175. https://doi.org/10.3390/agronomy11061175 https://www.mdpi.com/journal/agronomy

Agronomy 2021,11, 1175 2 of 26

urban waste as a resource is essential, and the use of locally available waste for the pro-

duction of organic recycling fertilizers and soil amendments is of particular economic and

political interest [

]. Composting is a globally widespread method of producing organic

recycled fertilizer from locally available biowaste. During composting, various organic

residues mixed with mineral components are aerobically and biochemically decomposed

by macro- and micro-organisms. This microbial process allows stabilization of the organic

matter (OM) and effective sanitization of the product if controlled and appropriately moni-

tored [

]. This well-established recycling process can, therefore, provide organic fertilizer

for urban cropland. Shrestha et al. [

] showed, for example, that compost-based urban

agriculture has a strong potential for P recovery and that careful compost application

targeted to crop nutrient demands can maintain high yields and minimize nutrient losses.

In the following paragraphs we further elaborate on the total recovery potential and

the fate of municipal organic waste; with particular focus on Germany, which is the regional

context of this study. Thereafter, we introduce two safe treatment options for recycling

nutrients within urban environments.

In 2016, about 0.9 Gt food and green waste was produced globally [

]. Against the

backdrop of urbanization, this fact is associated with a significant potential to capture

and recycle nutrients within urban areas. However, only 4% of global urban P and N

sources are estimated to be recycled within agriculture [

], and hence, only a minor

proportion of nutrients in urban biowaste is recaptured. In Germany, waste is typically

collected by disposal companies and treated depending on its type. Easily degradable

OM, such as food waste, is fermented in biogas plants and used for energy production.

The resulting digestate can be directly used as fertilizer or, after further composting, as a

soil amendment, with an overall positive environmental and economic impact [

]. Green

waste from gardens, parks, and landscape management is processed in municipal or private

composting plants to produce what is known as green compost [

]. According to the

German Circular Economy Act (Kreislaufwirtschaftsgesetz-KrWG), all private households

are obliged to collect the organic waste they produce separately from other waste streams

(§11 (1) KrWG) and to transfer it to the local disposal sites (§17 KrWG) [

]. Exemptions

are possible if a minimum of 25 m

garden area per person is used to establish their own

recycling processes, e.g., composting [

]. In 2017, 53.8 kg of organic food waste and

71.3 kg of garden waste per person were collected separately, representing just 48.4% of

the potential amount [

]. This means that despite the regulation being in effect for

several years, German organic waste collection remains insufficient. One explanation is

that in cities and metropolitan areas, not all households have access to an organic waste

bin. For Berlin (2016 data), the proportion of households with access to organic waste

disposal facilities was 80% in inner-city districts and only 20–25% in suburban areas [

]. A

large amount of organic waste ends up in the residual waste, which is then incinerated in

waste-to-energy plants. The full potential for nutrient recycling is therefore not achieved.

Against this backdrop, decentralized organic waste recycling could serve as a good

alternative or supplement for existing centralized systems in urban areas and help supply

locally produced fertilizers of controlled quality to urban food producers. Two promising

approaches to recycling urban nutrients on a local level include (i) small-scale vermicom-

posting of urban organic waste and (ii) composting of human excreta.

Small-scale vermicomposting is a low-odor, space-saving process that requires little

effort and can therefore be carried out even in households lacking gardens [

]. Vermicom-

posting is a method whereby the stabilization process of organic waste is achieved with

earthworms [

]. The worms (E. eugeniae,E. fetida,E. andrei, and P. excavatus) ingest and

fragment the OM, simultaneously aerating and mixing it by moving through the substrate.

Thus, they optimally prepare the OM for decomposition by micro-organisms, enhancing

its microbial decomposition rate [

]. Vermicompost (VC) stimulates plant growth due to

the nutrient supply [

], as well as the presence of growth-promoting substances in the

vermicast, e.g., auxin and cytokinin [

] or humic acids [

]. Compared to thermophilic

Agronomy 2021,11, 1175 3 of 26

compost (TC), VC of the same input material results in a more homogeneous product with

smaller particles and a higher content of nutrients [30,31].

Human excreta is an inevitable urban waste source, which is also seen as a key, but

yet untapped, resource for ‘urban mining’ of nutrients, particularly P and N [32]. Human

urine and feces contribute 70–80% of N and up to 60% of P as well as other macro- and

micronutrients in urban municipal wastewater [

]. However, in the sewage system,

these nutrients are often contaminated with heavy metals and microplastic from sources

other than toilets. Due to this contamination, field fertilization with sewage sludge has

been restricted or banned by many national governments, including Germany, due to the

2017 revised German Sewage Sludge Ordinance (Klärschlammverordnung-AbfKlärV) [

Currently, 65% of the sewage sludge produced in Germany is dried, incinerated, and

deposited in landfills [

]. The most common processes that are studied and applied for

recovery of nutrients from wastewater are mainly ones that focus on individual elements,

e.g., wastewater P recovery by struvite precipitation or P extraction from sewage sludge

ash [

]. However, removal of N from wastewater via aeration from combined nitrification

and denitrification processes requires 50–80% of the electricity consumed by wastewater

treatment plants and further leads to significant gaseous N losses due to nitrous oxide

emissions from activated sludge processes [

]. On the other hand, it has been shown

that human excreta can be treated and sanitized effectively by composting [

–

] and

can improve crop productivity when used as a soil amendment (e.g., [

]). Most

approaches for recycling-oriented sanitation services include the usage of waterless (or dry)

toilets, which result in significantly less freshwater consumption than flush toilets [

–

These toilets either collect urine and feces together or separate them ‘at the source’ and

treat them separately. The collected solid waste is thermally composted with added carbon

from toilet paper, sawdust, or other additives such as green waste [

–

]. For successful

sanitation of fecal compost, the World Health Organization recommends treatment at

55–60

◦

C for several days to one month, depending on conditions [

]. Longer periods are

recommended, for example, when continuous temperature monitoring is not possible [ibid.].

Specific regulations for human excreta treatment are not yet in place in most countries,

including Germany [

–

]. The EU fertilizer ordinance promotes bio-based recycled

fertilizers, but does not explicitly mention (processed) human excreta—neither urine, nor

feces [

]. Furthermore, in many European countries, the agricultural use of excreta is not

yet covered by the national legal framework [

–

]. A paradigm shift with changes

in policies is needed to integrate this resource into recycling-oriented waste management

and fertilizer legislation at a European and national level. Further research is required to

support these policies, including studies assessing the suitability of the recycled products

from human excreta for use as fertilizer in horticulture and agriculture. Moreover, existing

studies on the potential of compost from urban waste, including VC or fecal compost, often

focus on N and P but not on other macronutrients (e.g., [

]. A comparative study of

urban composts can help assess their potential for integration into urban food production,

ultimately promoting a more sustainable regional nutrient cycling economy.

1.2. Objectives and Hypothesis of the Present Study

The objectives of this study were: (i) To characterize the physicochemical composition

of different types of urban organic waste stream-derived compost, including green compost,

VC and fecal compost, and (ii) to examine and assess the fertilization potential of these

urban organic fertilizers when used as pot substrate for lettuce, with particular focus

on plant growth and macronutrient availability. It was hypothesized that fertilization of

lettuce with VC or fecal compost would lead to increased plant biomass, shoot mass and

macronutrient uptake compared to thermophilic garden waste compost. The rationale

behind this hypothesis was that green compost often contains high levels of carbon (C),

resulting in a high C:N ratio, thus increasing the risk of N immobilization in the soil,

whereas feedstocks rich in N and P, such as food waste or human excreta, can produce

compost with lower C:N ratios and higher nutrient availability.

Agronomy 2021,11, 1175 4 of 26

2. Materials and Methods

2.1. Experimental Design

2.1.1. General Set-Up

Between June and July 2019, a five-week pot experiment with lettuce was carried out

under greenhouse conditions at the Leibniz Institute of Vegetable and Ornamental Crops

(Großbeeren, Germany; 52

◦

N, 13

◦

E; alt. 40 m). Seeds were germinated on 23 May

2019, healthy seedlings were transplanted into cultivation trays on 11 June and into the

treatment media ten days later (planting date, 24 June; defined as day 1 of the experiment).

The complete plant biomass was harvested on 31 July. Each of the seven treatments had

five replicates (n= 5; 35 pots in total), and the pots were set up in randomized positions on

a table in a 60 m2greenhouse.

2.1.2. Growth Conditions

Seeds of lettuce (Lactuca sativa var. capitata) cultivar ‘Lucinde’ (Bingenheimer Saatgut

AG, Echzell, Germany) were germinated in quartz sand and kept at 7

◦

C for the first 24 h

to accelerate germination. ‘Lucinde’ cultivar was chosen for rapid shoot development and

its suitability for year-round cultivation. At the three-leaf stage, the seedlings were pricked

to a peat-filled cultivation tray and fertilized with 16 mg N, 4 mg P, 13 mg potassium

(K), and 1 mg magnesium (Mg) per plant, using a solution of 3 g L

−1

MANNA LIN M

Spezial (Wilhelm Haug GmbH & Co KG, Germany). After ten days, the seedlings were

transplanted into 35 prepared pots: 3.5 L plastic pots filled with a mixture of quartz sand

as a base plus the test compost. Quartz sand was used as a neutral, non-reactive substrate

that does not influence nutrient release through its chemical and microbiological properties.

For consistent conditions in the substrate, the same amount of compost was added to

all treatments by preparing a homogenous mixture of 100 g dry matter (DM) compost

and

1.67 kg DM

of washed quartz sand (grain size, 0.5–1.2 mm) for each, adding up to

2.2 L filling per pot. For the control pots, 1.93 kg DM (2.2 L) sand filling was added to

compensate for the missing compost.

The greenhouse climate data were recorded continuously throughout the experiment

(Figure A1). The average air temperature during the day was 26.6

◦

C (max, 43.3

◦

C; min,

15.1

◦

C) and 20.7

◦

C at night (38.9

◦

C max, 14.8

◦

C min); the average relative humidity was

57.9%. During the growth period, plants were irrigated with deionized water, maintaining

a substrate water content of 60% of the maximum water holding capacity. Water was

added each day according to the daily water loss due to evapotranspiration, which was

determined gravimetrically several times per week. The applied water volume ranged

from 50 to 350 mL per day, supplied either at once, or split into several doses, depending on

the size of plants, the temperature and the evapotranspiration rate during the experiment

(Figure A1). Irrigation was carried out close to maximum water-holding capacity to avoid

phases of drought stress.

2.2. Fertilizer Trials

2.2.1. Treatments

In total, seven treatments were tested in this study, including six compost treatments

and a fully synthetic control fertilizer (Figure 1). The studied compost treatments included

two VCs and four TCs. The VC treatments differed in the bedding material used during

vermicomposting; one was based on coir/coconut fiber (VC-C) and one on paperboard and

soil (VC-P). The TC treatments differed in the main input material and included one based

on fecal matter (TC-F) and one on green waste (TC-G). In addition, two TC-G treatments

with increased levels of added mineral N (N

min

) (TC-G2 and TC-G3) were included to

compare the nutrient contributions of the different composts. A synthetic mineral fertilizer

without compost was included as a control (sand control; SC).

Agronomy 2021,11, 1175 5 of 26

Agronomy 2021, 11, x 5 of 26

based on fecal matter (TC-F) and one on green waste (TC-G). In addition, two TC-G treat-

ments with increased levels of added mineral N (N

min

) (TC-G2 and TC-G3) were included

to compare the nutrient contributions of the different composts. A synthetic mineral fer-

tilizer without compost was included as a control (sand control; SC).

Figure 1. Experimental design of the pot experiment. The green boxes refer to the seven tested treat-

ments, including one SC as a reference and six different sand/compost mix treatments. The blue,

red, and yellow boxes refer to the mineral fertilization solution added per pot: Blue, P and K; red,

N; yellow, micronutrients. DM, dry matter; SC, sand control; TC-F, thermophilic compost from fecal

matter; TC-G, thermophilic compost from green waste; TC-G2 and TC-G3, thermophilic compost

from green waste with additional N fertilization; VC-C, vermicompost with coir bedding material;

VC-P, vermicompost with paperboard bedding material.

2.2.2. Tested Composts

The two VCs tested in the present study were produced at Technische Universität (TU),

Berlin for ten weeks between January and March 2019 in household-sized batch systems

(10-l buckets) using Eisenia fetida earthworms. The bedding material was 1.5 kg wet coir

per bucket for the VC-C, or a soil-paperboard mixture containing 1 kg soil (‘Einheitserde’,

Einheitserde Werkverband e.V. Typ Classic) and 110 g corrugated paperboard for the VC-P.

Twice a week, 1140 g fresh matter (FM) from the TU canteen kitchen waste was fed to the

earthworms in each bucket. This material contained 16.5% coffee grounds and 83.5% fruit

and vegetable peels, and other food preparation waste [55].

The TC-F was provided by the Birkenhof compost-producing company (Kompostier-

und Lohnunternehmen Schulze-Kahleyß GmbH, Lindendorf, Germany). Its production in-

volved a six-month process using ~40 t (~30 m

) residue from dry toilets mixed with 4.25

t (~10.2 m

) green cuttings and 0.7 t straw (~3 m

) as a structural material. To provide

moisture, 650 L urine was added while preparing the composting windrows. The dry toi-

let waste was collected from more than 15 different festival sites in Northern Germany by

three companies that rent and operate dry toilets for public events. The organic matter

was a mix of human feces (~30% vol.), toilet paper (~13% vol.), straw used as drainage

material (~7% vol.), and sawdust (~50% vol.). Sawdust was initially added immediately

after use, and subsequently during storage and before delivery to the composting plant,

to reduce odor. The dry toilets used a combined collection system for urine and feces, from

which the liquid was subsequently drained. Hence, a certain amount of urine was mixed

with the collected fecal matter. At the composting site, the material was mixed and ar-

ranged in two compost windrows. The temperature was monitored constantly at three

locations inside the compost windrow and three on the surface; this remained at >55 °C

Figure 1.

Experimental design of the pot experiment. The green boxes refer to the seven tested

treatments, including one SC as a reference and six different sand/compost mix treatments. The blue,

red, and yellow boxes refer to the mineral fertilization solution added per pot: Blue, P and K; red, N;

yellow, micronutrients. DM, dry matter; SC, sand control; TC-F, thermophilic compost from fecal

matter; TC-G, thermophilic compost from green waste; TC-G2 and TC-G3, thermophilic compost

from green waste with additional N fertilization; VC-C, vermicompost with coir bedding material;

VC-P, vermicompost with paperboard bedding material.

2.2.2. Tested Composts

The two VCs tested in the present study were produced at Technische Universität (TU),

Berlin for ten weeks between January and March 2019 in household-sized batch systems

(10-l buckets) using Eisenia fetida earthworms. The bedding material was 1.5 kg wet coir

per bucket for the VC-C, or a soil-paperboard mixture containing 1 kg soil (‘Einheitserde’,

Einheitserde Werkverband e.V. Typ Classic) and 110 g corrugated paperboard for the VC-P.

Twice a week, 1140 g fresh matter (FM) from the TU canteen kitchen waste was fed to the

earthworms in each bucket. This material contained 16.5% coffee grounds and 83.5% fruit

and vegetable peels, and other food preparation waste [55].

The TC-F was provided by the Birkenhof compost-producing company (Kompostier-

und Lohnunternehmen Schulze-Kahleyß GmbH, Lindendorf, Germany). Its production

involved a six-month process using ~40 t (~30 m

) residue from dry toilets mixed with

4.25 t (~10.2 m

) green cuttings and 0.7 t straw (~3 m

) as a structural material. To provide

moisture, 650 L urine was added while preparing the composting windrows. The dry toilet

waste was collected from more than 15 different festival sites in Northern Germany by

three companies that rent and operate dry toilets for public events. The organic matter was

a mix of human feces (~30% vol.), toilet paper (~13% vol.), straw used as drainage material

(~7% vol.), and sawdust (~50% vol.). Sawdust was initially added immediately after use,

and subsequently during storage and before delivery to the composting plant, to reduce

odor. The dry toilets used a combined collection system for urine and feces, from which

the liquid was subsequently drained. Hence, a certain amount of urine was mixed with

the collected fecal matter. At the composting site, the material was mixed and arranged

in two compost windrows. The temperature was monitored constantly at three locations

inside the compost windrow and three on the surface; this remained at >55

◦

C for the first

42 days, and >40

◦

C for the remaining duration of the process. The bulk density of the final

compost was determined to be 472.5 kg m−3.

The TC-G was a commercially available product from the local Berliner Stadtreini-

gung waste disposal company and consisted of green cuttings from park and landscape

maintenance.

Agronomy 2021,11, 1175 6 of 26

2.2.3. Nutrient Availability and Supplementation

The amount of compost applied per pot was calculated such that each plant would

receive sufficient P and K to produce a per-head yield of 15 g DM or 250 g FM and a

target content of 0.45% P and 4.2% K in the shoot DM, with an average of 6% DM content

(see [

]). According to the calculated minimum P and K contents in the composts, 100 g

DM compost was applied to each pot, ensuring the availability of a minimum of 0.07 g total

P (P

tot

) and 0.63 g total K (K

tot

) per plant. As this quantity of compost would not cover the

plant’s N demand, additional mineral N(N

min

) was supplied, so that the uptake of other

elements (K, P, Mg and Ca) was not limited by N deficiency. Approximately 30% of N from

organic fertilizers is accessible by plants in the first vegetation period after application [

Since the present study’s growth period was only five weeks, an availability of 10% was

assumed. To calculate the amount of N available from the composts, the following formula

was used:

Navailable =Nmin +Ntot −Nmin

To exclude micronutrient deficiency, the plants grown in compost treatments were

supplied with a solution of essential nutrients (excluding P and K) on days 1 and 2 of the

experiment (Table 1). The compost-free control treatment SC received complete mineral

fertilization. An additional 188 mg NH4NO3-N was applied to each plant on day 21.

Table 1.

Total quantities of nutrient salts applied to each pot via a solution, split into three doses,

supplied on days 1, 2, and 21 of the growth period. SC, sand control; TC-F, thermophilic compost

from fecal matter; TC-G, thermophilic compost from green waste; TC-G2 and TC-G3, thermophilic

compost from green waste with additional N fertilization; VC-C, vermicompost with coir bedding

material; VC-P, vermicompost with paperboard bedding material. S was supplied through a range of

salts, noted as s.a. = see above. P and K were solely supplied by the composts and only added to the

SC treatment, noted as n.a. = not applied.

Element Supplied Form VC-C, VC-P,

TC-F, TC-G TC-G2 TC-G3 SC

mg pot−1

NH4-N NH4NO3188.0 283.0 378.0 188.0

NO3-N NH4NO3188.0 281.0 377.0 187.0

∑NH4NO3376.0 564.0 755.0 376.0

KKH2PO4

K2SO4n.a. n.a. n.a. 114.0

191.0

P KH2PO4n.a. n.a. n.a. 90.1

Mg MgSO480.9 80.9 80.9 161.0

Ca Ca(SO4)230.2 30.2 30.2 130.0

Fe C10H12N2NaFeO810.4 10.4 10.4 10.4

Mn MnSO45.0 5.0 5.0 5.0

Zn ZnSO45.1 5.1 5.1 5.1

B H3BO45.1 5.1 5.1 5.1

Cu CuSO42.0 2.0 2.0 2.0

Mo MoO32.1 2.1 2.1 2.1

S s.a. 131.0 130.8 131.0 356.0

Na C10H12N2NaFeO84.0 4.0 4.0 4.0

2.3. Analysis, Sampling and Harvesting

2.3.1. Compost Analysis

Unless otherwise stated, all compost analyses were based on national or international

compost analysis standards. The chemical analysis of the VCs and the green compost were

carried out at TU, Berlin. The samples were sieved to <11.2 mm to remove non-degraded

plant parts and prepared using a sample splitter (Retsch GmbH, Haan, Germany) according

to DIN 19747:2009 [

]. According to DIN EN 15933:2010 [

] the pH was measured in a

Agronomy 2021,11, 1175 7 of 26

suspension of 5 mL material in 25 mL CaCl

-solution (0.01 mol L

−1

). Electrical conductivity

(EC) was measured in the fresh samples in a 1:25 (m

/vol.) solution with ultrapure water

according to the standard DIN CEN/TS 15937:2013 [

]. The gravimetric water content

was determined after oven-drying for 24 h at 105

◦

C, and OM was determined through loss

on ignition of the dry sample at 550

◦

C according to the standard DIN EN 15935:2012 [

Soluble nitrate (NO

3−

), ammonium (NH

), and Mg were extracted in a 1:25 (m

/vol.)

solution of the samples with 0.0125 mol L

−1

CaCl

for 1 h (modified VDLUFA, 2012a

Ch. A6.1.4.1, and Ch. A6.2.4.1). The solutions were analyzed for NO

3−

, NH

and

Mg using ion chromatography (DX-500; Dionex Corporation Sunnyvale, CA, USA), flow

injection for atomic spectroscopy (FIAStar 5000 System; Foss GmbH, Hamburg/Hilleroed,

Denmark) and inductively coupled plasma optical emission spectrometry (ICP-OES; iCAP

6000 Series; Thermo Fisher Scientific GmbH, Dreieich, Germany), respectively. Calcium

acetate/lactate (CAL)-extractable P (P

CAL

) and K (K

CAL

) were analyzed by ICP-OES using

a 1:25 (m

/vol.) CAL-solution according to the modified method from the VDLUFA

methods book I, chapter A 6.2.1.1 [

]. To analyze the elements, composts were dried at

105

◦

C and ground to a particle size of <250

m with a planetary ball mill (PM 400; Retsch,

Haan, Germany) according to DIN EN 16179:2012 [

]. The samples were subjected to

microwave-assisted aqua regia digestion following the standard DIN EN 16174:2012 [

]

in a MARS 5 microwave (CEM GmbH, Kamp-Lintfort, Germany), followed by ICP-OES to

determine the content of macronutrients Ca, K, Mg and P, micronutrients Cu, Fe, Mn, Zn

and Na, and heavy metals As, Cd, Cr, Ni and Pb.

For the fecal compost, analyses were carried out at the Helmholtz Centre for Environ-

mental Research, Leipzig, Germany, for the following parameters: DM content following

the standard DIN EN 12880-2001-02 [

]; pH and EC measured in the fresh samples in a 1:10

/vol.) solution with ultrapure water according to the standards EN 15933:2012 [

]

and DIN CEN/TS 15937:2013 [

], respectively; and contents of NO

3−

according to the

method from the VDLUFA methods book II.1 for organic fertilizers, chapter 3.4.2. [

] and

of NH

according to the method described by Strach [

]. K

tot

and Mg

tot

were measured

at TU, where samples were digested as described above and analyzed by flame atomic

absorption spectrometry (AAS 400; Perkin Elmer, Waltham, MA, USA). EC and amounts of

tot

, and Ca

tot,

were measured according to the standards DIN EN 13038:2011 [

], and DIN

EN ISO 11885:2009 [

]. The fecal compost was further subject to microbiological testing at

an external laboratory (LUFA Nord-West, Oldenburg, Germany), including counts of total

aerobic bacteria and colonies of fecal coliforms Escherichia coli (E. coli), fecal enterococci and

Salmonella spp. according to the methods C1 to C4 in chapter four of the methods book for

organic fertilizers of the German Federal Compost Quality Association (Bundesgütegemein-

schaft Kompost e.V.—BGK) [

]. In all compost types, C

tot

and N

tot

were measured using a

vario EL III CHNS Elemental Analyzer (Elementar Analysensysteme GmbH, Langenselbold,

Germany) and according to the standard DIN EN 15104:2011-04 [71].

The maximum water-holding capacity of the composts was determined using a modi-

fied version of the method described by Alef [

]. For this purpose, 0.5 L plastic pots were

filled with the substrates in triplicate, saturated with water, and placed on a sand bed to

drain for 1 h. The water content of what remained in the pot was then determined by the

difference in weight before and after drying at 105

◦

C, corresponding to the maximum

water absorption capacity, which was used to adjust the optimum irrigation volume per

pot (see Section 2.1).

Measurement of pH was not performed on the pot substrates. As opposed to natural

sandy soils, the quartz sand used was a mineral substrate and had, therefore, no pH-

buffering capacity [73]. The tested compost additions would have a stronger effect on the

pH in the sand substrate than in soil. Therefore, the pH in the pot substrate mix was likely

close to the original pH values of the added compost.

Agronomy 2021,11, 1175 8 of 26

2.3.2. Plant Harvest and Biomass Measurement

The lettuce shoots, including the stems, were harvested after a 37-day growth period.

The roots were washed manually from the substrate, and the fresh weight of the roots and

shoots was measured. Root and shoot samples were oven-dried at 60

◦

C until a constant

weight was reached, and the dry weight was recorded as the total biomass. Subsequently,

the samples were ground to <250

m in a centrifugal mill (ZM 200 by Retsch, Haan,

Germany) and homogenized for further analysis.

2.3.3. Plant Analysis

From the ground plant material, ~250-mg samples were digested with 5 mL nitric

acid and 3 mL hydrogen peroxide according to the VDLUFA methods book III, chapter

10.8.1.2 [

] and subjected to pressure digestion with microwave heating (MARS 5 Xpress;

CEM GmbH, Kamp-Lintfort, Germany). P, K, Ca, Mg and Na contents were determined by

ICP-OES (iCAP7400; Thermo Fisher Scientific GmbH, Dreieich, Germany) at the following

wavelengths: 178.284 nm (P), 766.490 nm (K), 315.877 nm (Ca), 279.553 nm (Mg), and

589.592 nm (Na). For N and C analyses, the samples were treated using the DUMAS dry

combustion method at 950

◦

C in an oxygen-enriched He atmosphere in a combustion tube

filled with W(VI) oxide (vario EL cube; Elementar Analysensysteme GmbH, Langenselbold,

Germany), using a thermal conductivity detector according to the VDLUFA methods book

I, chapter A 2.2.5 [75].

2.4. Statistical Analysis

The measured nutrient contents and DM and FM of the roots and shoots across the

samples were analyzed for statistical significance. For this purpose, the mean values and

standard deviations were calculated. Provided that the data met the condition of normal

distribution (Shapiro-Wilk-Test), they were tested using one-way analysis of variance

(one-way ANOVA; p< 0.05 was considered to denote statistically significant differences).

The significance levels between groups were determined using Tukey honest significant

difference (HSD) test with

= 0.05 and p< 0.05 was considered to denote statistical

significance. Pearson correlation analysis was used to determine any association between

the compost nutrients and the nutrient uptake or content in the shoots. To assess the

effect of different N

min

levels, the correlation of N uptake or yield with supplied N

min

level was analyzed for all samples. All statistical methods were conducted using the

STATISTICA version 13 software (StatSoft Inc., Tulsa, OK, USA) and SAS 9.4 (SAS Institute,

Cary, NC, USA).

3. Results and Discussion

In this section, we present and discuss the results of our study. First, we describe the

composition of the different types of studied compost derived from various urban organic

waste sources (Section 3.1). Secondly, we present and discuss the results of examining

and assessing the fertilization potential of these urban organic fertilizers when used as

pot substrate for lettuce, with particular focus on plant growth and nutrient availability

of macronutrients (Section 3.2). Finally, we discuss the suitability and relevance of the

studied composts for P-recycling regarding practical applications in urban horticulture

(Section 3.3).

3.1. Physicochemical Characteristics of the Urban Organic Waste-Derived Composts

3.1.1. Nutrient Composition

The nutrient composition of the tested composts is presented in Table 2. TC-F exhibited

the highest content of N, P, Mg, and Ca. It also contained high amounts of K, but this

was 2-fold higher in the VC-C. The contents of the above elements in TC-F were similar

to those reported by Krause et al. [

] for fecal compost enriched with biochar. TC-G and

VC-P appeared to have a similar composition, with numerically higher P and lower Na

content in the former. The two types of VC exhibited very different N, P, K, Ca, Mg and Na

Agronomy 2021,11, 1175 9 of 26

contents. VC-C contained more N, P, and Mg and almost twice as much K and Na than

VC-P. Only the Ca content was higher in the latter. These findings indicate that the bedding

material significantly affects the compost quality in terms of nutrient content.

Table 2.

Physicochemical characteristics and composition of the studied composts. All values refer to DM contents. As

100 g

DM of organic fertilizer were applied per pot, 1/10 of the values equals the nutrient supply per plant in this experiment.

CAL denotes elements extracted by calcium acetate/lactate; DM, dry matter; EC, electric conductivity; FM, fresh matter;

n.d., not determined; OM, organic matter.

Compost Ctot Ntot Ptot PCAL Ktot KCAL Mgtot MgCaCl2 Catot Natot

g kg−1

VC-C 436 16.2 1.7 0.46 14.7 12.2 3.0 1.1 9.7 1.3

VC-P 227 10.3 1.0 0.14 6.3 4.2 2.0 0.4 15.0 0.5

TC-F 299 19.4 4.0 1.99 12.8 6.3 3.9 n.d. 21.2 2.4

TC-G 185 10.4 1.5 0.41 6.4 5.0 1.9 0.6 16.8 0.3

Compost NO3-N NH4-N Fe Mn Cu Zn As Cd Cr Ni Pb

mg kg−1

VC-C 690.2 10.7 2250 124 15 54 <1.5 0.16 8.8 3.9 3.8

VC-P 3.6 6.2 6498 163 12 39 3.0 0.29 12.1 5.6 10.2

TC-F 470.0 0 5900 n.d. 26 103 2.1 <0.50 7.1 4.7 13

TC-G 0 7.9 6189 252 29 146 2.9 0.50 19.5 9.1 31.8

Compost pH EC OM DM C:N C:P N:P Ca:P

in CaCl2µS/cm % DM % FM wt ratio molar ratio

VC-C 6.1 773 81.3 26.3 26.9 264 9.8 4.5

VC-P 7.2 276 42.0 44.7 21.9 229 10.4 11.7

TC-F 8.8 1977 29.1 49.6 15.4 75 4.9 4.1

TC-G 7.6 376 30.3 72.2 17.9 124 7.0 8.7

The range of N

tot

measured in the composts (10–19 g kg

−1

) was in agreement with

the amount typically found in composts (12 g kg

−1

; [

]), and comparable to findings by

Hernández et al. [

] on TCs and VCs (14–16 g kg

−1

) and Arancon et al. [

] on different

VCs (10–19 g kg

−1

). The levels of N

min

were higher in VC-C and TC-F than in VC-P

and

TC-G

. As thermophilic composting and vermicomposting are predominantly aerobic

processes, N

min

was present mainly as NO

3−

, due to nitrification and a small proportion

as NH

. Only TC-F did not contain any NH

, which can be related to its alkaline pH,

whereby NH

presumably volatized as NH

during composting [

]. The NO

3−

content in

VC-C and TC-F were comparable to those reported by Hernández et al. [

]. The differences

in N

min

and N

tot

between the two VCs indicate that the bedding material affected microbial

N mineralization during the composting process. Presumably, coconut fiber, with its high

water-holding capacity comparable to peat [

], provided more balanced moisture and a

more favorable oxygen level than paperboard; these characteristics are required for aerobic

processes as microbial N mineralization. Furthermore, enhanced N mineralization in VCs

was associated with the direct excretion of excess N by earthworms [

]. As the N input

from food waste was similar for both VCs, it appeared that coir offered more favorable

conditions for the earthworms. According to the estimated N availability (see Section 2.2),

approximately 0.23 g kg

−1

of N in VC-C and TC-F, and 0.1 g kg

−1

in VC-P and TC-G, were

assumed to be available per plant. Hence, none of the composts could provide sufficient N

as pot substrate, and additional mineral N supply was therefore required.

Finally, a positive correlation was found between the contents of Ptot and extractable

CAL

(r= 0.998). The highest proportion of P

CAL

in P

tot

was found in TC-F (49%), followed

by VC-C, TC-G (both 28%), and VC-P (14%), suggesting a higher P availability and value as

a fertilizer in the fecal compost compared with the other composts. A correlation was also

found between P

tot

and Ca (r= 0.478) and between P

tot

and Mg (r= 0.809), in agreement

Agronomy 2021,11, 1175 10 of 26

with the results of 17 different composts reported by Vandecasteele et al. [

]. In TC-F, with

its alkaline pH, it can be assumed that Ca and P

tot

were in the form of calcium phosphate

(Ca3(PO4)2)—it is known that a large fraction of P in human feces is present as Ca3(PO4)2

and iron phosphates [81].

3.1.2. OM Content and Nutrient Ratios

Mature compost OM composition varies from 25 to 40% of DM, and should not fall

below 25% [

]. All four tested composts were above this threshold; the proportion was

particularly high in VC-C (>80%). Comparably high OM content was reported in substrates

containing 100% coconut fiber [

]. Therefore, the notably higher OM share in VC-C

is likely attributed to the coconut fiber used as bedding material. Furthermore, VC-C

exhibited the highest C:P ratio among all tested composts. These characteristics indicate

that this VC is a beneficial soil conditioner for humus reproduction, particularly suitable

for the amendment of soils already high in P, consequently requiring soil amendments

with reduced P content [53].

To minimize the priming effect of compost when applied to soil or potting substrate,

the C:N ratio should not exceed a value of 18 for standard compost [

] and 22 for VC [

Three of the tested composts fulfilled these guidelines; however, the VC-C ratio exceeded

the recommended threshold (C:N, 27).

The C:P ratio, which describes the relationship between C and P input [

], ranged

between 75 and 264 in the studied composts. In this context, the C input may also refer

to the application of OM in general. The two TCs exhibited lower C:P ratios (

≤

124) than

the two VCs (

≥

229). The C:P ratios measured in TC-F and TC-G were comparable to those

reported by [

] for composts based on vegetable, fruit, garden, or farm waste. McLaughlin

et al. [

] argued that no C:P ratio could adequately describe P release from OM. Similarly,

in the present study, no clear association between C:P ratio and P availability was detected

by regression analysis.

The molar Ca:P ratio in composts has also been suggested as an indicator for P use

efficiency [

]. All tested composts exhibited Ca:P > 2, which has been reported to favor

apatite-like structures and, therefore, lower P availability [

]. Since all treatments received

additional N

min

to adapt to the plant’s needs, the N:P ratio was not a focus of the present

study.

3.1.3. Additional Compost Characteristics Relevant for Plant Nutrition—pH, EC and

Salt Content

The compost pH values were close to neutral for VC-P and TC-G, whereas VC-C was

slightly acidic (Table 2). The pH of all three composts was within the range of

5.5–8.0

considered adequate for plant growth in potting media or soils [

]. TC-F, however,

exhibited an alkaline pH, comparable to that reported for VCs and composts produced

from farmyard or cattle manure (e.g., [

]. Vandecasteele et al. [

] noted a substantial

decrease of readily available P in composts when the pH (measured in H

O) was >8.5.

Given that pH measured in CaCl

usually provides a value lower than that measured in

H2O, TC-F with pH>8.5 (measured in CaCl2) may be affected by restricted P availability.

The EC in compost is related to the amount of ions (mostly Na

, K

, Ca

, Mg

−

, SO

42+

, CO

32−

, HCO

3−

, NO

3−

), also referred to as soluble salts [

]. Thus, EC reflects

on nutrient concentration, which may be governed by essential plant nutrients (Ca

, K

)

or Na

and Cl

−

[ibid.] and explaining the high EC of VC-C and TC-F. Soil is considered

saline at an EC > 4000

S cm

−1

[

]. As high soluble salt contents in soil amendments

(with high share of Na

and Cl

−

) can induce salt stress in plants, the EC of such material

should not exceed 1000–2000

S cm

−1

[

]. This criterion was fulfilled by all four composts

(Table 2): the EC was <1000

S cm

−1

in all samples, except TC-F, in which it was close to

2000 µS cm−1

. By comparison, Papathanasiou et al. [

] reported an EC of 2650

S cm

−1

for VCs from cattle manure, whereas Coria-Cayupán et al. [

] found the EC of VCs from

urban food waste to range from 400 to 500

S cm

−1

, similar to that of VC-P and VC-C of

the present study.

Agronomy 2021,11, 1175 11 of 26

Furthermore, VCs are generally low in Na, because, for successful vermicomposting,

only substrates with low NaCl content should be used. Earthworms cannot survive under

conditions of Na content >5 g kg

−1

in the mixture of feeding and bedding material, resulting

in a failed composting process and no product [

]. Since human nutrition, and therefore

human excreta, often contain relatively high table salt content (i.e., NaCl), the level of Na

in TC-F was two to seven times higher than in the other composts (Table 2), leading to high

EC. A strong positive correlation was found between Na concentration and EC among

composts (r= 0.97). Higher amounts of Na were also found in VC from cattle manure in the

study by Arancon et al. [

]. However, those results and our findings herein indicate that

the higher Na content in fecal compost is still within a range suitable for plant cultivation.

3.1.4. Heavy Metal Content

The content of heavy metals As, Cd, Cr, Cu, Ni, Zn, and Pb (Table 2) in all investigated

composts met the quality criteria for composts according to the German Federal Compost

Quality Association BGK [

], and the value limits for organic fertilizers according to

the German Fertilizer Ordinance (Düngemittelverordnung) and European regulations (EU

2019/1009) (see Table A1). The content of heavy metals, except for As, was highest in

the TC-G. This is likely due to large-scale waste collection systems being susceptible to

contamination from misthrows and impurities which are difficult to avoid. Furthermore,

cross-contamination is possible due to ‘urban dust’ (e.g., tire wear) collected with green

cutting wastes from landscape maintenance or other sources, e.g., tools used for conveying

or transportation of green cutting wastes [96,97].

3.2. Fertilizing Potential of the Studied Composts

3.2.1. Plant Growth Parameters for Lettuce

The compost type appeared to affect the FM yield of the lettuce heads. Overall, our

results agreed with those reported by Rubatzky & Yamaguchi [

], who found that lettuce

yields can vary from 100 to 400 g FM per plant under greenhouse conditions. All compost

treatments improved the marketable yield compared with the synthetic SC treatment

(Table 3).

Table 3.

Marketable yield of lettuce heads (FM per plant), shoot and root (DM per plant), and

root:shoot ratio of lettuce plants. Values represent mean

standard deviation (n= 5). Different

letters reflect that means significantly differ from one another (HSD, Tukey test,

= 0.05; n= 5).

* Marketable

yield (shoot FM) was not normally distributed; therefore, no indications of statistically

significant differences are included here. DM, dry matter; FM, fresh matter; SC, sand control; TC-F,

thermophilic compost from fecal matter; TC-G, thermophilic compost from green waste; TC-G2 and

TC-G3, thermophilic compost from green waste with additional N fertilization; VC-C, vermicompost

with coir bedding material; VC-P, vermicompost with paperboard bedding material.

Compost

Treatment

Shoot FM Respectively

Marketable Yield * Shoot DM Root DM Root:Shoot

g per Plant

VC-C 244 ±16 15.7 ±1.4 a 2.70 ±0.53 a 0.17 ±0.03 a

VC-P 157 ±11 10.1 ±1.0 c 1.77 ±0.19 b 0.18 ±0.01 a

TC-F 187 ±25 11.4 ±2.1 bc 1.47 ±0.32 b 0.13 ±0.04 ab

TC-G 161 ±12 9.5 ±1.0 c 1.28 ±0.16 b 0.14 ±0.02 ab

TC-G2 208 ±15 13.2 ±0.4 b 1.36 ±0.44 b 0.12 ±0.03 b

TC-G3 209 ±11 13.4 ±1.0 ab 1.20 ±0.26 b 0.10 ±0.03 b

SC 104 ±8 10.8 ±0.6 c 1.53 ±0.42 b 0.11 ±0.02 b

Shoot FM yield per plant was observed to follow the descending order VC-C>TC-

F>TC-G

≥

VC-P. No difference was observed between TC-G2 and TC-G3, despite TC-G3

receiving more N

min

fertilization. The marketable yield was similar to that reported by

León et al. [

], in VC added to silty loam. The shoot DM was also higher in VC-C, followed

Agronomy 2021,11, 1175 12 of 26

by TC-F (bc) and then SC, VC-P and TC-G (c), which resulted in similar shoot DM biomass

production.

The root DM in the VC-C treatment was significantly higher than the others. For the

four compost treatments that had received the same N

min

addition, the root:shoot ratios

were comparable, but the VC treatments ratios were significantly different from those of

TC-G2, TC-G3, and SC. Kang & van Iersel [

100

] demonstrated that root:shoot ratios can

decrease with increasing N supply, as shoot growth is enhanced more than that of the

root. It is also known that root mass can be increased by deficiency of N or P in particular

(e.g., [

101

102

]). Although the shoot N content and uptake were relatively low in VC-C, a

substantial nutrient deficiency was unlikely to cause the increased root growth. The higher

root DM of VC-C was presumably related to the significantly faster shoot DM growth. This

can be corroborated by the comparable root:shoot ratios among VC-C, VC-P, TC-F, and

TC-G. It should be noted that the limited volume within the pots could have suppressed

root growth in general.

3.2.2. Plant Nutrient Uptake

The total uptake of macronutrients by the lettuce plants (in mg per plant DM) is listed

in Table 4, and nutrient contents in the lettuce shoots (in mg kg

−1

of DM) are presented

in Figure 2(exact numbers see Table A2). In the following paragraphs, we discuss the

availability of each nutrient in the studied treatments.

The P content in lettuce shoots grown in TC-F and SC was within the range of

4.5–7.0 g kg−1

, within Bergmann’s recommendations [

] (Figure 2). All other treatments

resulted in P contents below the optimal nutritional level, the lowest measured in VC-P. Fur-

thermore, P levels in the shoots from VC-C, TC-F, and SC were comparable, all significantly

higher than those in the TC-G/G2/G3 and VC-P plants (Table 4). The higher N levels in

TC-G2 and TC-G3 compared with TC-G, increased shoot growth and resulted in a higher

P uptake. TC-F resulted in the highest shoot P content and total P uptake by the plants,

suggesting that this compost type had the most value as a P fertilizer. VC-C led to a similar

P uptake as TC-F, but lower P contents in the shoot due to a dilution effect induced by an

increased shoot growth. Compost P content was correlated with shoot uptake (

r= 0.746

p= 0.002) and shoot content (r= 0.931, p< 0.001), as was the amount of extractable P

CAL

from the composts (r= 0.718, p= 0.004 and r= 0.919, p< 0.001, respectively).

Table 4.

Uptake of macronutrients N, P, K, Mg, Ca, and Na in lettuce shoots, expressed as mg per plant shoot (dry matter)

(mean

standard deviation, n= 5). Different letters reflect that means significantly differ from one another (HSD, Tukey test,

= 0.05; n= 5). SC, sand control; TC-F, thermophilic compost from fecal matter; TC-G, thermophilic compost from green

waste; TC-G2 and TC-G3, thermophilic compost from green waste with additional N fertilization; VC-C, vermicompost

with coir bedding material; VC-P, vermicompost with paperboard bedding material.

Compost

Treatment

Ntot

[mg]

Ptot

[mg]

Ktot

[mg]

Mgtot

[mg]

Catot

[mg]

Natot

[mg]

VC-C 354 ±32 c 57.0 ±4.7 a 871 ±14 a 153.0 ±7.6 a 268 ±15 ab 78.7 ±3.9 b

VC-P 333 ±25 cde 20.2 ±1.5 c 382 ±16 c 59.3 ±9.2 c 206 ±30 c 51.1 ±7.2 cd

TC-F 353 ±34 cd 63.8 ±10.5 a 834 ±86 a 53.7 ±10.2 c 157 ±32 cd 115 ±17.2 a

TC-G 305 ±13 de 30.0 ±3.3 c 534 ±33 b 58.7 ±8.0 c 213 ±27 bc 38.6 ±3.9 d

TC-G2 465 ±11 b 42.3 ±1.2 b 523 ±10 b 98.9 ±9.9 b 321 ±49 a 46.5 ±10.9 c

TC-G3 537 ±31 a 43.5 ±4.6 b 513 ±16 b 92.9 ±5.2 b 299 ±17 a 53.7 ±31.4 c

SC 298 ±7 e 64.4 ±3.4 a 210 ±9 d 98.7 ±11.0 b 130 ±21 d 49.9 ±1.1 cd

Agronomy 2021,11, 1175 13 of 26

Agronomy 2021, 11, x 13 of 26

Figure 2. Contents of nutrient (a) N, (b) P, (c) K, (d) Mg, (e) Ca and (f) Na in lettuce shoots grown in all tested treatments

(n = 5). Error bars indicate standard deviation. Different letters reflect that means significantly differ from one another

(HSD, Tukey test, α = 0.05; n = 5). The area highlighted in green represents the optimum range of contents, according to

Bergmann [40]. SC, sand control; TC-F, thermophilic compost from fecal matter; TC-G, thermophilic compost from green

waste; TC-G2 and TC-G3, thermophilic compost from green waste with additional N fertilization; VC-C, vermicompost

with coir bedding material; VC-P, vermicompost with paperboard bedding material.

Figure 2.

Contents of nutrient (

) N, (

) P, (

) K, (

) Mg, (

) Ca and (

) Na in lettuce shoots grown in all tested treatments

(n= 5). Error bars indicate standard deviation. Different letters reflect that means significantly differ from one another

(HSD, Tukey test,

= 0.05; n= 5). The area highlighted in green represents the optimum range of contents, according to

Bergmann [

]. SC, sand control; TC-F, thermophilic compost from fecal matter; TC-G, thermophilic compost from green

waste; TC-G2 and TC-G3, thermophilic compost from green waste with additional N fertilization; VC-C, vermicompost

with coir bedding material; VC-P, vermicompost with paperboard bedding material.

Agronomy 2021,11, 1175 14 of 26

According to Fricke [

103

], 30–50% of P in compost is used up by crops within the first

vegetation period after compost application. However, the P availability measured in the

composts (described by P

CAL

) included in this experiment was much lower than expected,

at only 16–35%. Three of the four tested types of compost could not supply P to the lettuce

plants sufficiently under the given conditions. Furthermore, the proportion of compost

tot

that the plants took up decreased as the pH value increased (r=

−

0.842, p< 0.001).

Lower P availability in more alkaline conditions is well described in the literature [

104

105

Different solubilities of P-salts at different pH values create competition with hydroxyl ions,

and the predominant presence of phosphate ions as poorly available HPO

42−

[

106

107

Frossard et al. [

108

] studied several urban waste-derived composts. They found that, in

those with alkaline pH, a range of Ca-P compounds with low solubility were present,

such as apatites or octacalcium phosphates. These forms of P result in a relatively low

plant availability, particularly in alkaline soils [ibid.], also accounting for the compost/sand

mixture. The high P uptakes and share of P

CAL

on P

tot

in TC-F suggest that available

P species were present, despite a pH of 8.8. Generally, the optimum range of pH for P

availability is around 6.0–6.5 [

], suggesting that lowering the substrate pH could further

enhance this availability in TC-F. Moreover, Vanden Nest et al. [

] reported that, in organic

fertilizers, P-use efficiency decreases with increased amounts of Ca, which could be noted

by the molar Ca:P ratio. Due to the different P-application amounts by compost addition,

the P-use efficiency for the tested composts could not be determined. Nevertheless, the

percentage of P uptake from applied P

tot

can be used as an indicator for available P. This

revealed a strong negative correlation with the Ca:P ratio of the composts (r=

−

0.946,

p< 0.001).

Regarding N uptake, the shoot tissues’ contents were broadly consistent among most

experimental groups (Figure 2). VC-C resulted in a shoot N content significantly lower

than the other treatments, including SC, indicating a dilution effect due to the higher shoot

DM yield. The highest N content was measured in plants grown in TC-G3, which had the

maximum N fertilization. It was the only treatment with sufficient N levels to supply the

lettuce plants, according to Bergmann [

], being within the range of 40–55 g kg

−1

. Despite

the shoots not achieving the N contents within Bergmann’s recommended nutritional range,

the lettuce heads did not show visual signs of N deficiency—the inner and outer leaves

were homogeneously green in color (Figure A2). The total shoot N uptake measured in the

plants grown in the tested treatments was observed in the following descending order: TC-

G3>TC-G2>VC-C=VC-P=TC-F

≥

TC-G

≥

SC (Table 4), and a correlation was found between

that and the treatment N

min

supply (i.e., N

min

of the compost plus mineral supplementation;

r= 0.946, p< 0.001). By contrast, no correlation was found with yield, despite that the

amount of N fertilization is often directly associated with plant growth [

108

]. The shoot N

uptake between the VCs and TC-F was comparable but was significantly higher for VC-C

than for TC-G. VC-P and TC-G demonstrated N-uptake levels comparable to that of SC,

suggesting a minimal N supply by these composts. Increased N uptake can be associated

with higher NH

fertilization levels, supported by the fact that the treatments with

added N (TC-G2 and TC-G3) resulted in a significantly higher N uptake compared with

the other treatments.

The calculated amount of available N in the composts (see Section 2.2) did not have a

detectable effect on N uptake in the lettuce shoots. None of the plant shoots took up more

N than that supplied by the mineral fertilization (Tables 2and 4). The mean N-uptake

values were in the range of 305–354 mg, whereas 376 mg N

min

as synthetic fertilizer was

applied. Furthermore, SC resulted in an N uptake lower than the N

min

supply (298 mg),

presumably due to the growth achieved in sand being limited compared to that in compost.

The fact that TC-G3 led to a significantly higher shoot N content, but not yield (FM

and DM) nor content of other macronutrients indicates that the available N in TC-G3

exceeded the plants’ demand. Hence, the maximum N fertilizer effect was already reached

in TC-G2. High N content in plant tissue is associated with high NO

3−

content, which can

be toxic to humans if consumed [

]. McCall & Willumsen [

107

] found that reducing

Agronomy 2021,11, 1175 15 of 26

the applied N

min

to lettuce plants to a certain amount did not affect yield but significantly

decreased leaf NO

3−

-N content. However, the leaf NO

3−

-N content was not measured in

the present study; therefore, the effect of N

min

fertilization on plant NO

3−

-N content could

not be assessed.

Concerning K levels, the shoot content was highest in plants grown in TC-F; those from

VC-C and TC-G were comparable (Figure 2, Table A2). Plants from these three treatments

met (VC-C and TC-G) or exceeded (TC-F) the target shoot K content recommended by

Bergmann [

], which ranges 42–60 g kg

−1

. Lower K content resulted from VC-P, TC-G2,

and TC-G3, which gave comparable measurements. Shoot K uptake per plant (Table 4)

ranged from 513 to 534 mg for all three TC-G treatments (p

≥

0.05), indicating a limited

availability of K at a higher growth rate. The higher DM gain at higher N levels (TC-G2 and

TC-G3) compared with TC-G was not linked with increased K uptake from the green waste

compost. The K content in the shoot tissue was much lower at higher N levels, while the

total shoot K uptake remained comparable. This indicates a limited availability of K from

the green waste compost, which TC-G already exploited. A significantly lower K uptake

per plant was observed for SC than the compost treatments, not having fully exploited

the mineral fertilizer application of 304 mg K per pot. In the lettuce plants grown in SC,

leaf tip necrosis, necrotic dots, and necrotic tissue spreading over large parts of the older

leaves were observed (Figure A3), and this was attributed to K deficiency. Since K supply

influences yield, K deficiency was considered as a critical factor in treatment comparison.

It can be presumed that SC was supplied with insufficient K, thus resulting in plant growth

limitation. When comparing SC and VC-P, which produced comparable shoot DM biomass,

VC-P had a K uptake of ~172 mg more than SC and still resulted in tissue K content below

the optimum levels. Therefore, the addition of K fertilizer to SC was insufficient. The

uptake of 60–83% of K observed in the present study agrees with the 50–80% of K from

compost reported being potentially available to plants within the first vegetation period

after application [

103

]. The availabilities determined for K were higher than expected, in

contrast to those for P.

Ca and Mg were supplied in an amount sufficient to achieve the optimal content

ranges of 12–21 and 3.5–6.0 g kg

−1

, respectively, by all treatments (Figure 2, Table A2).

The highest shoot Ca content was observed in the plants from VC-P and the three TC-G

treatments. The lower contents from VC-C, TC-F and SC were comparable. The Ca uptake

rates among treatments followed a similar trend as the tissue contents. Mg content was

significantly higher in VC-C and SC than in the other groups, at 9.82 and

9.24 g kg−1

respectively, exceeding the optimal content by ~3 g kg

−1

. The remaining compost treat-

ments demonstrated generally comparable content. The uptake of Mg was below the level

of mineral fertilization in most treatments, only slightly exceeded in TC-G2 and TC-G3

(Table 4)

. Uptake in the SC plants was similar to that observed for TC-G2, whereas the

mineral Mg fertilization was twice as high in SC. On the other hand, Mg uptake in plants

grown in VC-C (Table 4) exceeded the Mg input by mineral fertilization (Table 1) by 200%.

Therefore, at least 50% of the Mg in lettuce grown in VC-C was compost-derived, whereas

no clear conclusion can be made for the other treatments. The differences in Mg uptake

rates among the composts VC-C, VC-P, and TC-G match those for the contents of available

MgCAL (Table 2).

The lowest Mg and Ca contents in shoots were measured in plants grown in TC-F,

despite this compost containing the highest amounts of both nutrients. The high K contents

could have had antagonistic effects on Mg and Ca uptake by the plants, as reported by

Rietra et al. [

109

] or Jakobsen [

110

]. Furthermore, as mentioned above for P availability, the

pH can be a major influencing factor on Ca availability from the substrate, if it is present

as Ca

(PO

)

. The uptake of Ca was found to be negatively correlated with compost pH

(

r=−0.831

,p< 0.001), exhibiting a similar trend to P uptake. Additionally, Vandecasteele

et al. [

] reported that high compost Ca content and pH (measured in H

O) >8.5 were

associated with reduced P availability. The compost with the highest Ca content was TC-F,

Agronomy 2021,11, 1175 16 of 26

yet it resulted in the lowest shoot tissue content and uptake. However, shoot Ca content

for this compost was still sufficient, though on the lower end of the optimum range.

Finally, regarding Na content, most treatments did not result in significantly different

content (Figure 2), except TC-F, which led to a markedly higher value. The Na content

resulting from the four compost treatments revealed a relatively strong correlation with

shoot Na uptake (r= 0.956, p< 0.001). Na is recognized as an element beneficial to plants,

an essential nutrient for certain (halophytic) species [

111

]. However, a high Na content in

plant tissue can inhibit the uptake of other cations, mainly Ca, and can negatively affect

yield if a critical content of >20 g kg

−1

is exceeded [

]. This critical content was not

reached in any of the tested treatments; however, in TC-F, the high Na content may have

exerted antagonistic effects on Ca uptake, as TC-F resulted in the lowest shoot Ca content

(Figure 2). This assumption is supported by the negative correlation between Na and Ca

content in the shoot tissues (r=−0.845, p< 0.001) in the compost treatments.

3.3. Practical Applications of the Studied Composts in Urban Horticulture

3.3.1. Substrate Suitability

Among all tested treatments, VC-C resulted in the highest yield in marketable lettuce

shoot FM and DM yield. One of the main factors influencing this could be the high avail-

ability of macronutrients P, K, Mg and Ca, compared to the other treatments. Furthermore,

in lettuce grown in VC-C, the tissue content of all nutrients, except P, was within a suitable

range. In addition to nutrient supply, the high OM content of VC-C may have further

stimulated the plant growth. The amount of OM in soils is a critical parameter influencing

soil productivity since it improves physical and chemical properties and enhances soil

microbial activity [

112

]. This can accelerate plant growth, e.g., by the production of plant

growth-promoting substances [

]. Ruiz & Del Carmen Salas [

] reported beneficial

physicochemical substrate properties that increased enzyme activity when combining VC

with coconut fiber mixture.

The lower yield of lettuce grown in VC-P or TC-G was likely caused by the compost’s

lower nutrient content. Neither treatment could supply sufficient P to the plants, and VC-P

was also unable to provide sufficient K. All treatments enhanced plant growth compared

with SC. However, additional K fertilization in SC may have yielded different results since

the plant tissue P, Mg, and Ca levels were sufficient. As the content per compost DM of

almost all examined macronutrients was lower in VC-P and TC-G composts, compared

with the other composts, application of higher quantities of these organic fertilizers may be

able to compensate for the lower nutrient availability.

TC-F was the only one of the four tested composts that produced lettuce shoots with

suitable contents of macronutrients K, P, Mg and Ca. Moreover, the resulting P and K

uptake were among the highest. The high potential for fecal compost as a nutrient resource

in horticultural production systems, particularly P, has been reported by Krause et al. [

]

and Sangare et al. [

113

], among others. Although sufficient P and Ca contents were found

in the plant tissue, the full fertilizing potential of the TC-F treatment as a fecal compost

may have been hampered. TC-F contained the highest P and Ca contents but led to the

lowest proportion of P uptake per applied P and the lowest Ca uptake of all four compost

treatments. Reasons for this could include its high pH, leading to the chemical bonding of

P and Ca as insoluble Ca3(PO4)2, as well as possible Ca and Na cation antagonism.

Overall, as hypothesized, the VC-C and TC-F treatments led to a higher plant yield

than TC-G, but VC-P did not. Regarding the shoot nutrient uptake and status, P and K

supply were significantly higher for VC-C-and TC-F, whereas Mg supply was higher for

VC-C. For Ca, our hypothesis was disproved, as TC-G achieved a higher shoot content

than the VCs and TC-F. Finally, none of the treatments were able to supply sufficient N to

the lettuce plants.

Overall, the results of this study clearly show the suitability of the tested composts

produced from urban waste as organic fertilizers for horticultural production, as demon-

strated with the example of lettuce. Hence, these findings highlight the need to support

Agronomy 2021,11, 1175 17 of 26

recycling optimization and policy development to integrate hitherto-untapped resources,

such as source-separated human excreta (i.e., urine diversion), for the purpose driving a

sustainable circular economy with nutrient recycling.

3.3.2. Urban Potential of Compost Production and P Recycling, Using the Example

of Berlin

Lastly, we estimated the recycling potential of urban compost and the contained P,

using Berlin as an example. We focused on composts with the most promising fertilizing

potential: VC-C and TC-F. On average, 81 kg food waste [

] and 80 kg feces [

114

] are

produced in Germany per person annually (wet mass). In Berlin, a considerable proportion

of the population do not have access to organic waste collection (see Introduction). Small-

scale vermicomposting, however, is, in principle, feasible for most urban citizens, as little

money and space are required. Assuming that 10% of the city’s 3.7 million inhabitants [

115

]

establish a vermicomposting system in their household, nearly 25,000 t FM of coir-based

VC could be produced every year. From this, ~1.7 t P

CAL

or 7.8 t P

tot

could be recycled

annually from kitchen waste used for vermicomposting, if the P input of the coir itself is

excluded (Table A3).

The recycling of P from feces collected in inner-city apartments is not as quickly appli-

cable as most household toilets are connected to the central sewage system. In allotment

gardens, on the other hand, human excreta are disposed of via water closets into garden pits

that are emptied regularly by pump trucks. Here, dry toilets could be implemented, and

subsequent recycling could be carried out, e.g., through the establishment of professional

composting hubs. Assuming such a technological (and legal) realization, a total of 5.5 t

CAL

or 11 t P

tot

could be recycled from human excreta. For this estimation, we assumed

that each of the 70,000 allotment gardens existing in Berlin is used by two people for ~25%

of the year. Given this usage, a total of ~2800 t FM feces could be collected to produce

~5620 t FM fecal compost (Table A4).

4. Conclusions

Our results demonstrate that the composts TC-F, TC-G, and VC-C fulfilled the require-

ments for organic NK-fertilizer according to the German Fertilizer Ordinance DüMV. All

tested composts met the quality criteria of the German Federal Compost Quality Associ-

ation BGK. All tested composts supported plant growth to a certain extent, despite the

nutrient content in the plant shoots not reaching sufficient values in all cases. VC-C and

TC-F showed the most fertilizing potential in terms of plant growth and nutrient supply.

Additionally, VC-C offered a higher OM input per applied P (indicated by the C:P ratio),

hence demonstrating even more beneficial soil conditioner properties than the fecal-based

compost. Using paperboard instead of coir as bedding material for vermicomposting

lowered the VC product nutritional value and substrate quality. However, since coir is

a byproduct of coconut production, it is not a regional product nor a locally available

resource. Future research could, therefore, focus on adequate substitutes for coconut fiber

to be used in vermicomposting. Promising alternatives could include biochar, miscanthus,

or hemp fiber.

Furthermore, none of the tested composts provided sufficient N for horticultural

production. Further research on alternative N sources for the substitution of synthetic

fertilizer with compost is needed. In the long term, this is critical to achieve a complete

circular economy system by utilizing urban nutrient sources for urban farming. For this

purpose, the combination of fecal compost with (processed) urine may prove efficient

in terms of short- and long-term P availability and ensure an improved N supply. The

addition of nitrified or acidified urine would increase available N and lower the pH of the

fecal compost substrate to a more suitable range. However, the combination and optimum

mixture of urine and fecal compost in pot culture for urban horticulture require further

study.

In conclusion, our finding of the adequacy of the studied composts as substrates for

urban farming supports urban waste processing in decentralized units or facilities and the

Agronomy 2021,11, 1175 18 of 26

creation of regional inner-urban nutrient cycling. Small-scale compost systems, such as

vermicomposting units, can supplement the existing centralized organic waste recycling

systems. In small-scale or household-based systems, maintenance of high-quality input

material can be incentivized by direct use of the waste to produce recycling fertilizers, such

as VCs. In addition, traffic and emissions related with the collection and transportation of

organic waste within the cities can be reduced. Such cycling concepts for urban horticultural

systems that use urban waste to produce compost for urban farming can directly contribute

to several of the United Nations Sustainable Development Goals (SDG). Cycling economies

ensure an efficient, resource-conserving, and responsible production of plant-based food

(SDG 12), which includes sustainable production in peri-urban and urban areas (SDG 11).

Moreover, circular food-production systems that achieve nutrient recycling and C recovery

reduce nutrient and energy imbalances and, in doing so, promote the sustainable use of

terrestrial ecosystems (SDG 15) with a smaller negative impact on the climate (SDG 13).

Author Contributions:

Conceptualization, C.S., F.H., O.C.L. and A.K.; methodology, C.S., F.H. and

A.K.; software, C.S.; validation, C.S., F.H. and A.K.; formal analysis, C.S.; investigation, C.S., F.H.

and A.K.; resources, A.K. and F.H.; data curation, C.S.; writing—original draft preparation, C.S.;

writing—review and editing, C.S., F.H., O.C.L. and A.K.; visualization, C.S.; supervision, F.H., O.C.L.

and A.K.; project administration, A.K.; funding acquisition, A.K. All authors have read and agreed to

the published version of the manuscript.

Funding:

This project has received funding from the European Union’s Horizon 2020 research and

innovation programme under grant agreement No 774233. We further acknowledge support by the

German Research Foundation and the Open Access Publication Fund of TU Berlin.

Data Availability Statement:

Data supporting reported results can be requested from the corre-

sponding author.

Acknowledgments:

The provision of vermicompost fertilizers was made possible by Esther Fel-

gentreff’s and Paula Braun’s bachelor thesis projects at TU Berlin. The authors want to thank the

team of gardeners at IGZ for their regular plant care and support during the experiment. Also, our

gratitude to the various people at IGZ and TU Berlin doing the lab analysis. Special thanks also to

our colleagues who helped us with the harvesting work: Aladdin Halbert-Howard, Oscar Rodrigo

Monzon, Anja Müller, Farina Sempel, Stefan Karlowsky. We also acknowledge Eckhard George’s

supervision, who gave valuable advice for the fertilizing strategy, analysis of plant nutritional status,

and data analysis in general.

Conflicts of Interest:

The authors declare that they have no material financial interests related to the

research described in this paper. Regarding non-financial interests, the basis of this article was the

master thesis of Corinna Schröder. The thesis was therefore thoroughly revised, modified, and edited.

The funders had no role in the design of the study; in the collection, analyses, or interpretation of

data, in the writing of the manuscript, or in the decision to publish the results.

Appendix A

Table A1.

Limit values for heavy metals and trace nutrients according to the German Fertilizer

Ordinance (Düngemittelverordnung, DüMV) [

116

] and EU 2019/1009 regulation [

]; quality criteria

for the certification of compost according to the German Federal Compost Quality Association

(Bundesgütegemeinschaft Kompost e.V., BGK) [95]; * labeling threshold acc. to DüMV.

Cu Zn As Cd Cr Cr(VI) Ni Pb

mg kg−1DM

DüMV 500 * 1000 *

40 (20 *)

1.5 (1.0 *) 2

80 (40 *)

150 (100 *)

EU 2019/1009 300 800 40 1.5 2 50 120

BGK 100 400 40 1.5

100

50 150

Agronomy 2021,11, 1175 19 of 26

Table A2.

Nutrient content in the lettuce shoots for the different treatments and optimum nutrient levels for lettuce leaves,

according to Bergmann [

]. Treatments: VC-C (coir + kitchen waste vermicompost), VC-P (paperboard + kitchen waste

vermicompost), TC-F (fecal compost), TC-G (green waste compost), TC-G2 and TC-G3 (green waste compost with additional

N fertilization), SC (sand control).

Compost

Treatment

Ntot

[g kg−1]

Ptot

[g kg−1]

Ktot

[g kg−1]

Mgtot

[g kg−1]

Catot

[g kg−1]

Natot

[g kg−1]

VC-C 22.6 ±1.4 d 3.7 ±0.3 b 55.9 ±5.6 b 9.8 ±1.1 a 17.2 ±1.8 bc 5.0 ±0.4 b

VC-P 33.2 ±1.2 b 2.0 ±0.1 c 38.2 ±3.1 c 5.9 ±0.6 bc 20.6 ±2.9 ab 5.1 ±0.4 b

TC-F 31.5 ±3.3 bc 5.7 ±0.6 a 74.4 ±6.6 a 4.7 ±0.3 c 13.8 ±1.1 c 10.2 ±1.2 a

TC-G 32.3 ±2.7 b 3.2 ±0.5 b 56.6 ±3.7 b 6.2 ±0.5 bc 22.6 ±2.8 a 4.1 ±0.1 b

TC-G2 35.1 ±1.3 b 3.2 ±0.2 b 39.5 ±1.6 c 7.5 ±0.7 b 24.2 ±3.7 a 4.6 ±0.4 b

TC-G3 40.0 ±1.8 a 3.3 ±0.5 b 38.4 ±3.1 c 6.9 ±0.5 b 22.4 ±2.3 a 4.6 ±0.3 b

SC 27.7 ±1.5 c 6.0 ±0.2 a 19.6 ±0.4 d 9.2 ±1.5 a 12.2 ±2.6 c 4.6 ±0.2 b

Bergmann 40–55 4.5–7.0 42–60 3.5–6.0 12–21 -

Table A3. Calculation of the P recycling potential for the TC-F composting process in the city of Berlin, Germany.

Parameter Value Unit Source or Calculation

Feces potential

Feces produced per person 0.22 kg FM day−1[114]

or 80.3 kg FM year−10.22·365 days

Installation potential of dry toilets in Berlin

Number of allotment gardens 70,000 [117]

No. of people per garden 2 Assumption

Time of usage of gardens 25 % of the year Assumption

Fecal mass obtainable per year 2,810,500 kg FM 80.3 kg·70,000·2·0.25

Mass calculation of fecal compost processing TC-F

Total input mass of feces 2,810,500 kg FM Calculation

Share of feces in fecal compost 25–30 % Practitioner information

Total compost input material * 936,833 kg FM 2,810,500 kg

0.3

Weight loss during composting process 40 %

Practitioner information; [

118

]

Finished compost product mass 5,621,000 kg FM 9,368,333 kg·(1−0.4)

Dry matter content of finished compost 49.5 % DM Measured; [119]

Total dry mass of fecal compost TC-F 2,782,395 kg DM 5,621,000 kg·0.495

Ptot content of fecal compost 0.00398 kg Ptot kg−1DM Measured

Psol content of fecal compost 0.001986 kg Psol kg−1DM Measured

Ptot mass recycled per year 11,074 kg Ptot year−12,782,395 kg·0.00398

Psol mass recycled per year 5526 kg Psol year−12,782,395 kg·0.001986

* for composition see Section 2.2 Material & Methods—Fertilizer treatments

Table A4. Calculation of the P recycling potential for the VC-C vermicomposting process in the city of Berlin, Germany.

Parameter Value Unit Source or Calculation

Food waste potential in Berlin

Food waste produced per person and year 81 kg FM [22]

Percentage of food waste digestible by compost worms 80 % Practitioner information

No. of inhabitants in Berlin 3,669,500 [115]

Share of inhabitants starting VC 10 % Assumption

Usable food waste per year in Berlin

23,778,360

kg 81 kg·3,669,500·0.8·0.1

Vermicomposting process for VC-C

Total input mass of food waste

23,778,360

kg Calculation

Share of food waste 39.2 % [55]

Share of bedding material coconut fiber 60.8 % [55]

Total mass of input material

60,659,082

kg FM 23,778,360 kg

39.2 ·100

Agronomy 2021,11, 1175 20 of 26

Table A4. Cont.

Parameter Value Unit Source or Calculation

Vermicomposting process for VC-C

Weight loss during the composting process 22.8 % [55]

Finished compost product mass

46,828,811

kg FM 60,659,082 kg·(1−0.228)

DM of processed compost 12 % Measured

Total dry mass of vermicompost VC-C 5,619,457 kg DM 46,828,811 kg·0.12

Ptot content of vermicompost 0.001646 kg Ptot kg−1DM Measured

Ptot mass from VC-C per year 9250 kg 5,619,457 kg·0.001646

Psol content of vermicompost 0.000462 kg Psol kg−1DM Measured

Psol mass from VC-C per year 2596 kg 5,619,457 kg·0.000462

Subtraction of P input from coconut fiber

total input mass coconut fiber

36,880,722

kg FM Calculation (Total input

mass—food waste input mass)

DM of coconut fiber 13.5 % [55]

total dry input mass of coconut fiber 4,978,897 36,880,722 kg·0.135

Ptot content of coconut fiber 0.000291 kg Ptot kg−1DM [55]

Ptot mass input from coconut fiber 1449 kg DMcoconut fiber * c(Ptot)

Psol content of coconut fiber 0.00017 kg Psol kg−1DM [55]

Psol mass input from coconut fiber 846 kg DMcoconut fiber * c(Psol)

Ptot mass recycled per year 7801 kg Ptot year−19250 kg −1449 kg

Psol mass recycled per year 1750 kg Psol year−12596 kg −846 kg

Agronomy 2021, 11, x 21 of 26

Figure A1. Evapotranspiration of the lettuce plants and climate chamber temperature data. The lower y-axis shows the

evapotranspiration of the treatments in g per plant and day for the different treatments (measured and interpolated). The

upper y-axis shows the temperature [°C] in the climate chamber during the experiment. The lower and upper lines indicate

the daily minimum and the daily maximum temperature. The middle line indicates the daily mean temperature. The

average air temperature during the whole experimental period was 26.6 °C at day and 20.7 °C at night, and the average

relative humidity was 57.9%.

(a) (b)

Figure A2. Lettuce shoots on day 13 (a) and day 37 (b) of the experiment. The inner and outer leaves were homogeneously

green in color. Some of the lettuce shoots suffered from leaf tip necrosis (see also Figure A3) or drought stress, the latter

indicated by light brown dry leaves.

Figure A1.

Evapotranspiration of the lettuce plants and climate chamber temperature data. The lower y-axis shows the

evapotranspiration of the treatments in g per plant and day for the different treatments (measured and interpolated). The

upper y-axis shows the temperature [

◦

C] in the climate chamber during the experiment. The lower and upper lines indicate

the daily minimum and the daily maximum temperature. The middle line indicates the daily mean temperature. The

average air temperature during the whole experimental period was 26.6

◦

C at day and 20.7

◦

C at night, and the average

relative humidity was 57.9%.

Agronomy 2021,11, 1175 21 of 26

Agronomy 2021, 11, x 21 of 26

Figure A1. Evapotranspiration of the lettuce plants and climate chamber temperature data. The lower y-axis shows the

evapotranspiration of the treatments in g per plant and day for the different treatments (measured and interpolated). The

upper y-axis shows the temperature [°C] in the climate chamber during the experiment. The lower and upper lines indicate

the daily minimum and the daily maximum temperature. The middle line indicates the daily mean temperature. The

average air temperature during the whole experimental period was 26.6 °C at day and 20.7 °C at night, and the average

relative humidity was 57.9%.

(a) (b)

Figure A2. Lettuce shoots on day 13 (a) and day 37 (b) of the experiment. The inner and outer leaves were homogeneously

green in color. Some of the lettuce shoots suffered from leaf tip necrosis (see also Figure A3) or drought stress, the latter

indicated by light brown dry leaves.

Figure A2.

Lettuce shoots on day 13 (

) and day 37 (

) of the experiment. The inner and outer leaves were homogeneously

green in color. Some of the lettuce shoots suffered from leaf tip necrosis (see also Figure A3) or drought stress, the latter

indicated by light brown dry leaves.

Agronomy 2021, 11, x 22 of 26

(a) (b)

Figure A3. Leaf tip necrosis on the SC lettuce shoots (a) documentation of symptoms on several shoots, (b) detailed view

of leaves; pictures were taken on day 35 of the experiment.

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Figure A3.

Leaf tip necrosis on the SC lettuce shoots (

) documentation of symptoms on several shoots, (

) detailed view of

leaves; pictures were taken on day 35 of the experiment.

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