Exploration of Agronomic Efficacy and Drought Amelioration Ability of Municipal Solid-Waste-Derived Co-Compost on Lettuce and Maize

Abstract

Organic soil amendments, such as composts, mitigate the negative impacts on the environment that are caused by poor waste management practices. However, in the sub-Saharan African region, and Malawi in particular, studies investigating the agronomical efficacy and their ability to ameliorate drought stress when used as a soil amendment are minimal. This study aimed to evaluate the efficacy of sewage sludge and municipal solid waste (MSW) co-compost to ameliorate drought stress and improve crop productivity. Three experiments were conducted (i) to determine optimal application rate for co-compost, (ii) to evaluate yield response of maize and lettuce to co-compost application under contrasting soils, and (iii) to assess the effect of co-compost under water-limited conditions. Our results indicate that an application rate of 350 g co-compost per station was the most effective. This rate is 50% and 37% lower than the currently recommended rate for applying conventional compost to green vegetables and maize, respectively. In addition, under drought conditions, the co-compost application enhanced growth in lettuce, with less wilting, increased biomass and yield, approximately 130% greater leaf yield, and a 138% improvement in root growth. Furthermore, the relative root mass ratio (RRMR) was enhanced with the co-compost application by 103% under drought stress. This suggests that the co-compost amendment resulted in a greater allocation of biomass to the roots, which is a crucial morphological attribute for adapting to drought conditions. The concentration of K in the leaves and roots of plants treated with co-compost was significantly increased by 44% and 61%, respectively, under drought conditions, which may have contributed to osmotic adjustment, resulting in a significant increase in leaf relative water content (RWC) by a magnitude of 11 times. Therefore, in light of the rising inorganic fertilizer costs and the limited availability of water resources, these results demonstrate the potential of MSW and sludge co-composting in ameliorating the drastic effects of water- and nutrient-deficient conditions and optimizing growth and yield under these constraining environments.

1. Introduction

Malawi is experiencing one of the fastest population growth rates especially in urban areas, and it is projected to quadruple in the next 80 years. Over the course of 50 years from 1970, Malawi’s population has seen a 400% increase [1], a consequence of which entails an enormous increase in waste generation resulting in pollution and greenhouse gas (GHG) emissions. Malawi’s urban population generates substantial amounts of municipal solid waste (MSW), amounting to 0.65 kg per capita per day grossing 4.8 M Mg per year. Unfortunately, Malawi’s waste management is still in its infancy, with only few households having access to proper waste collection and disposal facilities. As a result, a big chunk of this waste is dumped in waterways and on roadsides or taken to landfills [2] where it is left to breakdown anaerobically thereby polluting the environment and generating large quantities of methane. While a number of policies, legislations, and regulations guide waste disposal, management, and utilization in Malawi, such as the National Environmental Policy (2004), National Environmental Action Plan, Environmental Management Regulations (2008), Environmental Management Act (1996), National Water Policy (2005), National Sanitation Policy (2008), and the National Management Strategy (2019–2023), there are no national standards that regulate the land application of sewage sludge and other composts [3,4]. As a result, untreated sewage sludge is often applied on home gardens and lawns, such that members of the public often manually excavate the sludge from wastewater treatment facilities for use in their agricultural fields [3]. Therefore, the co-composting of sludge with MSW would provide economic, environmental, and social advantages to all areas involved, including waste generation, waste management, and urban planning. It would allow for the recycling of a substantial amount of organic carbon found in MSW, thereby effectively closing the carbon cycle. In pursuit of a circular economy and the identification of viable alternatives to supplement fertilizers, municipal solid waste is a potential resource that can be transformed into valuable wealth through composting, thereby providing the much-needed nutrients for plant growth.

Composting facilitates the recycling of nutrients, promoting the transition toward a circular economy, a concept now promoted in various international forums. Both waste management and agriculture can receive advantages from adopting a circular approach to agriculture, which involves the recovery and reuse of waste resources. Therefore, in order to address the increasing production of organic waste worldwide, the implementation of recycling and composting methods has been proposed [5]. In addition, composting is also gaining popularity as an alternative to traditional waste processing processes. Currently, it is being employed to process several types of organic waste, including farm manures, sewage sludge, industrial sludge, and the organic component of municipal solid waste (MSW) [6,7]

Composting municipal solid waste (MSW) has become increasingly popular since it has the ability to avoid environmental degradation, decrease the need for landfill space, lower incineration expenses, and create more options for agricultural output. However, the usage of MSW compost is limited because crops do not thrive when it is used exclusively due to its generally low levels of essential plant nutrients. The co-composting of municipal solid waste (MSW) with nutrient-rich biosolids is becoming increasingly popular. The advantages associated with co-composting are easily observable. MSW serves as a filler material, while biosolids offer a readily available source of nitrogen and other nutrients. Co-composting differs from typical composting procedures and has proven to be a highly efficient way to improve the physiochemical properties of soil. It provides a sustainable option for maintaining soil health.

Additionally, it cleans up the environment and minimizes pollution. Co-composting of sewage sludge and organic solid waste is advantageous because the two materials complement each other well. Sewage sludge has a relatively high nitrogen content, while organic solid waste is high in organic carbon and has good bulking properties. Pathogens found in excreta can be eliminated by the co-composting process’s thermophilic conditions, which are defined as temperatures higher than 50 °C. Consequently, both wastes are transformed into a fertilizer and soil conditioner that are safe for human health. The use of co-composts in agriculture can supplement, complement, or substitute chemical fertilizers while replenishing soil health. Long-term trials with co-compost show high bioavailability of macro- and micro-nutrients. Composts are also slow-release fertilizers that ensure a steady supply of nutrients over a long time with an additional positive effect of reducing greenhouse gas emissions when compared to inorganic fertilizers.

In Malawi, although the use of compost-derived from MSW is not widespread, some studies on the continent have attested to the fertilizing and soil conditioning efficacy of co-compost. The main focus of this study was to assess the quality and possible functions of compost derived from municipal solid waste (MSW) in Malawi agricultural systems. The main objectives of the study were (1) to determine the optimal application rates for co-compost in maximizing the growth and yield of maize and lettuce, (2) to evaluate the agronomic efficacy of co-compost in maize and lettuce and changes in soil physicochemical properties under varying compost amendments, and (3) to evaluate the ability of co-compost in ameliorating adverse effects of drought stress in lettuce. The study has shown that with a proper composting process and pre-fortification with a N-rich sewage sludge, co-compost offers a remarkable potential in increasing crop yields, in a manner similar to, or even more efficient than inorganic fertilizers, while also reducing the adverse effects of drought stress, by enhancing soil moisture retention and crop water status.

2. Materials and Methods

2.1. Site of the Study

The study was conducted under both field and greenhouse conditions at the Lilongwe University of Agriculture and Natural Resources (LUANAR)’s Bunda College Horticultural Farm, Lilongwe district, central region of Malawi. Malawi is found in the southeastern part of Africa and is characterized by subtropical conditions with a subhumid climate. Based on variations in altitude and climate, the nation is split into three primary agroecologies: the Shire Valley, the lakeshore, and the mid-altitude or plateau regions (900–1200 m above sea level). Lilongwe district lies in the mid-altitude region, and the farm is situated at 14°35′ S, 33°50′ E, 1158 m above sea level, Figure 1. The site receives an average rainfall of 1030 mm/year and has an average temperature of 20 °C.

2.2. Description of Agronomic Experiments

Experiment 1.

Determination of optimal compost application rates for the growth of Maize.

This experiment was conducted to determine the optimum application rates for the growth and yield of maize (Zea mays). The experiment was conducted under field conditions at LUANAR’s Bunda College Horticultural Farm (Figure 1). The soil at the site was classified as Alfisol using the USDA classification system. The site is frequently cultivated with horticultural crops, predominantly vegetables. Maize was selected for this study because it is staple crop in the Malawian food system. The experiment was laid out in Randomized Complete Block Design (RCBD) with four treatments (none as a negative control, 200 g, 350 g, and 500 g per planting station of one plant) and replicated three times. The co-compost was applied 14 days before planting in ridges on planting stations to allow mineralization, while top dressing was added three weeks after planting. Plants were harvested after physiological maturity. Certified seed of Kanyani maize variety was used in this study.

Experiment 2:

Evaluation of growth, yield response of maize and lettuce, and changes in soil physicochemical properties under varying compost amendments.

Two experiments were conducted for this study. For lettuce, the experiment was a greenhouse study in which Alfisol soil was collected from cultivated fields and adjacent forest to assess agronomic efficacy of various soil amendments on growth and yield of lettuce in contrasting soils (i.e., difference in management). The soils from cultivated fields were characterized by low organic matter, while virgin soils from forest were characterized by high organic matter. The soils were filled in 10 L pots. Soil was filled to 80% capacity of the pot by volume. The treatment conditions were as follows: no fertilizer application (none/control), 350 g of composts (conventional and co-compost), 350 g sludge, and 8 g of inorganic NPK fertilizer comprising 23% N, 10% P, and 5% K (426 kg/Ha). The amount of 350 g per station translates to 11.7 Mg/ha for the used spacing of 75 cm by 40 cm. The experiment was laid out as Completely Randomized Design with 3 replications. Certified seed of lettuce variety Great Lakes was used for this experiment.

The maize field experiment was conducted at LUANAR’s Bunda College Horticultural Farm (Figure 1), and the soil was characterized as Alfisols using the USDA 2022 classification system. The experiment was laid out as Randomized Complete Block Design (RCBD) with five treatment conditions as follows: no fertilizer application (none/control), 350 g of composts (conventional and co-compost), 350 g sludge, and 8 g of inorganic NPK fertilizer comprising 23% N, 10% P, and 5% K replicated 3 times. The plants were planted on ridges produced by conventional primary tillage using hand hoeing at a depth of 30 cm. The plants were planted at a spacing of 25 cm within ridges and 75 cm between ridges. A total of 350 g of compost/sludge per station translates to 18.7 Mg/ha for the used spacing. Certified seed of Kanyani maize variety was used in this study.

Experiment 3.

Response to drought stress of lettuce grown on compost-amended and non-amended soils.

This experiment was conducted to evaluate the response of lettuce to drought stress in composted and non-composted soil under greenhouse conditions. This experiment was based on the known property of organic matter in enhancing the water-holding capacity of soils. It was hypothesized that soils enriched with composted material would have better water retention capacity and thus could improve plant survival under drought conditions. In this experiment, plants were grown in pots filled with low organic matter soil as described in experiment 2. The potted soil was uniformly enriched with 350 g co-compost, 350 g conventional compost, 350 g sewage sludge, or no fertilization (none/control). Under field practices, the used rate of 350 g/station for compost/sludge is equivalent to 11.7 Mg/ha. Five pots, each containing two plants, were used for each treatment. The experiment was arranged in Completely Randomized Design (CRD) with 5 replicates. As growth progressed, the number of plants was reduced to one. Water capacity of the pots was determined prior to planting using a gravimetric method previously described in [8]. Drought stress was induced three weeks after planting until harvest at 35–40% of pot water capacity (PWC). The pots were weighed on a daily basis, and watering was only performed when the weight of the pot was below the specified range for control (75–80% PWC) and drought (35–40% PWC). Certified seed of lettuce variety Great Lakes was used for this experiment.

2.3. Used Composts and Composting Processes

In this study, fertilizer sources used overall were as follows: (1) NPK (inorganic fertilizer/positive control), (2) conventional compost made from a mixture of plant residues and boosters (i.e., animal manure), (3) co-compost made from MSW and sewage sludge, and (4) processed sewage sludge (sludge) obtained from Kauma sewer processing unit by Lilongwe city council. The conventional compost was made by mixing of plant residues (maize stalks, sugarcane remains, banana leaves, groundnut leaves, and rice husks), soil (as microbial source), and water. These materials (except water) were put into a heap and a layer of soil was added to the heap. Then, more organic waste was added to the heap on top of the soil; this step was repeated until the heap attained the required size. Once the required heap size was attained, some water was added to the heap to moisten the material. The heap was turned every 7 days for up to 30 to 36 days, and at every turning, additional water was added to moisten the material further. MWS-derived co-compost in this study was made by mixing municipal solid waste and sewage sludge. The MSW used to make the compost were as follows: banana leaves, cabbage leaves, food waste, groundnut shells, fruits, and other vegetable waste. The processed sewage sludge was collected by city assembly treatment plants. The procedure for making the co-compost was as follows: A pile was made using municipal solid waste (MSW), groundnut shells, sewage sludge (10%), green plant wastes, and dry plant waste samples making a 12 m³ heap measuring 3 m × 2 m by 2 m (surface to volume ratio is key). Then, 1000 L of water was added to the pile. After 3 days, the pile was turned when temperature reached 70–80 °C (to ensure O₂ circulation and avoid anerobic respiration), after which another 1000 L of water was added. After 3–4 days, the heap was turned again. These steps were repeated while reducing the amount of water applied due to shrinkage of the pile, to about 600 L. The turning was performed up to 7th time, after which no more water was added. Thereafter, the co-compost was spread out to air-dry and sieved on a 1 cm diameter sieve to remove large and undecomposed residues. The maturity of the compost was determined using physical methods by observing changes in the color, odor, and temperature according to [9]. Composts were considered mature for use when the color of the composted material changed to dark or greyish black, when the composted material had completed a thermophilic phase with a temperature of 72 °C and attained a stable ambient temperature of 27–30 °C during maturation phase, and when the compost attained an earthy odor. The undecomposed residues were used to make a new heap.

2.4. Chlorophyll–SPAD Measurements

The chlorophyll concentration in the leaves was measured using a chlorophyll meter (SPAD-502 Chlorophyll meter, Minolta Camera Co., Ltd., Osaka, Japan). The measurements were taken on fully expanded leaves from the center locations in all replicates for each treatment combination. Three measurements were obtained for each sample, and the final recorded value was determined by calculating the average of the three measurements collected from distinct places inside the same leaf.

2.5. Measurement of Leaf Relative Water Content (RWC)

Leaf water status was assessed by quantifying leaf relative water content (RWC). Leaf relative water content (RWC) was measured on the third inner leaf of each lettuce plant on the final day of harvesting. To accomplish this objective, the leaf was promptly measured to ascertain its fresh weight (FW). Subsequently, it was fully submerged in distilled water within a 15 mL falcon tube and left overnight under illuminated settings. After a period of 24 h, the turgid leaves were delicately extracted from the tubes, wiped with a gentle fabric to eliminate any moisture on the surface, and subsequently measured to determine their turgid weight (TW). They were then subjected to a drying process in an oven at a temperature of 80 °C for a duration of 72 h to obtain their dry weight (DW). The leaf relative water content (RWC) was subsequently determined using the method described in [10].

$$RWC={\displaystyle \frac{(FW-DW)}{(TW-DW)}}\times 100$$

2.6. Plant Tissue Analysis

Elemental analysis was performed according to a method in [11]. For this purpose, leaf and root samples were oven-dried at 80 °C for 72 h. About 2 g each of the leaf and root samples were ashed in a muffle furnace at 450 °C for 3 h. The ash was transferred to 100 mL measuring flasks, containing 50 mL deionized water and 10 mL hydrochloric acid (25%). The solutions were heated to 90 °C for 2 h in a water bath, cooled, transferred to 100 mL volumetric flasks, and filled up to the mark with deionized water. Samples that were not clear were filtered before measurements. Concentrations of macronutrients (K, Mg, Ca, and Na) and micronutrients (Fe, Zn, Mn, and Cu) were determined using an atomic absorption spectrometer (Varian, SpectrAA–20, Melbourne, Australia).

2.7. Soil Analysis

Soil analyses were performed following standard methods of soil analysis for the parameters of interest. These are indicated in

Table 1. Summary of methods used for soil analyses.

Nutrient Variable	Units of Measurement	Method Used	Reference
pH (water)	pH	1:1 (soil–H2O)	[12]
Organic carbon	%	Walkley and Black	[13]
Total N	%	Kjeldal method	[14]
Available P	mg/kg	Mehlich 3 method, spectrophotometry	[15]
Exchangeable K, Ca, Mg	mg/kg	Mehlich 3 method, spectrophotometry	[15]
Cu, Zn	mg/kg	1 M NH4Cl, spectrophotometry	[12]
EC		Electrical conductivity meter	[16]

below. All laboratory measurements were conducted in quadruplicate.

2.8. Statistical Analysis

Data were analyzed using R statistical package, version 3.6.0. Inferential statistics were performed using analysis of variance (ANOVA) at 0.05 level of significance. Separation of means was performed using Tukey’s test (Honestly Significant Difference) selected for its higher power to correctly interpret the statistical significance of the difference between means with extreme values/outliers. Results represent means and standard errors from 6 biological replicates. Letters indicate results from multiple comparison tests using Tukey’s test. Different letters indicate significant differences between means at 0.05 level of significance. Similar letters indicate lack of statistical significance at 0.05 level of significance.

3. Results

3.1. Determination of Optimal Compost Application Rates for Growth of Maize

In order to determine the optimum application rates for the growth and yield of maize, plants were provided with four rates of MSW-derived co-compost. These were 0 g, 200 g, 350 g, and 500 g per planting station. In the vegetative growth parameters (shoot and root dry weights, plant height, and stem diameter), there were hardly any significant differences between the composted and non-composted soils (

Table 2. Effect of compost application rates on plant growth parameters and chlorophyll content of maize.

Treatment	SDW (g)	RDW (g)	SD (cm)	Plant Height (cm)	SPAD
0 g	90 + 7.1 a	10.1 + 0.8 a	1.7 + 0.07 a	170 + 9.6 a	23.1 + 1.8 a
200 g	115 + 9.8 b	14.4 + 1.3 a	1.9 + 0.07 a	195 + 5.6 b	36.7 + 1.9 b
350 g	9 + 5.1 a	16.2 + 1.0 a	1.9 + 0.07 a	193 + 4.3 b	39.7 + 1.9 b
500 g	98 + 4.7 a	22.4 + 2.2 b	2.1 + 0.06 a	192 + 8.3 b	44.1 + 2.6 b

SDW = Shoot dry weight; RDW = Root dry weight; SD = Stem diameter. Different letters indicate significant statistical differences at 0.05 level of significance using Tukey test. Similar letters indicate lack of statistical differences at 0.05 level of significance using Tukey test.

). However, for grain yield parameters (cob fresh weight, cob dry weight, grain dry weight, and average grain size), co-compost incorporation resulted in a significant increase in grain yield parameters by almost two folds (

Table 3. Effect of compost application rates on grain yield components of maize.

Treatment	CFW (g)	CDW (g)	GDW (g)	AGS (mg)
0 g	134 + 7.3 a	62 + 4.9 a	49 + 4.5 a	175 + 6.6 a
200 g	246 + 8.4 b	117 + 4.8 b	97 + 4.3 b	225 + 6.6 b
350 g	245 + 7.5 b	122 + 4.5 b	101 + 4.2 b	221 + 6.5 b
500 g	261 + 8.3 b	128 + 5.5 b	105 + 5.0 b	228 + 8.1 b

CFW = Cob fresh weight; CDW = Cob dry weight; GDW = Grain dry weight; AGS = Average grain size. Different letters indicate significant statistical differences at 0.05 level of significance using Tukey test. Similar letters indicate lack of statistical differences at 0.05 level of significance using Tukey test.

). For example, control plants had an average cob fresh weight of 134 g per plant, whereas the amendment with 200 g, 350 g, and 500 g of co-compost produced 247 g, 246 g, and 261 g per plant (Table 3). Similar patterns were observed for cob dry weight, grain dry weight, and average grain size (Table 3). Within the three co-compost application rates (200 g, 350 g, and 500 g per station), there were no significant differences in grain yield parameters, although 500 g per station produced a relatively higher yield. However, from an economic perspective, 350 g per planting station yielded nearly similar results, so it was recommended that 350 g be the ideal application rate. This application rate is 30% less than the current recommended practice in Malawi of 500 g per station.

3.2. Evaluation of Growth and Yield Responses of Lettuce to Varying Compost Amendments Under Greenhouse Conditions

To evaluate the growth and yield responses of lettuce to different compost and co-compost amendments, lettuce plants were treated with 350 g of compost and co-compost in both greenhouse and field conditions. Two different soil types were used in the greenhouse: virgin soil from the forest, which was richer in organic matter, and soil collected from a frequently cultivated farm. It was hypothesized that lettuce would respond more strongly to compost amendments in relatively poorer soils as observed by [17]. In general, plant growth was much higher in lettuce plants grown in virgin soil than in cultivated soil (Figure 2 and Figure 3). There were no significant differences in plant height and stem diameter between the different fertilizer sources in both cultivated and virgin soils (Figure 2A,D). However, root length was significantly increased by the co-compost amendment especially in the cultivated soil, by a magnitude of 1.5, while differences were insignificant in the virgin soil (Figure 2B). Leaf fresh weight was also significantly increased by the co-compost application, followed by sewage sludge, and these responses were similar in both virgin soil and cultivated soil (Figure 2C). Biomass accumulation as measured through shoot dry weight almost doubled in the co-compost-treated plants, in both virgin soil (139 g vs. 63 g per plant) and cultivated soil (130 g vs. 45 g per plant) (Figure 3A), followed by sludge (99 g and 107 g under cultivated and virgin soils, respectively), whereas in NPK-treated plants, differences were insignificant from the untreated plants. The magnitude of increase in shoot dry weight by the co-compost application in cultivated soil was 2.9 g, whereas in virgin soil, it was 2.2 g, indicating a greater growth response in the impoverished cultivated soil than in the organic-matter-rich virgin soil (Figure 3A). Root dry weight was also considerably increased by the co-compost soil amendment in both cultivated (5.0 g) and virgin soils (3.9 g) compared to none (1.4 g and 1.9 g per plant in the cultivated and virgin soils, respectively) and conventional compost- and NPK-treated plants (Figure 3B), whereas no significant differences were shown for leaf relative water content (Figure 3C). Chlorophyll concentration was measured using an SPAD meter showed a significant increase by co-compost (82%) and sewage sludge (87%), especially in cultivated soils relative to the “none” treatment, whereas in virgin soil, the NPK application showed the highest chlorophyll concentration, which was increased by 53% (Figure 3D).

3.3. Evaluation of Growth and Yield Responses of Lettuce to Varying Compost Amendments Under Field Conditions

Under open field conditions, lettuce was cultivated under rainfed conditions, and it was shown that the co-compost amendment exhibited the highest yield in terms of leaf fresh weight of 395 g and leaf dry weight of 17.7 g per plant (

Table 4. Comparative agronomic efficacy of different composts and NPK fertilizer on growth and yield of lettuce.

Treatment	LFW	LDW (g)	RDW (g)	RFW (g)	# Leaves
0 g	190 + 30 a	10.9 + 8.3 a	0.8 + 0.1 a	5.7 + 0.8 a	17.9 + 1.7 a
Co-compost	394 + 43 c	17.7 + 1.6 b	1.2 + 0.2 b	12.8 + 1.6 b	26.5 + 1.2 b
Compost	204 + 31 a	11.2 + 1.0 a	0.9 + 0.2 a	7.8 + 1.1 a	19.8 + 1.0 ab
NPK	322 + 36 c	16.9 + 1.9 b	1.2 + 0.2 b	10.8 + 1.6 ab	23.2 + 1.0 ab
Sludge	307 + 38 c	13.0 + 1.2 ab	1.1 + 0.1 ab	9.5 + 1.0 ab	22.3 + 1.5 ab

LFW = Leaf fresh weight; LDW = Leaf dry weight; RDW = Root dry weight, RFW = Root fresh weight, # Leaves = Number of leaves. Different letters indicate significant statistical differences at 0.05 level of significance using Tukey test. Similar letters indicate lack of statistical differences at 0.05 level of significance using Tukey test.

). This was followed by the NPK fertilizer application with an LFW of 323 g and an LDW of 16.9 g per plant and sewage sludge with an LFW of 307 g and LDW of 13.0 g per plant. Furthermore, root fresh and dry weights were also significantly enhanced by the co-compost application with a magnitude of 125% and 62%, respectively, followed by the NPK application and sewage sludge, in a manner similar to leaf fresh and dry weights (Table 4). Co-compost-amended plants also exhibited the highest number of leaves with an average of 27 leaves, compared to 18 leaves for the negative control and 19 leaves per plant for conventional compost. Also, root and stem diameters were significantly enhanced by the co-compost application with a magnitude of 1.3 and 1.7, respectively, followed by the NPK fertilizer application with magnitudes of 1.2 and 1.4, whereas the conventional compost treatment was not significantly different from the non-compost-amended treatment (

Table 5. Comparative agronomic efficacy of different composts and NPK fertilizer on growth and chlorophyll content of lettuce leaves.

Treatment	RD (cm)	SD (cm)	Root length	SPAD
Control	1.2 + 0.1 a	1.3 + 0.1 a	14.0 + 0.9 a	29.7 + 1.8 a
Co-compost	1.5 + 0.1 a	2.2 + 0.1 b	13.1 + 1.2 a	38.7 + 1.3 b
Compost	1.1 + 0.1 a	1.4 + 0.1 a	12.2 + 1.2 a	36.0 + 2.0 ab
NPK	1.4 + 0.1 a	1.8 + 0.6 ab	14.0 + 1.1 a	39.8 + 1.6 b
Sludge	1.3 + 0.1 a	1.8 + 0.1 ab	12.1 + 0.8 a	36.0 + 1.3 ab

RD = Root diameter; SD = Stem diameter. Different letters indicate significant statistical differences at 0.05 level of significance using Tukey test. Similar letters indicate lack of statistical differences at 0.05 level of significance using Tukey test.

). Strikingly though, the control plants (none) produced the longest roots (14 cm long), albeit with the differences being insignificant from the treatments (Table 5). The chlorophyll concentration was the highest in the NPK fertilizer treatment (39.8 SPAD units) and co-compost-amended plants (38.7 SPAD units), significantly higher than the negative control (29.8 SPAD units). However, the differences were also not as pronounced within the treatments (Table 5).

Tissue element analysis was then conducted in dried leaf and root samples to evaluate if growth and yield parameters could be attributed to tissue nutrient concentration. It was shown that in leaf samples, plants applied with co-compost had the highest K concentration of 3730 µg/g DW followed by sewage sludge of 2501 µg/g DW, whereas the lowest was in the none treatment group with a concentration of 2030 µg/g DW. In roots, a similar observation was made for the co-compost-treated plants, which also had the highest K concentration of 2159 µg/g DW compared to 1176 µg/g DW for the negative control plants (

Table 6. Effect of compost types on macro- and micronutrient accumulations in leaves and roots of lettuce plants under open field conditions.

Concentration (µg/g DW)
Organ	Compost	K	Na	Ca	Mg	Zn	Mn	Fe	Cu
Leaf	NPK	2355	237	2618	314	49.4	277	1717	4.2
	Sludge	2501	279	2893	215	66.6	312	2552	6.3
	Control	2030	183	2063	206	50.9	206	2096	6.4
	Co-compost	3731	168	2686	253	63.6	260	2961	9.2
	Compost	2246	202	2335	234	60.4	268	2345	7.5
Root	NPK	1110	370	1730	49.5	45.5	189	1864	1.9
	Sludge	1552	943	2677	65.7	54.5	194	1942	12.2
	Control	1177	497	1827	74.3	42.4	183	1790	9.8
	Co-compost	2159	186	2437	71.0	57.0	170	2440	12.5
	Compost	2034	476	2349	70.4	48.4	174	1974	10.3

). A striking observation was the very high Na concentration in plant tissues from sewage sludge-treated plants (279 µg/g DW in leaves and 943 µg/g DW in roots), especially in the roots. The Ca concentration was also higher in the co-compost (2686 µg/g DW)-and sewage sludge (2893 µg/g DW)-treated plants. These also had the highest concentrations of Fe (2962 and 2552 µg/g DW) and Cu (9.2 and 6.3 µg/g DW) (particularly pronounced in co-compost), whereas Zn and Mn were notably higher in sewage sludge-treated plants in both leaves (with concentrations 66.6 and 312 µg/g DW) and roots (with concentrations of 54.5 and 194 µg/g DW).

In order to evaluate the residual efficacy of the soil amendments in lettuce grown under field conditions, various soil chemical attributes were evaluated. No significant differences were found in soil pH among the soils applied with various amendments. However, soils amended with sewage sludge had significantly higher electrical conductivity of 162 compared to 94.4 in the negative control, 89.6 in co-compost, and 107 in conventional compost (

Table 7. Effect of soil amendments on residual mineral concentrations and soil chemical properties in lettuce grown under field conditions.

Treatment	pH	EC	OM %	N	Concentration (mg/kg)
					P	K	Ca	Mg	Cu	Zn
Control	5.8	94	4.1	0.13	12.3	313	2038	334	0.07	0.8
Co-compost	6.1	90	5.7	0.15	26.0	1221	3700	675	0.12	1.8
Compost	6.2	107	5.0	0.14	16.2	586	3388	672	0.30	1.6
Sludge	5.5	163	4.7	0.14	15.4	351	3144	618	0.09	1.3

). The application of co-compost significantly improved the residual organic matter content in the soil by 39%, whereas no significant improvements were observed in N content. Regarding the mineral element analysis, soils amended with co-compost had considerably higher P (26.0 mg/kg vs. 12.3 mg/kg in control) and K (1221 mg/kg vs. 313 mg/kg in control) concentrations, whereas Ca and Mg were significantly improved by all amendments (conventional compost, co-compost, and sewage sludge, Table 7). The micronutrient element Cu was also significantly improved by the co-compost and compost by a magnitude of 1.7, whereas Zn was significantly improved by all amendments by at least a magnitude of 1.5 (Table 7).

3.4. Evaluation of Growth and Yield Responses of Maize to Varying Compost Amendments Under Field Conditions

In order to evaluate the growth and yield responses of maize to soil amendments, maize plants were grown on soil amended with none (negative control), co-compost, conventional compost, sewage sludge, and inorganic fertilizer (positive control) and various vegetative and reproductive growth parameters were evaluated. Plant height and root length did not show any significant differences among the soil amendments (Figure 4A,B). However, shoot and root dry weights were significantly higher in maize grown in soils amended with co-compost (149 g and 22.6 g per plant), whereas conventional compost (88.0 g and 14.9 g per plant) did not significantly differ from non-amended soil (73.4 g and 12.2 g per plant) (Figure 4C,D). Root dry weight was also significantly higher under the NPK fertilizer (21.7 g per plant) and sewage sludge (19.6 g per plant) (Figure 4D). Secondary growth, measured as stem diameter, was not significantly different in all treatments (Figure 4E). The chlorophyll concentration showed considerable increases by all soil amendments and was especially more pronounced in the NPK (57.4 SPAD units)- and co-compost-amended soils (56.7 SPAD units) (Figure 4E). Yield and yield components were also evaluated and showed significant increases in cob fresh and dry weights, by all soil amendments, especially co-compost (275 g and 145 g per plant) and NPK fertilizer (281 g and 136 g per plant) (Figure 5A,B). Cob weight represented the weight of a whole maize cob including the cob, grains, and sheath. Grains were then separated from the cob and measured separately, and the grain weight per cob was also considerably higher in all soil amendments, especially co-compost (Figure 5C). The cob grain dry weights were 56.1 g, 118 g, 100 g, 112 g, and 114 g in the control, co-compost, conventional compost, NPK, and sludge, respectively.

In order to compensate for the differences in cob size that may likely influence grain weight per cob, we determined the weight of 50 grains (Figure 5D) and the pattern was also similar to cob fresh weight, cob dry weight, and cob grain weight shown in Figure 5A–C, albeit the differences were smaller. The grain fresh weights were 9.1 g, 12.4 g, 108 g, 12.1 g, and 12.1 g for the control, co-compost, conventional compost, NPK, and sludge, respectively. This prompted the measurement of average grain size, which also showed a similar pattern, and was highest in the co-compost soil-amended plants (Figure 5E). A striking observation, however, was the harvest index (HI), which measures the ratio of grain to total shoot dry matter and represents a plant’s reproductive efficiency [18]. Here, the HI was the highest in plants grown under NPK, which had an HI of 1.2, and the lowest in plants grown on soils without any soil amendment (Figure 5F), which had an HI of 0.76, and those amended with co-compost (0.79), signifying less reproductive efficiency in the co-compost amendment.

Plant tissue analysis showed the highest K concentration in leaves whose plants were applied with sewage sludge (299 µg/g DW) and co-compost (288 µg/g DW). In roots, a higher K concentration was observed in co-compost (380 µg/g DW) and NPK (361 µg/g DW) (

Table 8. Effect of compost types on macro- and micronutrient accumulations in leaves and roots of maize plants under open field conditions.

Concentration (µg/g DW)
Organ	Compost	K	Na	Ca	Mg	Zn	Mn	Fe	Cu
Leaf	Sludge	299	59.5	2035	214	33.5	135	1685	4.7
	NPK	240	41.1	1634	88	29.5	155	1309	5.2
	Control	166	43.4	1360	87	28.6	118	1242	4.4
	Co-compost	288	26.3	1973	137	27.0	102	1894	7.9
	Compost	220	49.4	1745	86	28.4	105	1358	5.3
Root	Sludge	315	34.8	1647	46.7	25.9	121	2326	11.6
	NPK	361	32.2	1375	47.1	20.5	135	1884	6.1
	Control	262	37.1	1048	47.3	24.6	100	2048	7.0
	Co-compost	381	21.4	1720	46.2	19.2	148	2981	12.5
	Compost	285	33.5	1234	46.9	22.3	113	2420	8.3

). Similar to the observation in lettuce (Table 6), the Na concentration was also significantly higher in plant leaves whose plants were applied with sewage sludge with a concentration of 59.5 µg/g DW) compared with 43.4 µg/g DW in the control (Table 8). The Ca concentration was also higher in plants amended with co-compost (1973 µg/g DW in leaves and 1720 µg/g DW in roots) and sewage sludge (2035 µg/g DW in leaves and 1647 µg/g DW in roots). Also, sludge-treated plants had an unusually higher Mg concentration in the leaves (214 µg/g DW), which was nearly thrice higher than the average values for the other treatments followed by co-compost (137 µg/g DW). Co-compost-treated plants exhibited the highest Fe and Cu concentrations in both leaves (1894 µg/g DW and 7.9 µg/g DW) and root tissues (2980 µg/g DW and 12.5 µg/g DW) followed by sewage sludge (1685 µg/g DW and 4.7 µg/g DW in leaves and 2325 µg/g DW and 11.6 µg/g DW in roots). No significant differences were shown for the Zn concentration in both leaf and root samples, whereas the Mn concentration was significantly higher in the leaves of plants applied with inorganic NPK fertilizer (155 µg/g DW), and in roots, Mn was the highest in co-compost-amended plants (148 µg/g DW).

3.5. Evaluation of Lettuce’s Response to Drought Stress Under Compost- and Non-Compost-Amended Soils

This experiment was conducted on the premise that soil amendment with compost would improve the soil’s water-holding capacity and subsequently enhance the plants’ response to drought stress. Figure 6 shows a photograph of plants grown under drought and well-watered soil conditions amended with compost, co-compost, and sludge. Under drought stress, growth in terms of plant height, root length, and number of leaves was reduced, albeit with little statistical significance (Figure 7A–C) and so were differences among soil amendments. Under drought stress, important differences among soil amendments were observed in total leaf area (Figure 7D), wherein two key observations were made. Firstly, plants amended with co-compost had a significantly higher leaf area (1526 cm²) than all other amendments, especially the control (278 cm²). Secondly, co-compost-amended plants growing under drought stress had comparable growth in terms of leaf area (1526 cm² under drought vs. 1689 cm² under well-watered conditions, P_0.05 > 0.05) and number of leaves (12.8 cm² under drought vs. 15.4 cm² under well-watered conditions) with those growing in well-watered conditions, signifying that the co-compost amendment ameliorated drought stress. Leaf biomass accumulation, on both fresh and dry weight bases, was significantly reduced by drought stress by 143% and 42% in plants without any soil amendment; however, the amendment with co-compost significantly improved both parameters by 110% and 35% followed by sewage sludge with 58% in both fresh and dry leaf weights, respectively (Figure 8A,B). In contrast, differences were less apparent in the roots. In fact, plants whose soil was amended with co-compost exhibited considerably higher root growth under drought stress, on both fresh and dry weight bases (Figure 8C,D). Relative water content (RWC) was significantly lower under drought stress in soils without any amendment (5.3%), whereas in co-compost-amended soils, RWC was exceptionally high (56.7%) and comparable between drought stress and well-watered conditions (57.4%) (Figure 9A). Differences in chlorophyll concentration were less pronounced between drought stress and well-watered conditions, but values were generally higher under sewage sludge (44.2 and 39.7 SPAD units in drought and WW, respectively) soil amendment compared to 27.8 and 28.8 SPAD units in drought and WW, respectively (Figure 9B). A clear sign of drought stress is a decrease in shoot growth, leading to a shift in the distribution of biomass between the roots and shoots. The reallocation of resources can be characterized by the root mass ratio (RMR). The study revealed that under drought stress, the root mass ratio (RMR) was significantly higher compared to the control condition. The average RMR was 6.5 under drought stress and 4.8 under control. This indicates that more biomass was allocated to the roots under drought stress. However, it is worth noting that plants treated with conventional compost under control conditions had a significantly higher RMR of 8.7, compared to the drought-stressed plants with an RMR of 6.8. Then, we obtained the relative root mass ratio (RRMR), a more objective indicator of biomass allocation under abiotic stress conditions. Using RRMR, lettuce plants whose soil was amended with co-compost had a much higher RRMR (2.5), which was double the value for the control, indicating that the co-compost amendment increased the allocation of biomass to the roots to a greater extent when subjected to drought stress (

Table 9. Effect of soil amendments and drought stress on biomass allocation.

Treatment	RMR		RRMR
	Drought	Control
Control	5.8 b	4.7 a	1.2 b
Co-compost	7.4 b	3.0 a	2.5 c
Compost	6.8 a	8.7 b	0.8 a
Sludge	6.0 b	2.7 a	2.2 c

RMR = Root mass ratio; RRMR = Relative root mass ratio. Different letters indicate significant statistical differences at 0.05 level of significance using Tukey test. Similar letters indicate lack of statistical differences at 0.05 level of significance using Tukey test.

), which is an important morphological attribute for drought adaptability.

In order to evaluate if soil amendments affected the uptake of macro- and micronutrients under drought stress, an elemental analysis was conducted in dried leaf and root samples. It was shown that under drought stress, co-compost and sewage sludge amendments improved the K concentration in both leaves (8820 and 8820 µg/g DW, respectively) and roots (2568 and 2404 µg/g DW, respectively). These observations were also made on well-watered conditions (

Table 10. Effect of compost types on macro- and micronutrient accumulations in leaves and roots of lettuce plants under drought stress and greenhouse conditions.

Concentration (µg/g DW)
Organ	Treatment	Compost	K	Na	Ca	Mg	Zn	Mn	Fe	Cu
Leaf	Watered	Control	7434	50.0	1542	221	44.5	219	897	11.3
		Sludge	8012	53.8	1574	233	95.7	266	706	10.7
		Co-compost	8697	43.8	1922	195	55.1	187	1117	12.5
		Compost	7875	52.4	1735	200	58.8	192	846	10.2
	Drought	Control	6119	52.2	1063	217	55.3	122	1078	10.3
		Sludge	8866	58.4	1187	217	59.7	17	1068	10.8
		Co-compost	8821	48.2	1357	200	57.6	178	1386	15.8
		Compost	7732	50.3	1220	203	55.3	165	1102	11.2
Root	Watered	Control	2085	53.9	485	76.6	30.9	120	833	13.4
		Sludge	3665	65.6	747	88.2	52.6	147	1357	12.3
		Co-compost	3585	61.3	772	63.6	41.1	138	1464	16.0
		Compost	2938	63.2	734	70.3	39.3	122	1123	12.3
	Drought	Control	1595	30.0	576	93.4	27.6	107	812	8.9
		Sludge	2404	61.8	1150	113.5	32.5	116	1218	11.2
		Co-compost	2568	27.5	1278	47.9	28.8	151	1288	19.8
		Compost	1845	34.3	849	69.5	37.4	123	1083	9.3

). Under drought stress, sewage sludge-treated plants had a significantly higher Na concentration in the roots (61.8 µg/g DW), consistent with observations in lettuce under open field conditions (Table 6) and maize (Table 8). The Ca concentration was significantly higher in leaves of co-compost- and compost-amended plants under both well-watered (1922 µg/g DW) and drought stress conditions (1357 µg/g DW). In roots, this was true for co-compost- and sewage sludge-treated plants under drought stress and all treatments (except none) under well-watered conditions. No significant differences in the Mg concentration were found except for roots under drought stress in which sewage sludge accumulated a significantly higher Mg concentration (114 µg/g DW) (Table 10). The micronutrient concentration was generally higher in both leaves and roots of the co-compost-amended plants under both well-watered and drought stress conditions, followed by the sewage sludge application, and this was particularly true for Fe and Cu, which were higher in the co-compost-treated plants, whereas Zn and Mn were higher in the sewage sludge-treated plants (Figure 8).

After harvesting the plants, soil samples were sampled and analyzed for various chemical properties and mineral elements to evaluate the residual effect of the soil amendments. Soil pH ranges were all within the slightly acidic range for all soil amendments under both well-watered and drought conditions, which were between 4.3 and 5.8 under well-watered conditions and between 4.4 and 5.8 under drought conditions (

Table 11. Effect of soil amendments on residual mineral concentrations and soil chemical properties in pot-grown lettuce under greenhouse conditions.

Treatment	Water	pH	EC	OM %	N	Concentration (mg/kg)
						P	K	Ca	Mg	Cu	Zn
Well-Watered	Control	4.8	846	4.5	0.12	10.9	425	1934	235	0.09	0.7
	Compost	5.8	947	6.3	0.17	20.5	1384	3381	652	0.16	1.8
	Co-compost	4.3	1280	7.4	0.22	25.0	1665	3847	690	0.17	1.8
	Sludge	4.4	2025	5.1	0.25	13.2	364	2035	277	0.11	1.5
Drought	Control	4.4	720	4.6	0.16	10.3	382	2034	348	0.13	1.1
	Compost	5.8	699	6.2	0.23	12.0	718	2142	308	0.13	1.3
	Co-compost	5.1	515	7.9	0.27	23.5	1177	3041	624	0.13	1.9
	Sludge	4.7	1211	4.5	0.28	13.9	811	2663	576	0.15	2.2

). Electrical conductivity (EC) was generally significantly higher in soils amended with sewage sludge (2025 and 1211) under well-watered and drought conditions, respectively. Organic matter (OM%) content was significantly enhanced by both compost applications: conventional compost (6.3%) and co-compost (7.4%) under well-watered conditions. N content was significantly higher in sewage sludge (0.25 mg/kg)- and co-compost-amended soils (0.22 mg/kg) under well-watered conditions. Mineral concentrations were also measured and showed significantly high P, K, Ca, and Mg concentrations in compost- and co-compost-amended soils under well-watered conditions and co-compost-amended soils under drought conditions (Table 11). No significant differences were shown in the Cu concentration among the soils amended with different amendments under both water conditions. The Zn concentration was significantly higher under well-watered conditions in soils amended with compost and co-compost, whereas under drought stress, the concentration was higher in sewage sludge- and co-compost-amended soils (Table 11).

4. Discussion

Agricultural intensification, with a drive to keep up with the increasing food demands has also meant indiscriminate use of agro-chemicals, excessive and deep tillage, and luxury irrigation. These agricultural practices have degraded soils in addition to polluting surface and groundwater while also causing immense air contamination [19]. Cognizant of these effects and in support of developing a circular economy, the application of composted organic material on cropped soils is being encouraged in degraded soils. Composted organic materials represents potential sources of nutrients for crops and can partially substitute the use of mineral fertilizers [20]. In addition, regular soil amendment with composted material restores soil organic matter content in intensively cultivated soils and contributes to carbon storage in soils.

In this study, we evaluated the response of various crops to various organic soil amendments. In order to effectively derive the optimal benefits of organic soil amendments, the determination of proper application rates is a critical step. Here, we evaluated different application rates using maize crop and determined that 350 g was the ideal application rate per station. This rate translates to a rate of 11.7 and 18.7 Mg/ha in lettuce and maize, respectively, at the spacings used in this study. Based on the current practice and recommendations in the Malawian agricultural systems, these rates are 50% and 37% lower, thereby significantly reducing the quantities of compost required per ha. It must be indicated that a rate of 500 g per station (16.7 and 26.7 Mg/ha in lettuce and maize, respectively) produced better results, but the lack of statistical significance with the 350 g rate entails a limited return on investment to justify a further increase in application rates. In addition, the bulkiness of organic fertilizers and difficulty in transportation have often been cited as major drawbacks discouraging farmers from widely using them as a soil amendment [21]; hence, if similar benefits of applying 500 g would be derived at the 350 g application, the adoption of the latter rate would be more economical and would reduce unnecessary bulkiness and transportation constraints. Moreover, excessive amounts of co-composts and sludge would encourage the accumulation of heavy metals; hence, caution must be exercised in choosing an application rate for sludge-based co-composts.

Consequently, the study evaluated the agronomic efficacy of various organic soil amendments: compost, co-compost, and sewage sludge. These organic amendments were made by different procedures with different substrates (refer to the Section 2). The compost was principally made from plant residues and had fewer turning cycles. The co-compost was made from a mixture of MSW, plant residues, and sewage sludge. There is a suggestion that composts made from various organic wastes vary in terms of their quality and stability. This variation is influenced by the composition of the raw materials used in the manufacturing of the compost. Compost, unlike fast-release fertilizers like mineral fertilizers and slurry, includes significant quantities of organic matter. This organic matter enriches the soil organic carbon (SOC) content, as noted by [22]. However, compost generally has lower levels of nutritional components. In order to fortify compost with nutrients and to speed up decomposition, 10% sewage sludge was added to the composting process, deriving a nutrient-dense material that we termed co-compost [23]. An analysis of the residual efficacy showed that soils amended with co-compost had increased concentrations of K, P, Ca, and Mg, as well as micronutrients (Table 7 and Table 11). This may suggest that the soil amendment enhanced the soil’s cation exchange capacity (CEC), which consequently led to higher electrical conductivity of the soil in tandem. These values were much higher than conventional compost and slightly higher than sewage sludge. The addition of sewage sludge to a composting process has been exploited to optimize a C:N ratio [7,24] Sewage sludge is a N-rich material; hence, it lowers the C:N ratio and reduces the time taken for the decomposition and mineralization processes during the aerobic fermentation phase. It has been observed that farmers often have a disdain for choosing compost relative to sludge and other fast-release organic sources. For example, ref. [21] showed that in areas with a ready supply of slurry/manure, the use of other organic soil amendments such as compost is a less attractive option due to the limited amounts of N and P that may be derived from them. Therefore, this study shows that enhancing the conventional compost quality through co-composting by the addition of sewage sludge may overcome the nutritional barriers, as well as reduce the composting time. These beneficial results were demonstrated in two different crops: lettuce and maize. In lettuce, the co-compost application enhanced leaf yield (Figure 2C and Figure 4A) and root growth (Figure 3B), in both virgin and cultivated soils, albeit comparative growth/yield gains with no compost amendment were higher in cultivated than virgin soils, whereas absolute growth/yield were higher in virgin soils. This observation suggests that nutrient-poor, degraded, and frequently cultivated soils with less organic matter would obtain maximal gains from co-compost amendment. These findings have also been reported in previous studies [17,25,26], hinting at the possibility that the efficacy of compost amendments may be dependent on inherent soil fertility levels. In maize, the co-compost application significantly improved both shoot and root dry weights (Figure 4C,D) as well as the chlorophyll concentration (Figure 4F). The high chlorophyll concentration may have primarily been linked to high Fe and Mg concentrations, which were considerably accumulated in leaves (Table 9). This study also reports significant enhancements in grain yield parameters by all amendments and applications, especially co-compost, sludge, and NPK fertilizer (Figure 5A–D). Strikingly though, the co-compost amendment resulted in a lower harvest index (HI) that was comparable to no amendment (Figure 5F). The HI is the ratio of grain yield to biological yield or biomass, which represents a crop’s success in partitioning total photosynthate to harvestable product. This finding is in sync with a study by [27] which reported that under a high maize grain yield, the economic yield is mainly dependent on an increase in biological yield (biomass) than the harvest index. This suggests that an increase in above-ground biomass beyond a certain threshold does not translate into a higher harvest index as plants may have reached their maximum capacity for photosynthetic partitioning and grain filling. This point of view is also supported by a previous study [28] which showed that while increasing the P application rate in maize beyond a certain threshold increased the maize grain yield, it did not result in a higher harvest index, signifying that increases in biological yield superseded gains in economic yield. These observations warn against absolute consideration of the HI as a selection criterion in predicting maize grain yield and rather advocate for careful evaluation of growth conditions such as soil fertility, which have been reported to significantly influence the harvest index in maize [27].

Moreover, composts significantly enhance soil organic carbon content, owing to their higher amounts of organic matter [22]. This is also demonstrated in the present study, where it is shown that soils amended with both compost and co-compost had a higher organic matter content than non-amended soil as well as sewage sludge in both greenhouse and open field conditions (Table 7 and Table 10). Under open field conditions, for example, co-compost-amended soils had an OM% of 5.7, whereas sewage sludge had an OM% of 4.7. Under greenhouse conditions, co-compost-amended soils had an OM% of 7.4, whereas sewage sludge had an OM% of 5.1 and non-amended soil had an OM% of 4.5. The relatively higher values under greenhouse conditions were due to the differences in soil type used in these experiments. Under greenhouse conditions, virgin soil was used which is higher in OM compared to open field conditions, which are intensively cultivated. Due to the significant increase in organic matter (OM) resulting from the addition of co-compost to the soil, some soil physical qualities, such as accessible water content [29] and aggregate stability [30], are enhanced. This, in turn, helps to safeguard the soil against erosion. Viaene et al. [21] found that organic carbon in co-compost exhibits greater stability and resistance to decomposition compared to fresh manure or plant residues. In the latter cases, a larger proportion of the carbon decomposes upon application. Similar sentiments were made in [7]. It was from this basis, therefore, that the benefits of co-compost application were also particularly pronounced under drought stress, where it reduced the wilting of lettuce plants (Figure 6) and enhanced total leaf area (Figure 7D), leaf yield (Figure 8A,B), root growth (Figure 8C,D), and relative water content (Figure 9A). In a study by [31], it was shown that compost soil amendment improved the growth and yield of corn cultivated under both drought and control conditions. Under drought stress, better growth in co-compost-amended soils may have been attributed to the enhanced water-holding capacities due to the high organic matter content. This property was also reflected by a higher leaf water status as shown by leaf relative water content, which was similar between the drought-stressed and control plants (Figure 7A). Zebarth et al. [26] observed that the amendment of sandy, infertile soils with compost significantly enhanced its water-holding capacity. In the present study, the amelioration of drought stress by co-compost amendment may be ascribed to increased soil organic matter content (Table 10) and aggregate stability [32]. These attributes increase soil micropores which are key determinants of a soil’s water-holding capacity. Furthermore, the enhancement of the relative root mass ratio (RRMR) under drought stress in the co-compost-amended treatment may also have been crucial in enhancing growth. High RRMR depicts that the co-compost amendment increased the allocation of biomass to the roots to a greater extent when subjected to drought stress. This enhancement in root growth is a key attribute for the exploration of limited water resources under drought stress. Therefore, in light of the increased drought incidences as a result of the changing climate scenario, farmers can considerably benefit from organic soil amendment to maintain crop growth and productivity in nutrient-poor and dehydrated soil conditions.