Mixing Sodium-Chloride-Rich Food Waste Compost with Livestock Manure Composts Enhanced the Agronomic Performance of Leaf Lettuce

Abstract

Food waste generated at the consumer level constitutes a gigantic portion of the total amount of food wasted/lost and valorisation is touted as the most sustainable way of managing the generated waste. While food waste valorisation encompasses several methods, composting is the cheapest technique that can produce stabilised carbon-rich soil amendments. The food waste generated at the consumer level, however, is laden with sodium chloride. The compost produced from such waste has the potential of inducing saline and or sodic conditions in the soil, resultantly impeding proper crop growth and yield. Due to the scarcity of plausible means of eradicating sodium chloride from the food waste before composting, the idea of mixing the composted food waste with other low sodium chloride-containing composts to produce a food waste compost-containing amalgam with a high fertiliser potential was mulled in this study. The study then assessed the effects of mixing sodium-chloride-rich food waste compost with the nutritious and low sodium chloride-containing livestock manure composts on the yield and quality of leaf lettuce. Mixing food waste compost with livestock manure composts in the right proportions created mixed composts that produced a higher lettuce yield than both the pure livestock manure composts and food waste compost. The mixed composts also produced leaf lettuce with higher chlorophyll content and, thus, better marketability and lower nitrate content (with higher health value) than the pure livestock manure composts.

1. Introduction

It is estimated that about one third of the food produced globally goes to waste every year [1] and that the lost quantum of food could be able to feed one billion people [2]. While the production to retail stages account for the largest chunk of the food lost and wasted even in the highly industrialised countries, the per capita loss and waste at the consumer level is far higher in the industrialised than in the developing countries [3]. At the consumer level, the per capita food waste stands at 95–115 kg/year in North America and Europe for example while in Sub-Saharan Africa and South/South-East Asia, the per capita waste amounts to only 6–11 kg/year [4]. Until recently, landfilling and incineration were common practices of managing the endlessly increasing amounts of food waste but, due to the dwindling amount of suitable land for the construction of new landfills and growing awareness about the dangers of landfilled food waste, especially methane gas emissions and nuisance odours, countries around the globe are increasingly restricting food waste landfilling [5,6,7,8]. Similarly, incineration is shrouded in a lot of controversy due to the perceived environmental pollution associated with it [9,10,11]. It is against that backdrop that a lot of countries are exploring more sustainable ways of managing their ballooning putrescible wastes and food waste valorisation for nutrient recycling is at the core of such plans [11].

Material valorisation can be achieved through composting, anaerobic digestion, dehydration, and pyrolysis [7,11]. Pyrolysis and anaerobic digestion are capital intensive ventures while the final product of dehydration is unstable and undergoes further decomposition when applied to the soil with concomitant undesirable consequences including immobilisation of nitrogen and discharge of phytotoxins, e.g., ammonia [11]. In light of the limitations of the aforementioned valorisation methods, composting remains the cheapest sustainable food waste valorisation method. The end product of composting is a stabilised organic matter rich in humic substances and essential plant nutrients, which makes it a good form of organic fertiliser [12]. However, compost made from the food waste generated at the consumer level contains elevated amounts of sodium chloride [13,14] and may induce salinity when continually applied to the soil. Indeed, Lee et al. [15] indicated that both the rice and red pepper yields decreased with increasing application rates of sodium chloride (NaCl)-rich food waste compost and attributed the decreases to the increasing salinity induced by the food waste compost. Several studies have highlighted the adverse effects of elevated salt levels in the soil. For example, an earlier study by Akbarimoghaddam et al. [16] examined the effects of elevating soil NaCl contents on the germination and growth of bread wheat seedlings and found that high NaCl content delayed germination, reduced shoot and root dry weights, and increased the root-to-shoot ratios of the seedlings. Another study by Neocleous et al. [17] assessed the effects of elevated concentrations of NaCl on the growth and yield of two leaf lettuce cultivars in a floating system and found that 20 mM of NaCl reduced the marketable yields of both cultivars.

Various explanations have been offered to account for the adverse effects of elevated soil quantities of NaCl on crop growth and yields. Reduced uptake and availability of essential nutrient elements, especially potassium and phosphorus, that ensue from the interference in the uptake of potassium by Na⁺ and precipitation of phosphorus by calcium ions in the soil have been reported by Bano and Fatima [18] and Akbarimoghaddam et al. [16]. Netondo et al. [19] indicated that salinity affects photosynthesis mainly by reducing the leaf area, chlorophyll content, and stomatal conductance as well as decreasing the photosystem II efficiency. Additionally, it has been demonstrated by Shrivastava and Kumar [20] that salinity impairs reproductive development through inhibiting microsporogenesis and stamen filament elongation and augmenting ovule abortion, senescence of fertilized embryos, and programmed cell death in certain types of tissue. Besides the adverse effects on crop growth and yield, high salt levels in the soil lead to a decrease in the population of decomposer microorganisms with a negative impact on carbon cycling as a review study by Rath and Rousk [21] intimated. In the light of the aforementioned undesirable attributes associated with soil salinity, it is imperative to search for sustainable methods of utilising NaCl-rich food waste compost so as to reduce the possibility of inducing soil salinity/sodicity. Due to the scarcity of plausible means of eliminating NaCl from the food waste before composting, food waste compost is usually laden with NaCl [13,14]. It is against that background that the current study explored the idea of mixing NaCl-rich food waste compost with low-NaCl-containing animal manure composts to produce compost amalgams with low NaCl contents. The objective of this study, therefore, was to assess the fertiliser potentials of the produced compost amalgams.

2. Materials and Methods

2.1. Preparation of the Compost, and Compost and Soil Analysis

The food waste compost was produced at the Chungnam National University agricultural research farm located at Eeoun-dong, Daejeon, South Korea following the method outlined by Lee et al. [14] and using saw dust as the bulking agent. The food waste used in the study was a dehydrated mixture of both cooked leftovers and uncooked food that went bad before it could be consumed. Three livestock manure composts, i.e., poultry manure (PMC), swine manure (SMC), and cattle manure (CMC) composts, were purchased from a local compost producer. Each of the livestock manure composts was mixed with the food waste compost (FWC) in three different ratios of 1:3, 1:1, and 3:1 (w/w), respectively. The mixed composts were tested for maturity before usage using both the solvita and germination index methods. The calcium (Ca), magnesium (Mg), potassium (K), and phosphorus (P) contents of the compost were extracted with the Mehlich 1 solution by following the extraction method espoused by Faithfull [22]. Phosphorus was then determined colourimetrically at 880 nm using a UV-Vis spectrophotometer (Thermoscientific, Genesys 50, Waltham, MA, USA) by strictly adhering to the method espoused by Murphy and Riley [23]. The cations were, on the other hand, quantified with ICP-OES (Thermoscientific iCAP 7000 series, Waltham, MA, USA). The easily oxidisable organic carbon was determined by strictly following the method developed by Tinsley [24] while total nitrogen was determined with the CHN analyser (LECO, Truspec, St. Joseph, MI, USA). The pH and EC of the composts were determined with the pH and EC meter (ORION Versa Star Pro, Thermo Scientific, Inc., Waltham, MA, USA) after extraction of 1 g of compost in 10 mL of distilled water. The NaCl content of the compost was measured with a salinity meter (PAL-SALT, ATAGO, Tokyo, Japan). The soil pH and EC were determined as mentioned above but 1 g of soil was extracted in 5 mL of water instead of the 10 mL used for the compost analysis. Available soil phosphorus was determined colourimetrically as stated above but the extraction was done with bray 1 solution. Soil cations were extracted with 1 M neutral ammonium acetate solution and determined with ICP-OES as mentioned above while the soil organic matter was determined colourimetrically at 660 nm using a UV-Vis spectrophotometer (Thermoscientific, Genesys 50, Waltham, MA, USA) following the method espoused by Nelson and Sommers [25]. The calibration curve was obtained from the different concentrations (0.00, 0.05, 0.15, 0.30, and 0.50 g L⁻¹) of glucose solution. The soil properties assessed are given in

Table 1. Selected properties of the soil used in the experiment.

Sample	pH	EC	Av. P2O5	OM	Exchangeable Cations (cmolc kg−1)
	(1:5)	(dS m−1)	(mg kg−1)	(%)	K+	Mg2+	Ca2+	Na+
Soil	6.52 ± 0.13	0.07 ± 0.00	13.45 ± 1.22	2.92 ± 0.09	0.04 ± 0.00	1.09 ± 0.17	0.01 ± 0.00	0.07 ± 0.01

while those of the compost are given in

Table 2. Selected chemical properties of the composts used in this experiment.

Compost Type	pH	EC	T-P	C	N	Ca	Mg	K	NaCl
	(1:10)	(dS m−1)	(%)
FWC	7.61 ± 0.00	7.98 ± 0.52	5.29 ± 1.53	37.70 ± 1.17	4.35 ± 0.13	2.65 ± 0.54	1.32 ± 0.04	0.58 ± 0.01	2.73 ± 0.05
PMC	8.67 ± 0.01	12.18 ± 2.13	5.47 ± 0.11	30.06 ± 0.41	2.45 ± 0.20	2.46 ± 0.84	1.57 ± 0.01	2.35 ± 0.11	0.52 ± 0.02
SMC	6.68 ± 0.01	5.13 ± 0.17	6.71 ± 0.12	40.21 ± 0.82	2.20 ± 0.06	2.73 ± 0.43	0.96 ± 0.03	0.63 ± 0.01	0.67 ± 0.10
CMC	8.35 ± 0.00	15.08 ± 0.22	5.95 ± 0.30	25.58 ± 1.15	1.81 ± 0.03	2.78 ± 0.54	1.40 ± 0.05	2.01 ± 0.04	0.87 ± 0.08
75% FWC + 25% PMC	7.78 ± 0.00	9.07 ± 0.54	5.70 ± 0.47	36.94 ± 0.74	3.95 ± 0.05	2.55 ± 0.27	1.37 ± 0.03	0.99 ± 0.01	2.02 ± 0.06
50% FWC + 50% PMC	7.97 ± 0.03	9.70 ± 0.16	6.49 ± 0.17	33.90 ± 2.32	3.17 ± 0.13	2.38 ± 0.11	1.47 ± 0.02	1.45 ± 0.03	1.29 ± 0.05
25% FWC + 75% PMC	8.24 ± 0.01	10.65 ± 0.21	6.56 ± 0.05	36.75 ± 0.21	3.92 ± 0.09	2.16 ± 0.37	1.51 ± 0.05	1.75 ± 0.07	1.00 ± 0.11
75% FWC + 25% SMC	7.41 ± 0.02	7.60 ± 0.28	6.24 ± 0.27	39.25 ± 0.70	2.88 ± 0.14	2.67 ± 0.21	1.26 ± 0.01	0.57 ± 0.01	2.15 ± 0.01
50% FWC + 50% SMC	7.25 ± 0.01	6.49 ± 1.72	6.69 ± 0.07	39.35 ± 0.54	3.37 ± 0.21	2.84 ± 0.49	1.12 ± 0.04	0.57 ± 0.02	1.09 ± 0.03
25% FWC + 75% SMC	7.04 ± 0.00	6.51 ± 0.06	8.09 ± 0.13	38.73 ± 0.92	3.75 ± 0.19	2.77 ± 0.58	1.10 ± 0.04	0.60 ± 0.02	1.02 ± 0.06
75% FWC + 25% CMC	7.72 ± 0.01	7.18 ± 1.07	5.92 ± 0.07	29.79 ± 0.40	2.55 ± 0.09	2.36 ± 0.30	1.35 ± 0.02	0.91 ± 0.01	2.86 ± 0.13
50% FWC + 50% CMC	7.87 ± 0.00	10.79 ± 0.63	6.77 ± 0.11	31.59 ± 1.52	3.01 ± 0.03	2.76 ± 0.61	1.38 ± 0.03	1.25 ± 0.05	2.13 ± 0.03
25% FWC + 75% CMC	8.00 ± 0.00	14.96 ± 0.29	7.10 ± 0.10	35.53 ± 1.11	3.67 ± 0.12	3.25 ± 0.41	1.40 ± 0.02	1.60 ± 0.03	1.39 ± 0.05

.

2.2. Experimental Setup and the Lettuce Yield and Quality Parameters Assessed

The experiment was conducted in Wagner pots (1/5000 a) from the glass house at the Chungnam National University agricultural research farm located at Eoeun-dong, Daejeon, South Korea. The soil used in this experiment was obtained from a forested hill where the top 5 cm of the soil was removed to exclude all the organic matter. The soil was then dug up to a depth of 20 cm, air dried, and sieved through a 2 mm sieve. The lettuce variety grown was the Cheongchima cultivar whose seeds were bought from Nonghyup, Seoul, South Korea. The lettuce was grown for 30 days after transplanting of 21-day-old seedlings. Besides the compost treatments, an amendment with conventional mineral fertilisers of nitrogen (N), phosphorus (P), and potassium (K) was included for comparisons as well as the control which received neither NPK nor the compost. The conventional fertilisers (NPK) and all the composts were applied at rates recommended by the Ministry of Agriculture of South Korea and that was 20 Mg/ha for the composts. The amounts of N, P, and K applied were 17.2, 7.1, and 24.8 kg/ha but N was split into two equal applications. The agronomic performance of the leaf lettuce was assessed based on three parameters, namely the root yield, including root length and fresh and dry root weights, the shoot yield, including leaf length, number, and width and fresh and dry shoot weights, as well as selected quality indices, including sweetness and chlorophyll and nitrate nitrogen (NO₃⁻N) contents of the leaves. To delineate the effects of mixing FWC with each of the livestock manure composts on lettuce yield, a nonlinear relationship between marketable lettuce yield (fresh shoot weight) and the quantity of the FWC in the compost used to produce it was established. Additionally, the effects of composts on the accumulation of the three major nutrient elements of N, P, and K in the leaf lettuce were assessed by establishing linear relationships between the contents in the lettuce and the quantity of FWC in the compost used to grow the lettuce. In the same vein, linear relationships between the different compost mixtures and soil available phosphorus (P), exchangeable calcium (Ca), and sodium (Na) as well as the relationships between soil available P and soil exchangeable Ca plus the one between the NaCl content of the food waste and the amount of potassium (K) that accumulated in the leaf lettuce were examined. The N contents of the leaf lettuce were determined by strictly adhering to the micro-Kjedhal method outlined by Kalra [26]. P and K were determined colourimetrically and with ICP-OES, respectively, after extraction with the nitric–perchloric acid wet digestion in an open vessel method espoused by Kalra [26]. The fresh root and shoot weights were determined at harvesting using a portable electric weighing scale (Daihan Scientific, South Korea) and roots were weighed after thorough washing to remove all the soil followed by gentle rubbing with the serviette to remove the water. The dry shoot and root weights were determined after drying the materials for 24 h at 105 °C in a forced convection oven (ON-12Gw L080125, Jeio Tech, Daejeon, South Korea). Root length and leaf length and width were measured with a metre rule. The chlorophyll content was determined using a chlorophyll meter (SPAD-502, Konica Minolta, Tokyo, Japan) as outlined in Yoo et al. [27]. Sweetness and NO₃⁻N were measured with a digital saccharimeter (HI 96801, Hanna Instruments Inc, Woonsocket, RI, USA) and a NO₃⁻N meter (S040, HORIBA Ltd., Kyoto, Japan), respectively, after extraction of the juice out of the leaves.

3. Statistical Analysis

The data obtained from the assessed yield and quality parameters were subjected to a one-way analysis of variance (ANOVA) using the Microsoft excel 16 data analysis toolpak and the data that returned a significant result at p ≤ 0.05 were put through a Tukey post hoc test using the studentised q tables to quantify the significant differences between the different treatments. The nonlinear regression equations obtained between the marketable lettuce yield and the quantity of the FWC in the compost employed were used to estimate the mixing ratios that can produce the highest possible marketable yield using the Microsoft excel 16 data analysis toolpak solver function.

4. Results

As seen from

Table 3. Yield attributes of the leaf lettuce.

	Root Parameters			Shoot Parameters
Treatments	Fresh Weight	Dry Weight	Root Length	Fresh Weight	Dry Weight	Leaf Length	Leaf Number	Leaf Width
	(g)	(g)	(cm)	(g)	(g)	(cm)	(EA/Plant)	(cm)
Control	3.15 ± 0.54 h	0.23 ± 0.01 h	24.43 ± 0.51 a	6.13 ± 2.25 i	0.52 ± 0.21 f	6.33 ± 0.72 d	9.00 ± 1.00 e	3.76 ± 0.27 b
NPK	4.98 ± 1.02 g	0.30 ± 0.03 h	16.20 ± 5.65 d	18.83 ± 1.60 h	1.15 ± 0.14 e	9.22 ± 1.97 c	14.00 ± 1.20 d	7.09 ± 2.39 a
FWC	9.10 ± 2.78 d	0.89 ± 0.14 d	20.77 ± 5.07 b	48.42 ± 5.44 de	2.53 ± 0.26 cd	12.91 ± 1.42 ab	17.67 ± 1.74 c	7.17 ± 1.03 a
PMC	11.96 ± 2.17 b	1.64 ± 0.17 b	24.90 ± 4.58 a	31.95 ± 5.29 g	1.98 ± 0.21 d	13.06 ± 1.07 ab	20.00 ± 1.00 b	7.41 ± 0.42 a
SMC	15.94 ± 4.83 a	2.44 ± 0.41 a	23.10 ± 2.53 a	49.58 ± 8.80 de	2.76 ± 0.38 cd	13.78 ± 0.45 ab	21.33 ± 0.58 b	7.39 ± 0.3 a
CMC	10.43 ± 3.26 c	0.88 ± 0.30 d	24.87 ± 1.97 a	38.99 ± 1.52 f	2.55 ± 0.07 cd	11.43 ± 0.68 bc	21.33 ± 1.53 b	6.76 ± 0.47 a
75% FWC + 25% PMC	6.93 ± 2.85 ef	0.49 ± 0.12 g	17.63 ± 1.53 c	50.55 ± 8.05 d	3.15 ± 0.78 c	11.32 ± 2.30 bc	17.00 ± 3.07 c	6.78 ± 1.99 a
50% FWC + 50% PMC	7.34 ± 0.44 e	0.57 ± 0.20 f	18.50 ± 2.11 bc	51.91 ± 4.01c	3.03 ± 0.63 c	11.97 ± 1.30 b	23.67 ± 4.04 ab	6.1. ± 1.05 a
25% FWC + 75% PMC	8.55 ± 3.27 de	0.61 ± 0.13 ef	19.30 ± 1.50 bc	53.07 ± 6.24 c	2.59 ± 0.50 cd	11.99 ± 1.31 b	21.67 ± 1.53 b	6.78 ± 0.65 a
75% FWC + 25% SMC	6.64 ± 1.80 f	0.45 ± 0.19 g	14.20 ± 2.23 e	69.33 ± 5.82 a	3.15 ± 0.65 c	13.43 ± 0.93 ab	23.00 ± 3.66 ab	7.58 ± 0.93 a
50% FWC +50% SMC	7.56 ± 2.86 e	0.57 ± 0.12 f	17.77 ± 2.60 c	62.27 ± 6.71 ab	4.01 ± 0.25 a	13.41 ± 1.70 ab	23.00 ± 4.00 ab	7.42 ± 0.87 a
25% FWC + 75% SMC	9.89 ± 4.29 c	0.76 ± 0.06 e	20.43 ± 1.62 b	59.70 ± 5.55 b	3.23 ± 0.42 bc	12.64 ± 1.10 ab	25.33 ± 3.21 a	7.26 ± 0.95 a
75% FWC + 25% CMC	8.8 ± 3.93 d	0.77 ± 0.01 e	18.17 ± 2.29 bc	58.28 ± 8.99 b	3.25 ± 0.69 bc	14.67 ± 2.47 a	21.00 ± 4.58 b	8.43 ± 2.03 a
50% FWC + 50% CMC	16.63 ± 6.85 a	2.57 ± 0.61 a	24.00 ± 1.57 a	62.23 ± 8.12 ab	4.14 ± 0.13 a	14.61 ± 1.59 a	26.33 ± 1.53 a	7.77 ± 0.97 a
25% FWC + 75% CMC	9.73 ± 8.25 cd	1.02 ± 0.08 c	22.50 ± 2.51 a	45.93 ± 3.36 e	3.35 ± 0.34 b	11.8 ± 1.46 b	21.67 ± 0.58 b	6.82 ± 0.98 a

The letters (^a–i) denote the significant differences between the different treatments based on the result of the Tukey posthoc test.

, all the compost mixtures produced marketable lettuce yields (fresh shoot weight) that were higher than the pure composts except for the compost mixture that contained three parts of CMC and one part of FWC, i.e., the 25% FWC + 75% CMC amendment that produced a marketable yield lower than those of the pure FWC and SMC. The 75% FWC + 25% PMC, 50% FWC + 50% PMC, and 25% FWC + 75% PMC compost mixtures produced 58.22%, 62.47%, and 66.10% more marketable lettuce yields than the pure PMC amendment. Similarly, the 75% FWC + 25% SMC, 50% FWC + 50% SMC, and 25% FWC + 75% SMC mixed composts increased marketable lettuce yields by 39.83%, 25.59%, and 20.41%, respectively, in comparison with the pure SMC amendment. In the same vein, the 75% FWC + 25% CMC, 50% FWC + 50% CMC, and 25% FWC + 75% CMC compost mixtures increased marketable yields by 49.47%, 59.61%, and 17.80%, respectively, as compared with the pure CMC amendment. However, Figure 1a–c indicate that the marketable lettuce yield increased with the increasing proportion of the FWC in the compost mixture until the peak yield beyond which additional FWC content in the mixture of the compost caused a reduction in the leaf lettuce yield. The nonlinear relationship between the marketable yield and SMC-containing compost mixtures produced the best fit (r² = 0.9386) followed by the PMC-containing mixtures (r² = 0.8044) while the least fit was obtained from the CMC-containing mixtures (r² = 0.5947). The estimated yields presented in Figure 1d indicate that the highest possible yields can be achieved by mixing 35% of SMC with 65% of the FWC, 45% of the FWC with 55% of PMC, and 45% of CMC with 55% of the FWC. Additionally, all composts produced higher marketable yields than the conventional amendment of NPK, which in turn outperformed the control. As compared with the NPK amendment, pure composts of FWC, PMC, SMC, and CMC increased lettuce yields by 157.14%, 69.68%, 163.30%, and 107.06%, respectively.

The compost amendments outperformed the conventional NPK amendment with regards to the other shoot yield parameters assessed, including leaf length, number, and width. The longest leaves came from the leaf lettuce grown on the 75% FWC + 25% CMC and 50% FWC + 50% CMC compost mixtures with no significant statistical differences between the two amendments. These were followed by the rest of the compost amendments except for the 25% FWC + 75% CMC compost mixture, whose leaves were significantly shorter than the other compost amendments as can be seen from Table 3. For the leaf numbers, the compost mixtures containing 50% livestock manure composts generally produced leaf lettuce with more leaves than the rest of the amendments except for the SMC-containing mixtures, whose leaf numbers increased with the amount of SMC in the mixture. For leaf width, all amendments produced leaf lettuce leaves whose width did not differ statistically from one another. The compost amendments produced heavier fresh roots than the conventional NPK and, generally, pure composts produced heavier fresh roots than the mixed composts except for the 50% FWC + 50% CMC compost mixture, whose fresh roots were the heaviest of all the amendments. Additionally, pure compost amendments produced longer roots than their respective mixed composts, but all composts produced longer roots than the conventional NPK amendment except for the 75% FWC + 25% PMC compost mixture.

As seen in

Table 4. Quality attributes of the leaf lettuce.

Treatments	Chlorophyll Content	NO3−N	Sweetness
	(SPAD)	(mg kg−1)	(Brix)
Control	23.66 ± 1.70 d	1000 ± 85.00 g	2.00± 0.11 c
NPK	27.12 ± 3.39 bc	1200 ± 100.00 d	0.40 ± 0.01 f
FWC	31.50 ± 6.34 ab	910 ± 155.88 d	0.77 ± 0.15 e
PMC	26.99 ± 2.24 bc	1090 ± 190.53 e	2.90 ± 0.40 b
SMC	25.51 ± 2.27 c	1500 ± 264.58 a	3.57 ± 0.14 a
CMC	29.07 ± 2.10 bc	1300 ± 100.00 c	1.40 ± 0.10 d
75% FWC + 25% PMC	32.66 ± 3.70 a	1063 ± 158.22 e	1.20 ± 0.36 d
50% FWC + 50% PMC	30.16 ± 3.11 b	1007 ± 152.75 f	1.80 ± 0.14 cd
25% FWC + 75% PMC	30.33 ± 2.30 b	1130 ± 455.74 de	2.01 ± 0.20 c
75% FWC + 25% SMC	31.81 ± 4.45 ab	1267 ± 404.15 d	2.33 ± 0.29 c
50% FWC +50% SMC	31.71 ± 2.42 ab	1346 ± 115.47 c	2.90 ± 0.20 b
25% FWC + 75% SMC	30.98 ± 3.41 b	1443 ± 191.4 b	2.00 ± 0.21 c
75% FWC + 25% CMC	31.44 ± 6.41 ab	1040 ± 196.98 f	0.94 ± 0.06 de
50% FWC + 50% CMC	27.72 ± 3.79 bc	1133 ± 57.74 de	1.10 ± 0.15 de
25% FWC + 75% CMC	26.3 ± 6.31 c	1230 ± 152.75 d	1.43 ± 0.11 d

The letters (^a–f) denote the significant differences between the different treatments based on the result of the Tukey posthoc test.

, the FWC and its mixtures produced leaf lettuce with higher chlorophyll content than the livestock manure composts. For example, leaf lettuce produced with the FWC amendment contained 16.71%, 23.48%, and 8.36% more chlorophyll than the lettuce grown with the pure livestock manure composts of PMC, SMC, and CMC, respectively. The chlorophyll content of the leaf lettuce grown on the mixed compost amendments reduced with increasing contents of the livestock manure composts in the compost mixture. For example, the chlorophyll content reduced from the SPAD value of 32.66 in the amendment of 75% FWC + 25% PMC to the SPAD values of 30.16 and 30.33 in the 50% FWC + 50% PMC and 25% FWC + 75% PMC mixture amendments, respectively. Similarly, the SPAD values of the leaf lettuce produced with the FWC + SMC amendments reduced from 31.81 in the 75% FWC + 25% SMC mixture to 31.71 and 30.33 in the 50% FWC + 50% SMC and 25% FWC + 75% SMC mixtures, respectively. Lastly, the SPAD values of the lettuce grown with the FWC + CMC amendments reduced from 31.44 in the 75% FWC + 25% CMC mixture to 27.72 and 26.31 in the 50% FWC + 50% CMC and 25% FWC + 75% CMC mixtures, respectively.

The FWC amendment produced lettuce with the lowest content of NO₃⁻N and the NO₃⁻N content of the leaf lettuce grown on the soil amended with the mixed composts increased with the reductions in the percentage concentration of the FWC in the mixture as can be seen in Table 3. Additionally, apart from the SMC, CMC, and SMC-containing compost mixtures and the 25% FWC + 75% CMC amendment, all other compost mixtures produced lettuce with lower NO₃⁻N than the conventional amendment of NPK. The conventional amendment of NPK produced leaf lettuce with the lowest Brix values of sweetness followed by the FWC amendment. Amongst the livestock manure composts, the SMC produced the sweetest leaf lettuce followed by the PMC and then the CMC amendment. Surprisingly, the control experiment produced sweeter leaf lettuce than the NPK, FWC, CMC, and all its mixtures as well as the 75% FWC + 25% PMC amendment. The sweetness of the lettuce produced with the mixed composts reduced with the reducing concentration of the FWC in the mixtures. For example, the brix values of the lettuce grown on the FWC + PMC mixtures containing 75%, 50%, and 25% FWC stood at 1.20, 1.80, and 2.10, respectively. In the same vein, the brix values of the FWC + SMC mixtures containing 75%, 50%, and 25% FWC amounted to 2.01, 2.33, and 2.90, respectively, while those of the FWC + CMC mixtures stood at 0.94, 1.10, and 1.43, respectively. Figure 1d shows the predicted variations in lettuce yields with varying contents of FWC in the compost mixtures.

5. Discussions

As shown in Table 3, apart from SMC, the FWC outperformed the rest of the composts as far as marketable lettuce yield was concerned. This observation concurred with one made by Kwon et al. [28], who found that FWC amendments produced higher red pepper yields than the pig manure compost amendments in a field experiment that lasted for four growing seasons. The study found, however, that the potato yield obtained with both composts was not significantly different in terms of statistics, which may allude to the fact that the yield responses to FWC and livestock manure composts depend on the crop type grown. In agreement with the observation made in the present study, Kwon et al. [28] also indicated that the yields obtained with the FWC amendments were higher than those produced by the conventional amendment of NPK in all the crops grown in the experiment. The observations by Lee et al. [15], however, contravene the results of our study because in their study, NPK produced a higher rice yield than the FWC even though the red pepper yields achieved with either treatment were not significantly different (statistically). All compost amendments in this study produced higher lettuce yields than the conventional NPK amendment in disagreement with the observation made by Vo and Wang [29], who recorded lower fresh yields of muskmelon grown with livestock manure composts than those produced with the conventional mineral fertilisers. The study observed, however, that combined applications of the livestock composts and mineral fertilisers resulted in the highest muskmelon fresh yields of all the treatments.

In private communications documented by Troeh and Thompson [30], Broyer demonstrated that chlorine is a beneficial nutrient element to a range of crops, including leaf lettuce, with their study reporting a 30% increase in yield of the leaf lettuce grown with chlorine supplementation. The composts used in this study were rich in chloride ions, which might explain the higher yields obtained with the compost amendments than with NPK. In a 3-year field study that involved growing maize, bell pepper, and small grains every year but in rotations, Reider et al. [31] indicated that there were no significant statistical differences between the yields of bell pepper produced with either mineral fertilisers or livestock manure composts because bell pepper is a less-N-demanding crop compared with the rest of the crops grown, for which mineral fertilisers produced higher yields than the compost. However, this line of argument may not be applicable to our study because leaf lettuce is a high-N-demanding crop (a heavy feeder). It is important to note, however, that both food waste and animal manures in both their raw and stabilised forms of biochar, compost, digestates, etc., are laden with both macro and micro crop nutrient elements [32,33,34,35] and can, therefore, adequately supply them to the growing crops.

Another plausible explanation for the poor agronomic performance of the leaf lettuce grown on the conventional NPK amendment in comparison with the compost amendments is the inability of the sandy soil used in the present study to avail nutrients, especially N (leached), P (fixed), and micronutrients, to the growing leaf lettuce as demonstrated by Luyima et al. [35]. The mixed composts produced from the amalgamations of FWC and livestock manure composts outperformed both the pure livestock manure composts and FWC up to the optimum mixing ratios, beyond which the agronomic performance of the lettuce decreased with increasing FWC in the mixture. That decrease in the fresh yield of the leaf lettuce might have resulted from salinity induced by the high NaCl content in the compost mixture containing higher percentage concentrations of the FWC. An early study by Tesi et al. [36] examined the effects of NaCl-induced salinity on the growth and yield of butterhead lettuce in a floating system and found that the fresh lettuce weight was reduced at a NaCl concentration as low as 10 mmol/L. A recent study by Neocleous et al. [17] studied the effects of salinity induced by NaCl on both red and green lettuce varieties in a floating system and discerned that 20 mM of NaCl reduced lettuce yields by 13% and 17% in green and red cultivars, respectively, in comparison with the control.

The attenuation of crop growth and yield under NaCl-induced salinity conditions is caused by a miscellany of factors. It has been demonstrated by several studies, such as Neocleous et al. [37], Shrivastava and Kumar [20], and Cruz et al. [38], that salinity reduces accumulations of important crop nutrient elements in the growing crops, which concomitantly reduces crop growth and yield. The high multiple r values of the negative linear relationships obtained between the NaCl content of the compost used in the present experiment and the amount of each of the macronutrients of P and K (see Figure 2b,c) that accumulated in the leaf lettuce confirm that indeed NaCl impinges on the absorption of those nutrients from the soil. However, with a very low multiple r value of the negative linear relationship between the amount of N that accumulated in the leaf lettuce and the NaCl content of the applied composts (r² = 0.3777) (Figure 2a), the likelihood of NaCl to interfere with the absorption of N by the leaf lettuce is quite low. Indeed, Neocleous et al. [37] demonstrated that salinity induced by NaCl did not have any statistically significant effect on the amount of N that accumulated in the leaf lettuce grown in a floating system but both P and K contents were significantly reduced. Because the physicochemical properties of Na are analogous to those of K, Na⁺ ions compete with K⁺ in plant uptake specifically through high-affinity potassium transporters and nonselective cation channels [39]. However, Na is nonessential for crop growth and development [30] and, thus, its increased uptake is a harbinger of poor crop growth and yield. In fact, a study by Cruz et al. [38] found abnormally higher Na:K ratios in the tissues of cassava grown in saline soil than those of the cassava grown in a normal soil.

Even though our study did not determine the Na:K ratios of the leaf lettuce, it is pertinent to assume that the low yields obtained with compost containing higher NaCl contents ensued partly from the replacement of K by Na during crop growth. In their review study, Grattan and Grieve [40] indicated that phosphorus availability in saline soils reduces because of both ionic strength effects that lessen the activity of phosphate ions and sorption of the phosphates by the calcium ions. Indeed, our study found strong linear relationships between soil available phosphorus, soil exchangeable calcium, and the content of the NaCl in the applied compost (percentage of the food waste in the compost mixture) (see Figure 2d,e). The relationship between soil available phosphorus and the content of the NaCl in the applied compost was negative while the one between soil exchangeable calcium and the content of the NaCl in the applied compost was positive. These observations, therefore, confirm the fact that the application of the mixed composts containing high amounts of the FWC increased the soil’s Ca content, which heightened phosphorus (P) adsorptions, resultantly reducing its absorption by the leaf lettuce. The moderately strong negative linear relationship between soil exchangeable Ca and the soil available P (Figure 2f) validates the aforementioned assertion.

Pigments including carotenoids and chlorophyll constitute the many bioactive compounds found in leafy vegetables. Their composition in the pigment–protein complexes gives the vegetables a specific colouration, which is used as a parameter for their maturity, quality, and freshness [41,42]. The colour is also important in defining the appearance of the vegetables and, hence, influences consumer choice [43]. Additionally, the photosynthetic potential of the living plants depends on the chlorophyll content and, hence, the chlorophyll content of the plant gives some indication of the physiological status of the plant [44]. Besides the colouration and physiological functions, the content of the pigments in vegetable crops is important due to their recognised roles in health [45,46]. For instance, chlorophylls play a vital role in the prevention of several diseases associated with oxidative stress, including cancer, cardiovascular diseases, and other chronic diseases [47]. In our study, FWC and mixed composts produced leaf lettuce with higher chlorophyll contents than the livestock manure composts, implying that the FWC and mixed composts produced lettuce with higher marketability and health value. Given the role of chlorophyll in photosynthesis, the higher chlorophyll content of the lettuce produced with the FWC and mixed composts than that of the livestock manure composts might explain the generally higher fresh yields obtained with the FWC and mixed composts.

Vegetables supply about 72% to 94% of the total daily intake of nitrates to human beings [48]. However, excessive consumption of nitrates is associated with numerous human health hazards due to nitrate toxicity ensuing from the reduction of nitrates to nitrite and subsequent conversions to nitrosamines and nitrosamides through the reactions with amines and amides, whose carcinogenic action is well known [49]. Additionally, there are indications that high nitrate accumulations in plants lead to the formation of peroxynitrite (ONOO⁻), which is highly toxic to plants [50,51]. Hence, excessive nitrate accumulation in plants is not only harmful to human health but also to plant growth. In this study, FWC and mixed composts generally produced leaf lettuce with lower nitrate content than the pure livestock manure composts with beneficial effects for human health. Santamaria et al. [52] opined that a reduction in the nitrate content of the vegetables can aid in adding value to vegetable products that are already popular for their nutritional and therapeutic properties. Therefore, the ability of FWC and mixed composts to lessen the accumulations of nitrates in the leaf lettuce is a desirable attribute. On the other hand, livestock manure composts produced higher sugar contents in the leaf lettuce than the FWC, possibly because they contained and supplied less nitrogen to the growing lettuce than the FWC. This is because a study by Becker et al. [53] found that the sugar levels in the leaf lettuce decreased with increasing doses of nitrogen supplied to the growing lettuce.

6. Conclusions

The results obtained from this study indicate that mixing FWC with livestock manure composts in the right proportions produces mixed composts with better potentials to support the growth and yield of leaf lettuce than both the pure livestock manure composts and FWC. The higher agronomic efficiencies of the mixed composts are likely due to the lessened NaCl toxicity of the FWC and the higher nutritive value than that of the pure livestock manure composts since FWC is highly nutritious. The presence of the chloride ions in acceptable levels in the mixed composts might have also contributed to the improved yield of lettuce since leaf lettuce is a chlorine-loving crop. The mixed composts also produced leaf lettuce with better marketability (due to the higher chlorophyll content) and health value (due to the lower nitrate content) than the pure livestock manure composts. The observations made in this study reveal that mixing NaCl-rich food waste with composts containing low amounts of NaCl is a sustainable way of utilising NaCl-rich food waste for the benefit of both agricultural productivity and environmental sustainability.