Evaluation of factors affecting the quality of compost made by smallholder farmers in Malawi

Abstract

In Malawian agriculture, the use of compost as a soil amendment has received much attention over the last few decades. Despite this, little is known about the commonly practiced composting systems in Malawi and their potential in mitigating soil fertility problems experienced by smallholder farmers. This study characterized the Changu (turned and watered regularly) and Chimato (covered with mud and static) systems and investigated optimum conditions required for effective composting. It further investigated nutrient release characteristics of the composts from these systems and their impact on maize crop establishment. Replicated compost heaps were formed from wheat (Triticum aestivum) straw and grass /clover (Lolium perenne/Trifolium repens) (in the UK) and maize (Zea mays L.) straw and green bean (Phaseolus vulgaris L.) residue (in Malawi) using the Changu and Chimato systems. Four initial C:N ratios of 20:1, 25:1, 30:1 and 60:1 were studied in the UK whereas two initial C:N ratios of 20:1 and 30:1, chopped into two lengths (5 or 10 cm) were used in Malawi. All the treatments were set in a randomized complete block design and the composting experiments ran for 112 days in the UK study and for 77 days in Malawi. Incubation-mineralization studies using the resultant composts were run for 42 days and 84 days for UK and Malawi respectively, followed by a maize establishment study run for 25 days. The Changu systems had significantly longer mesophilic phases (19 days) and active composting periods (24 days) compared to the Chimato systems (14 and 22 days respectively). The temperature profiles for the two systems were similar in the glasshouse, but differed in the field due to reduced insulation in the Changu (uncovered) system. The composting processes in these systems contributed to the production of compost with as high as 1.1% total N. A higher concentration of NO3-N (406 mg/kg dwt.) was produced in the Changu system cf. Chimato (359 mg/kg dwt.) whereas a higher concentration of NH4-N (36 mg/kg dwt.) was produced in the Chimato system cf. Changu (34 mg/kg dwt.) for the Malawi compost. Similarly, Changu system resulted in greater concentrations of TON (61 mg/kg dwt.) cf. Chimato (24 mg/kg dwt.) whilst Chimato contained high concentration of NH4-N (61 mg/kg dwt.) cf. Changu (8 mg/kg dwt.) for the UK compost. No differences were observed in the concentration of extractable-P and extractable-K in the two systems for the UK studies whereas Changu treatments and those from initial C:N had more P in Malawi. Resultant compost pH ranged between 6.8 and 8.6 for the UK-based studies and between 7.2 and 8.9 for the Malawian-based study. Incubation-mineralization studies indicated temporal differences when the resultant compost from the two systems (Changu and Chimato) was incubated in the soil with respect to nutrient release. Initial feedstock C:N ratio had a significant effect, treatments with C:N 20:1 mineralized nitrogen whilst those with initial C:N 30:1 and 60:1 immobilized nitrogen compared to the control for the UK experiments. No immobilization was observed for Malawi resultant compost. This was reflected in the maize establishment trials when compost from the two systems was used as a soil amendment. Treatment with materials from initial C:N 20:1 produced significantly larger plant stalks and high plant biomass (0.92 g/plant (dry basis)) than the other treatments. Varied differences were observed between UK and Malawi with respect to composting system on plant growth. The use of compost from this study increased CEC of the soil by 2.1 cmol/kg. Efficient composting requires low C:N material and the required compost time and resultant quality is dependent upon the C:N ratio of the initial feedstock. The longer active composting time in the Changu systems appeared to influence production of TON compared to the Chimato. It is suggested that to optimise the compost quality there is a need to encourage the smallholder farmers to grow green leguminous crops which they can mix with the straw to reduce the initial C:N ratio to improve its compostability. It is also important to increase the number of aeration holes in the mud coat of the Chimato heap in order to improve the oxygenation process of the material and to use them for moisture adjustments. A cost benefit analysis conducted suggested that the lower the initial C:N ratio and the longer the chop length (≤ 10 cm), composting using the Changu system, the higher the net benefits which can be attained.