The effect of the nature and ratio of dry to green organic material on decomposition and quality of Compost

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Kaiba, Munsanje Kelvin
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This study investigated the effect of nature and ratio of grass to legume matter on decomposition and quality of compost. Composting is the process of breaking down organic materials of plants and animals to produce humus. The quality of compost varies depending on the nature of materials used and how the process is managed. One of the major limitations to composting is sourcing enough material of good quality to yield a consistent good quality end product. The requirements for composting are, organic matter, microorganisms, moisture, aeration, temperature control and labour. The hypothesis considered was that the nature of organic materials used would affect the decomposition rate, mineralisation rate and quality of the compost. In this study, the treatment materials used were the green materials sunhemp [Crotalaria juncea] and velvetbean [Mucuna pruriens]. These were composted in heaps in combination with dry grass and cattle manure at ratios of dry grass to green material of 1:1 and 2:1. The heaps were arranged in a Completely Randomised Design and replicated twice. A total of eight heaps were made and composted for twelve weeks. Decomposition rates were estimated by measuring change in volume of the heaps per week. Mineralisation rates were estimated by extracting nitrate-nitrogen (NO3") and ammonium-nitrogen (NH4"^) with potassium chloride (2M KCl) from samples taken at three-week intervals. The composts were tested for quality by estimating Cation Exchange Capacities (C.E.C) using the centrifugation method, carbon to nitrogen ratio(C/N ratio) calculated from values of total nitrogen determined by the Macro-Kjeldahl method and organic carbon by the Walkley & Black method. Available phosphorous and potassium were also determined using Bray 1 method and leaching with ammonium acetate respectively. The data were statistically analysed. The nature of organic material had no effect on the decomposition and mineralisation rates and quality of compost. This result suggests that velvet bean and sunnhemp used in combination with natural grass gives equally suitable compost. With the materials used in this study, maturity of the compost was achieved at six weeks of incubation. Varying the ratio of dry grass to fresh legume significantly (p=0.05) increased the decomposition and mineralization rates irrespective of the legume type. However, there was no effect on compost quality. Decomposition rate was fastest with 1:1 ratio, but there was greater reduction (65 %) in volume of the compost heap than the 2:1 ratio (48 %). In practice, this would mean that one needs a larger number of heaps in order to yield the same amount of compost. Therefore, composting at a 2:1 ratio would be desirable because of the greater yield of compost.
Soil Management , Compost