Effect of decomposing crop residues on soil properties and crop productivity in the semi−deciduous forest zone of Ghana

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A study was carried out to examine recycling of crop residues as a major component of integrated soil fertility management. The objectives of the study were to determine the quality, rates of decomposition and nutrient release patterns from residues of cassava leaves, cowpea haulm, groundnut haulm, maize stover, mucuna and soyabean haulm typical of cropping systems in Ghana as sources of plant nutrients. The study aimed at i). estimating the time course of nutrient release from crop residues to improve synchrony and increase maize yield, as well as ii). examining the changes in soil physico – chemical properties by applying some of the residues (maize stover and cowpea haulm) either alone or in combination with NPK (15–15–15) fertilizer. Laboratory and field experiments were conducted. The field experiments were conducted at the Central Agricultural Station, Kwadaso, Kumasi, in the semi – deciduous forest zone of Ghana. The laboratory experiments were conducted at the Soil Science laboratory, KNUST, Kumasi. The field studies were conducted on Asuansi soil series (Ferric Acrisol) in the minor season of year 2006 and repeated in the major and minor seasons of 2007. Total N content of the crop residues ranged from 0.74% in maize stover to 2.95% in cowpea haulm. Organic carbon ranged between 44.70% in groundnut haulm and 47.50% in maize stover. Total phosphorus ranged from 0.22% in groundnut haulm to 0.38% in maize stover. The C: N ratio of the residues ranged from 15.3 in mucuna to 65.1 in maize stover. In the major season, the decomposition rate constant (k) was 0.224 and 0.305 week-1 for surface – placed and buried maize stover respectively and 0.475 and 0.502 week-1 for cassava leaves. In the minor season, the k values were 0.210 and 0.240 week -1 for maize stover, 0.421 and 0.523 week-1 for cowpea haulm and 0.541 and 0.659 week-1 for cassava leaves. The values recorded in the dry season were 0.145 and 0.158 for maize stover, 0.206 and 0.258 for cowpea haulm and 0.233 and 0.290 for cassava leaves. Burying of the crop residues reduced the half – life (t50) from 4.37 to 3.88 weeks for maize stover, 3.07 to 2.77 weeks for cowpea haulm and 2.62 to 2.34 weeks for cassava leaves. Microbial biomass C and N were higher under the various treatments compared with microbial biomass P. Most of the microbial biomass P was locked up in the microbial cells. Combined application of the high quality cowpea haulm and low quality maize stover was able to increase the microbial biomass P. Mean grain yield was highest (1.69 t ha-1) under maize stover + ½ NPK treatment followed by cowpea haulm + ½ NPK treatment (1.62 t ha-1) with the control treatment recording the lowest (0.83 t ha-1). The results showed that all the treatments had significant effects on soil pH after cropping for three seasons. The level of organic matter increased under fertilizer treatment, while it decreased under the control treatment. Total N remained almost unchanged in all the plots after cropping. Available P decreased from initial value of 26.00 mg kg-1 to 25.09 mg kg-1 where no fertilizer was added but increased by 35 – 80% under fertilizer treatments. Exchangeable Ca, K and Na decreased in all the plots irrespective of the type of amendment. The combination of maize stover + ½ cowpea haulm increased the ECEC by 3%, remained unchanged under maize stover + ½ NPK treatment while it decreased in all other plots. Though maize grain yield increased under the combined application of crop residues and NPK fertilizer in this study, data on changes in soil properties after cropping suggest the need to increase the duration and the level of organic material added to sustain the fertility of the soil.
A thesis submitted to the Department of Crop and Soil Sciences, Faculty of Agriculture, College of Agriculture and Natural Resources, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana, in partial fulfillment of the requirements for the award of degree of Doctor of Philosophy in Soil Science.