Effect of Leucaena leucocephala biomass and sawdust on physical and chemical properties of the soil and on growth and yield of soyabean (Glycine max)

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The effect of sawdust and Leucaena leucocephala biomass on soil physical and chemical properties and on growth and yield of soyabean (Glycine max) were studied in two field experiments. The study was conducted to determine optimum combination level of sawdust and Leucaena biomass that can sustain growth and yield of soyabean and to assess treatment effect on the soil and earthworm population. Sawdust and Leucaena were applied at rates 0, 5, 10 and 20 t/ha for experiment I and at 0 and 10 t/ha for experiment II. The stem height, root length, root dry weight, nodule number and nodule diameter of soyabean were determined. Growth analysis involving leaf area index (LAI), leaf area ratio (LAR), leaf area duration (DO, net assimilation rate (NAR) and crop growth rate (CGR) were carried out. Soil samples were analysed for N, P, K, C and pH at planting and after harvesting. Soil bulk density, porosity, moisture content and temperature were determined. Germination was reduced significantly at 20 t/ha application of sawdust. Leucaena application showed significant increase in the LAI (P <0.01), LAR (P <0.05), Df (P <0.01) and a slight increase in soil nitrogen levels. Leucaena treatments produced soyabean plants which were almost twice as tall (64cm) as those of the sawdust treatment (37cm), and also produced almost twice more pods (26.4 pods) than the sawdust treatments (12.8 pods). Seed yields from Leucaena plots were twice (1.88 t/ha) as that of sawdust plots (0.83 t/ha). Sawdust and Leucaena combination yield was higher (1.52 tlha) than that of the control (1.04 t/ha). The main determinant of seed yield was number of pods per plant. At 10 t/ha, sawdust reduced both LAI and LAR significantly (P <0.05). Earthworm population increased with sawdust application up to 10 t/ha level and declined at 20 t/ha. Soil temperature decreased with increasing levels of sawdust and Leucaena application. Sawdust reduced bulk density, increased porosity and soil water content more than Leucaena. The study revealed that the optimum level of sawdust that can sustain crop production is 10 t/ha.
A thesis submitted to the Board of Postgraduate Studies, Kwame Nkrumah University of Science and Technology, Kumasi, in partial fulfilment of the requirement for the award of the Degree of Master of Science in Agroforestry, 1997