Effects of manure, lime and phosphorus fertilizer on soil properties and soybean (glycine maxl.) yields in Embu county, Kenya
Benvindo, Serafim Manuel Mario Verde
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Agricultural sector in Kenya, contributes with 25% on GDP, yet this sector is far to supply adequate food for an increasing population due to declining soil productivity. Soybean (Glycine max L. Merrill) is relatively new crop being introduced in smallholders farming system in the Central Highlands of Kenya as source of nutrition, income generation and soil fertility improvement. However, the crop yields are below the national average. Soil acidity and depletion of nutrients affect negatively performance and yields of soybean in the region. The study was carried out for two seasons in Embu County to determine the effects of manure, lime and phosphorus (P) fertilizer on: (i) soil chemical properties and soybean yields; (ii) nitrogen (N) and phosphorus (P) uptake, and biological nitrogen fixation (BNF) by soybean; (iii) soil microbial biomass. The treatments included goat manure (0,5 and 10 ton ha"), lime (0 and 2 ton ha"), and P fertilizer (0, 30 and 60 kg P20S ha"). The experiment was laid out in a Randomized Complete Block Design (RCBD) with 4 replicates. Soils samples collected before and after the experiment were analyzed for soil chemical properties and microbial biomass (MB). Soybean Nand P uptake, N fixation and yields were determined. Data generated was subjected to Analysis of Variance (ANOVA) and means were separated using Least Significant Difference (LSD) at 95% significance level (p<0.05). The results showed that lime alone (2 ton ha") increased significantly soil pH (15.9%), extractable Ca (64.4%) and Mg (23.1 %), and reduced exchangeable acidity by 3.5 times. Goat manure alone (10 ton ha") increased significantly extractable K by 6.8 times, CEC (55.8%), available P (37.9%), N and P uptake (98.2% for N and 120.8% for P) and microbial biomass (73.5%). Integrated application of 5 ton ha" of goat manure with 2 ton ha' oflime plus 30 kg ha" P20S increased significantly soil pH (14.1 %), Ca (87.7%), Mg (30.8%), K (3.7 times) CEC (73.7%) available P (38.0%) and 59.3% on microbial biomass. The integrated application of 5 ton ha' of goat manure with 30 kg ha' P205 increased significantly the uptake ofN (99.2%) and P (153.2%). Soybean grain yields were significantly increased by 2.5 and 2.3 times under sole application of goat manure (10 ton ha') and integrated application of 5 ton.ha' of goat manure with 2 ton.ha' oflime plus 30 kg P20S ha', respectively. Goat manure proved to be an important soil amendment by providing nutrients, increasing soil pH, which in turn enhanced nutrient uptake, N fixation, microbial biomass, and increased soybean yields. Integrated application of manure with lime and P fertilizer contributed to improved soil chemical properties and soybean yields. Therefore, manure can substitute or supplement the inorganic fertilizers and could be recommended for the smallholder farmers ofEmbu County, Central Highlands of Kenya.