Effects of plant density and phosphate levels on growth, yield and yield components of field beans phaseolus vulgaris l.
Field bean (Phaseolusvulgaris)grown mainly for its protein rich dry bean seeds, is the most important legume and the second most important crop in Kenya. Bean yields have been shown to be increased by phosphate fertilizers particularly in areas which are low in available phosphorus which is the case in most farming areas of Kenya. Increasing bean plant populations under optimal growth conditions has resulted in increase in bean yields mainly due to the increase in number of pods per unit area. The objective of this study was to investigate the effects of phosphate fertilizer, plant population density, and population density x fertilizer interaction on growth and yield of Phaseolus vulgaris. Two experiments were conducted at the University of Nairobi, Kabete in 1979 and 1980. Kabete has an annual average rainfall of 950 mm and mean maximum and minimum temperatures of 230C and 130C respect/ively. The soils are deep red friable loarns. Bean seeds (cultivar Rose Coco) were planted at 25 x 25 cm, 20 x 20 cm, 15 x 15 cm and 10 x 10 em to give population densities of 160,000, 250,000, 444,444 and 1,000,000 plants/ha respectively. Triple superphosphate fertilizer (46% P20s) was applied at the rates of 0 (control), 50, 100 and 150 kg P/ha. The 4 x 4 treatment combinations were replicated four times in a randomized complete block design. The crop was grown following the recommended agronomic practices. Six sequential harvests, each of which comprised 10 and 5 plants per plot in 1979 and 1980 respectively, were harvested at 2 week-intervals. From the samples, leaf, stem, pod and root dry weights, nodule number and nodule weight per pLarit; were determined. Leaf Area Index (LAI) was also determined using the disc method. Seed yield, pods/ plant, seeds/pod, 100 seed weight, total dry matter and Harvest Index (HI) were determined at the final harvest. Growth rate was also monitoredthroughoutthe gr~ving seasons. Phosphate fertilizer increased LAI, plant growth rate, rate of plant senescence, and number and weight of nodules. Pod number per plant, se~ds/ pod, and lOO-seed weight in 1980, were also higher in the fertilized than in the control plants. There was 13% and 9% increase in seed yield over the cOntrol in 1979 and 1980 respectively at the 150 kg P/ha treatment and this increase could be attributed to the increase in the number of pods per plant Yield differences resulting from the different fertilizer rates (50-150 kg P/ha) were insignificant, consequently application of 50 kg P/ha appeared the best. With increase in plant population, growth rate, dry matter yield and rate of senescence per plant, pods/plant and seeds per pod all decreased, while LAI, rates of growth, dry matter yield, rate of senescence and number of pods per hectare increased. lOa-seed weight was apparently unaffected. Seed yield decreased with increase in plant density hence this did not reflect increases in vegetative dry matter with rise in plant population. High populations favoured vegetative rather than reproductive growth as indicated by decreasing HI values with increase in plant density. Therefore no yield benefit was obtained by increasing plant population of this particular variety beyond 160,000 plants/ha. Population density x fertilizer interactions were largely insignificant. Where the interactions were significant, responses in growth and yield to the different plant densities occurred mainly where fertilizer was applied. This seemed to indicate that phosphorus can be limiting in Kabete soils.
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