The influence of nitrogen and phosphate fertilizer levels on growth, development and yield of potatoes (solanum tuberosum L.)

Abstract

This thesis is the subject of work conducted during 1980 short rains and 1981 long rains to investigate the influence of nitrogen and phosphate fertiliser rates and their interaction on growth, development and yield of potatoes under Kabete conditions with a view to evaluating the validity of current fertiliser recommendations, A completely randomised block 32 x 2 factorial experiment was laid in three replicates with three rates of nitrogen: 0, 75, 150 KgN/ha; three phosphate rates: 0, 100,200 KgB205/ha (0,44,88 KgP/ha), and two potato varieties: Anett (early maturity, 3-3~ months) and B53 (medium maturity, 3~-4 months). Plants were grown under field conditions and sampled at intervals of two weeks from the time of emergence to maturity and various growth parameters and yield components assessed. Results showed that provided tubers were adequately sprouted and environmental conditions favourable, application of moderate and proportionate rates of nitrogen and phosphate fertilisers would promote crop emergence, growth and early tuber formation. Both nutrients enhanced tuber bulking rates during the early stage of growth (upto eight weeks after emergence) b~t phosphate tended to lower bulking rates towards maturity. Nitrogen was found to have a beneficial effect of ehhancing both leaf area index and leaf area duration ii and increasing both tuber number per hill and size. Moderate phosphate application increased both tuber number per hill and size, but the highest phosphate rate caused premature senescence of plants and tended to reduce the number of tubers per hill. The number of stems per hill was mainly a varietal character and was determined by the initial degree of sprouting rather than the application of fertiliser. During both short and long rains, the recommended phosphate rate of 200KgP20S/ha (88KgP/ha) was found to be in excess of optimal requirement. During the long rains the recommended nitrogen rate of 75KgN/ha was optimal, but d~ring the short rains, heavier ni trogen application produced statistically higher yield in the early variety only, than did the recommended rate. Uptake of the greater proportion of the total nitrogen and phosphorus taken up was found to occur early in the growing season, upto four weeks after emergence. Uptake of nitrogen and its accumulation in tubers was enhanced by phosphate application, while that of phosphorus was decreased by nitrogen application. The levels of these nutrients taken up varied with the season. From these results it was.concluded that both nitrogen and phosphate fertilisers need be applied

at the time of planting as late applied fertiliser may not be beneficially utilized since uptake of ~these nutrients takes place early in the growing season. The recommended nitrogen rate of 7SKgN/ha was found optimal for Kabete but that of phosphate of 200KgP20S/ha (88KgP/ha) was found to be excessive. However, climate, nutritional and varietal influences appeared to be superimposed on each other with respect to potato productivity, so that heavy fertiliser applications particularly phosphate, may be a waste under unfavourable conditions, such as drought. Thus, although phosphate is known to promote root growth, thereby increasing water uptake by many plants under a drought environment, its negative effect of promoting plant senescence and producing few and small tubers, particularly when nitrogen level is low, tends to annul this benefit. Similar experiments, with more closely-spaced fertilizer rates and a wider range of potato varieties are therefore, recommended to ascertain the consistency of the results of this experiment under different locations.