Effect of Source, Time and Method of nitrogen application on growth and yield components of potato in Kenya.

Gathungu, G.K., 


Irish potato variety Dutch Robjin was planted in the Faculty of Agriculture Farm, Kabete Campus, University of Nairobi, in 1997-1998 short rains (Expt I), and 1998 long rains (Expt II). The experiment was laid out in a randomised complete block design in a split plot arrangement, and replicated three times. The three sources of nitrogen (N), [(calcium ammonium nitrate (CAN, 26% N); Urea (46% N) and ammonium sulphate nitrate (ASN, 27%N)] constituted the main plot treatments, and time [early application, split application (half of the fertilizer at planting and half applied 5 weeks after emergence), and late application  (5 weeks after emergence)] and method of application (placement and broadcast within the furrow) the subplots treatments. The highest potato growth and yields were observed where N was applied as CAN followed by ASN. High growth was manifested in the establishment of a greater leaf area index (LAI) with the application of CAN or ASN early in the growth. The large LAI probably provided more of the assimilates which enabled early and greater tuber bulking. Lower LAI observed where urea was applied early in the growth season indicated that less assimilates were available for crop growth and consequently lower tuber bulking and yields. The only benefit observed with urea was a higher LAI later in the season a time when the tubers had already formed and this did not contribute to tuber yield. However, urea delayed the onset of senescence unlike CAN and ASN where early senescence can be associated with the greater supply of the assimilates. However there was no significant difference in total N accumulation in the leaves and tubers between the sources of N. Early application of N followed by split applied N fertilizer led to fast early growth (shoot, tuber, root and total dry matter, LAI development, and stem height) and highest potato tuber yield. A fast early growth (especially the high LAI development) with early and split N application provided assimilates required for potato growth and development and consequently greater number of tubers and tuber yield. Late N application enhanced growth of the shoots (leaves and stem height) later in the season and therefore less assimilates were available for growth and development and consequently lower yield of tubers/plant was realised. However high total N accumulation in both leaves and tubers was observed with late N application compared to early N application. Broadcast or placement of N in the furrow had no significant effect on growth, development, total N accumulation in leaves and tubers and yield of potato. It is concluded that application of CAN or ASN especially early in the growth season could be beneficial. This is especially so considering that urea is costly and split application of the fertilizer only increases the labour costs. The fertilizer can either be broadcast or placed in the furrow.

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