Effect of source, time, and method of application of nitrogen on growth and yield of potato (solanum tuberosum l)
Gathungu, G K
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Irish potato variety Dutch Robjin was planted in the Faculty of Agriculture Farm, K.abeteCampus, 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 unmonium nitrate (CAN, 26% N); Urea (46% N) and ammonium sulphate nitrate (ASN, n%N)] constituted the main plot treatments, and time [early application, split application (half bf the fertilizer at planting and half applied 5 weeks after emergence), and late application (5 eeks 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 mdex (LAI) with the application of CAN or ASN early in the growth. The large LAI probably irovided more of the assimilates which enabled early and greater tuber bulking. Lower LAl ibserved where urea was applied early in the growth season indicated that less assimilates vere available for crop growth and consequently lower tuber bulking and yields. The only ienefit 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 enescence unlike CAN and ASN where early senescence can be associated with the greater upply of the assimilates. However there was no significant difference in total N accumulation a the leaves and tubers between the sources ofN. Early application of N followed by split applied N fertilizer led to fast early growth of tuber, root and total dry matter, LAl development, and stem height) and highest potato oeryield, A fast early growth (especially the high LAI development) with early and split N \ ). Iication 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.