Reduction of fertilizer use by planting an early variety of cassava (Manihot esculenta Crantz)

S. L. Tan

Abstract

Fertilizer use for cassava (traditionally a 12-month crop), particularly when planted on drained peat, constitutes a large component (34%) of production costs. The recommended rate has been 250 kg N, 30 kg P₂O₅ and 160 kg K₂O per hectare, supplemented by CuSO₄.5H₂O (either soil-applied at 10 kg/ha, or supplied by dipping cassava cuttings in 2% solution before planting). MARDI released an early variety of cassava named MM 92 in 1992. This variety is unique in its capability of producing a 6-month fresh root yield of 30–35 t/ha, which is comparable with what is produced by the commercial variety Black Twig in 12 months. In an attempt to reduce the high rate of fertilizer application, a fertilizer trial was conducted, in which the rate or the frequency of fertilizer application was reduced during six cycles of a 6-month crop of MM 92. Nine fertilizer practices were tested, two of which were controls. The first control comprised the full rate of fertilizers, applied at each crop cycle (100%). The second control was a treatment receiving no fertilizers over the six cycles (0%). Three treatments received half of the full rate of fertilizers over the six cycles (50%). One treatment received one-third (33.3%), two others received one-quarter (25%), while the remainder received 16.7% of the full rate, all through various combinations of rate and frequency. Results of the trial showed that fertilizers may be reduced substantially without compromising root yield in MM 92. The first and second cycles did not reveal any significant treatment differences, presumably due to residual effects of past manuring history on the site. Significant differences among treatments showed up by the third cycle. Significantly higher yields (ranging from 29.2 to 30.6 t/ha per season) were recorded by three treatments, namely the first control of full rates applied every season, half the rate applied every season (50%) or every other season (25%). Dry matter content of the roots was not affected by the fertilizer treatments throughout the six cropping seasons. Thus, fertilizer costs, when growing variety MM 92 for 6-month harvests, may be reduced by as much as 73.3% (a saving of RM3 216/ha) over a six-cycle cropping without jeopardizing root yields. Leaf N level has a greater influence on root yield than leaf P or K, both of which were above the reported critical levels for optimum yield in practically all the treatments. The critical leaf N (at 95% maximum yield) in this study was found to be around 5.5%.

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