Urea-N transformations in the River Estate series (Fluventic eutropepts) and its efficiency in dasheen (Colocasia esculenta L. schott) cropping systems

Investigated the effects of N fertilization in upland dasheen. Three field experiments were conducted between July 1988 and March 1991 to determine the fate of applied CO(NH2)2 in the River Estate Series (Fluventic Eutropepts) and its effect on growth, development and N efficiency in dasheen. In two of these experiments, the effects of N rate and method of dasheen production were assessed. In the first experiment, three methods of dasheen production were investigated i.e. monocrop dasheen grown for corms only (MC system), monocrop dasheen grown for leaves and corms (MLC system), and intercrop dasheen (I system). Four sequential cropping practices were investigated primarily to determine their effects on the CO(NH2)2-** (15N) behaviour and use in dasheen. Hydrolysis of CO(NH2)2 was generally complete within three days, but was prolonged where it was banded at a rate of 450 kg N/ha. Although the pH only increased just above neutrality for broadcast CO(NH2)2, volatilization was probably also an important mechanism of N loss since the fertilizer was applied over a large surface area of soil. Nitrification proceeded at a slower rate than CO(NH2)2 hydrolysis and exchangeable NH4+-N levels in the soil were generally slightly higher than NO3- -N. Application of CO(NH2)2-N increased the rate of photosynthesis of the crop which resulted in a doubling of total fresh corm yields. Leaf cutting reduced corm yields by an average of 39 % while there was only a 14 % reduction due to intercropping. However, leaf yields were increased by more than 160 % with N application and the productivity of the intercropping system was superior to that of the monocrop. There was also great consistency in corm yields and the productivity of the intercrop from one experiment to the next despite a much less consistent rainfall pattern. There was no significant effect of N rate or cropping system on CO(NH2)2-N efficiency in dasheen which averaged 17.4 % and 16.9 % for band and broadcast applications respectively. Timing of applications did not affect the efficiency. Incorporation of pigeon pea residue in the field did not affect the efficiency of CO(NH2)2-N nor total N uptake. The rate of decomposition of pigeon pea residue was slightly slower than that of rice and maize straws which can lead to a better conservation of added N through biological immobilization. Remineralization during crop growth improves N uptake efficiency through the development of a slow release system. Results show that dasheen can successfully be cultivated on an upland soil even in areas with an annual rainfall less than 2000 mm. Furrow cultivation affords this possibility but more careful management of the applied CO(NH2)2 must be practiced so as to reduce losses mainly through volatilization, dentrification and leaching.