Fate of CO(NH₂)₂-¹⁵N applied to taro (Colocasia esculenta var. esculenta) in an acid Vertisol of Trinidad

Abstract

Vertisols are an important natural resource, particularly in the developing world, but a greater understanding of their chemical characteristics with special reference to the fate of applied N is needed for their greater sustainable use. The Bejucal clay (Chromic Dystraquerts, very fine, mixed, acid) is an important agricultural soil in North Trinidad and a series of studies were undertaken on it to determine the optimum rate and the best time of application and to quantify the efficiency of applied N using CO(NH₂)₂-¹⁵N and taro, commonly called dasheen locally, (Colocasia esculenta var. esculenta), as a test crop.

The optimum rate was found to be 280 kg N ha^-1 for corm production from a field experiment, laid out in a randomised incomplete block design. This corresponded to an apparent recovery at 10 per cent when CO(NH₂)₂ was applied at 14 days after planting (DAP). Generally, a decrease in N uptake with increasing levels and later application times was observed. Another field experiment which employed ¹⁵N applied at 14, 56, and 112 DAP showed that ¹⁵applied at 112 DAP corresponded to the highest ¹⁵N level in the plant tissue.

A green house experiment conducted to determine the efficiency of the use of the applied N showed the highest recovery of approximately 49 per cent at a rate equilavent to 280 kg N ha^-1 with efficiencies of 43 and 40 per cent at the 140 and 420 kg N ha^-1 of applied N levels, respectively. A single application at 14 DAP resulted in an efficiency of 41 per cent compared to 44 and 47 per cent for the 2 split and 3 split applications, respectively. Split applications increased N uptake but without a corresponding increase in yield.

These studies showed that N applied as CO(NH₂)₂ at 280 kg N ha^-1 at 14 DAP is optimum for upland taro production under conditions of the Bejucal clay. Under normal field conditions surface washing may be the most important N loss pathway but when excessive fertilizers are used then the gaseous pathways may also contribute to N losses.