THE RECOVERY OF N-15 FROM LABELED UREA FERTILIZER IN CROP COMPONENTS OF SUGARCANE AND IN SOIL PROFILES

Trash conservation measures associated with burnt and green harvested cane, and minimum tillage, are being adopted by canegrowers. These new management systems pose questions about how to apply N fertilizers. Experiments were conducted to evaluate the efficiency of fertilizer N uptake by the crop. Urea, labelled with N-15, either broadcast or buried in three trash management systems: with and without trash after harvesting cane burnt, and with trash after harvesting cane green. The proportion of applied fertilizer-N recovered in a cane crop was 33% when labelled urea was buried, and 18% when broadcast. The presence of trash mulches from burnt or green harvested cane had negligible effects on the uptake of fertilizer-N. Of fertilizer-N applied, 25% was detected in soil 12 months after application, and there was no difference between burying and broadcasting urea. The fate of the lost fertilizer-N was not determined. Leaching did not appear to be a significant loss process in this gleyed podzolic soil, but ammonia volatilization probably occurred when urea was broadcast and it is suspected that denitrification accounted for the majority of the fertilizer-N losses.