RECOVERY IN PLANTS AND SOILS OF N-15 APPLIED AS SUBSURFACE BANDS OF UREA TO SUGARCANE

The recovery of fertiliser N by sugarcane crops is low in comparison w ith most other field crops. Application of urea in subsurface bands in stead of by broadcasting can greatly reduce loss of fertiliser N due t o ammonia volatilisation, but the fertiliser N is still susceptible to loss from leaching or denitrification, which could be affected by soi l internal drainage, trash management, or tillage practice. The recove ry of fertiliser N in crops and soil from N-15-labelled urea applied a s subsurface bands was measured in ratoon crops in southern Queensland and northern New South Wales. Two soil types, with contrasting intern al drainage, were used in each region. In Queensland, the cultural pra ctice was either trash burnt with inter-row cultivation or trash retai ned on the surface ('trash blanket') with no cultivation. In northern New South Wales, where the trash was burnt prior to harvest, the pract ice was either inter-row cultivation or zero tillage. Crop recovery of fertiliser N was nearly always in the range 20-40% of the amount appl ied. Residual fertiliser N in the soil at crop maturity ranged from 13 to 42% (average 26%). Total recovery of fertiliser N in the soil-plan t system ranged from 35 to 76% (average 52%) at 6 months after applica tion, and from 35 to 96% (average 56%) at crop maturity. Urea fertilis er supplied only 20-40% of the crop N uptake in a given season. Neithe r crop recovery nor loss of fertiliser N from the soil-plant system we re related to the soil type or cultural practice used, indicating that compensatory effects occurred.