FOLIAR UPTAKE OF N-P-K SOURCES AND UREA BIURET TOLERANCE IN CITRUS

Commercial application of foliar urea in the winter-time and NPK sprays in the spring-time commonly occur in Florida citrus production because of reported benefits of increased flowering and yield. There were no measurements of actual nutrient uptake accompanying the early work on yield benefits. Biuret toxicity from contaminants in commercial urea applications in the spring have occurred also. Therefore, the uptake of N, P and K and the level of tolerance of citrus orange (Citrus sinensis) leaves to biuret under these spray conditions were evaluated. Leaf N from urea sprays (applied at N rates of 15.7 kg/ha) was increased about 9% at the maximum spray deposit location in the tree. Leaf P and K were increased from sprays of 7.9 kg/ha of P or K only if leaf levels of P or K were low before spraying. Biuret concentrations below 0.5% in urea did not lead to phytotoxic symptoms, but concentrations of 0.8% or higher were phytotoxic when at least 15.7 kg/ha of urea N was sprayed two times, four to five weeks apart, in the spring. After one year of spray treatments in a commercial trial, yields were not significantly different from the non-sprayed control for treatments of two spring foliar NPK sprays (N-P-K = 15.7-7.0-7.0 kg/ha) with either 0.4 or 0.8% biuret in the urea and either of 2 sources of P (ammonium polyphosphate or mono-potassium phosphate).