Application a 32P bioassay for P-fertilization of citrus

Uptake of phosphorus (as 32P) by excised root samples from Citrus trees growing in the soil with originally less than 30 mg kg-1 available P was significantly lowered after P-fertilization. This effect became more prominent in the 2nd and 3rd year of the experiment. High concentration of available P in the soil (80mg kg-1) resulted in a higher P-content in the excised roots and therefore decreased P-uptake. Application of phosphate fertilizer to such soil increased the content of available P but P concentration in Citrus leaves was not significantly changed. Branch length, fruit yield, and Brix sugar in fruit juice were also not influenced. These data show that response to P-fertilization can be tested by leaf analysis, growth or yield measurement. P-uptake of excised roots harvested from the soil with available P above 150 mg kg-1 reached a level of 400 ∼ 500 pg P/mg root, which indicates that P-fertilization is unnecessary at the soil of P-content above this limit