PHOSPHORUS AND POTASSIUM SOIL TEST AND NITROGEN LEAF ANALYSIS AS A BASE FOR CITRUS FERTILIZATION

A network of six NPK long-term field trials was carried out on differe nt soils of citrus-producing regions of Sao Paulo state, Brazil, in or der to estimate quantitative relations of fruit yield to NPK fertiliza tion and to determine parameters for fertilizer recommendation based o n soil testing and leaf analysis. The experiments were set up in an in complete factorial design 1/2 4(3) with 32 treatments, with four yearl y rates of N (30, 100, 170 and 240 kg N/ha), P (9, 27, 45 and 63 kg P/ ha) and K (25, 91, 157 and 223 kg K/ha). Four to seven harvests were r ecorded for the six experiments. Response surfaces of the type y = bo + b(1)N + b(11)N(2) + b(2)P + b(22)P(2) + b(3)K + b(33)K(2) + b(12)NP + b(13)NK + b(23)PK were adjusted to the average yields of each trial. Correlation were established for yield increases, expressed as relati ve yields, and results of soil analysis of P and K, and leaf analysis of N. Soil samples taken at 0-20 cm depth in the beginning of each exp eriment were analyzed for resin extractable P and exchangeable K using an ion-exchange resin procedure. Yield responses for phosphorus and p otassium applications were observed respectively in soils with less th an 20 mg dm(-3) of P and 20 mmol, dm(-3) of KS. Yield responses to nit rogen were related to the total content of nitrogen in leaves, being l argest for N values of 23 g kg(-1) and smallest for N of 28 kg(-1). Wi th these field information, a practical approach for fertilizer recomm endation for citrus, based on soil analysis for P and K and leaf analy sis for N, was developed.