Ja. Quaggio et al., PHOSPHORUS AND POTASSIUM SOIL TEST AND NITROGEN LEAF ANALYSIS AS A BASE FOR CITRUS FERTILIZATION, Nutrient cycling in agroecosystems, 52(1), 1998, pp. 67-74
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.