The new planting areas of cacao in the Amazon basin of Brazil were established on eutrophic soils, especially structured Purple Earth, and on Yellow Latosol or dystrophic podzolic soils. The nutrient requirements of cacao trees cultivated on these soils were still unknown. The effects of N, P and K on the productivity of cacao were determined in two experiments conducted in the counties of Medicilândia, along the Transamazonian Highway, and Benevides, Pará, Brazil. The cacao crop of the hybrid Sca 6 x Be 10 was established on soil units of structured Purple Earth (TR) and Yellow Latosol (LA), after the slash and burn of the natural vegetation. The experimental design was a NPK 2³ factorial with additional treatments of P. Results showed that P (P < 0,01) increased the productivity on TR and LA soils by 13,7% (110 kg ha-1) and 44,3% (214 kg ha-1), respectively, on average, during the period 1987 to 1993. Potassium (P < 0,01) increased yield on LA. Significant N x K and P x K interactions (P < 0,05) on yield were also obtained in this soil. The linear response of cacao to P application and the increase or decrease in the infection of witches' broom disease due to N, P or K applications suggest the need for additional research to define economically optimum rates of P and K and to elucidate the effect of the reaction of these nutrients on witches' broom infections.
cacao; fertilization; macronutrients; witches' broom