Eficiência da adubação fosfatada e potássica na cultura da soja.

Abstract

Soybean (Glycine max) crops had spread in 70's due to increasing international demands and credit availability by the Brazilian government. Major soybean crops had establisted in Southern and Southeastern Brazil which, besides of having appropriate soil and climate conditions for soybean cultivation were closer to export shipping ports. In the past ten years, soybean crops have been expanding to low latitude regions of Brazil, reaching Amazon region. Those new extent a neas have led to the need of generating appropriate technologies and management for soybean cultivation in that new agricultural frontier. Howover application of fertilizer should be made based on the response of soybean varieties to fertilizers, in local conditions in order to maximize economical return per area unit. In the state of Para soybean cropping area have been increasing, especially in the Southern and Weastern Para. Researches on fertilization and mineral nutrition are incipient that in by fertilizer application has been recommended based on the results obtrined in other cropping areas in Brazil. The present research was carried out in order to establish response curves of soybean to phosphorus and potassium. The experiment were conducted at Embrapa Amazônia Oriental experimental fields. in Paragominas and Belterra counties using soybean. Sambaiba cultivar growin in representative soil type, for each area. Soil analysis were performed prior to the experiment istallation, soppling layers from 0 to 20 cm depth, for physical and chemical characterization. The experimental design was a randomized blocks with four replications, with the treatments arranged in a 4 x 4 factorial design, corresponding to four phosphorus levels (0, 80, 160 and 240 kg ha¹ P205) using simple superphosphate as P source and four potassium levels (0. 60. 120 and 180 kg ha K20) as potassium chloride were tested. Phosphorus fertilizer was applied once in planting groove while potassium was split in two dosis, one third applied at time of planting and two third in coverage, 30 days after germination. The fertilizer effects were recordes as plant height first legume insert, bending, days to flower onset. 100 seed weight. seed moisture, at harvest, yield. pests. diseases and chemical analysis at soil and plants. Data obtained were analysis by ANOVA and regression analyses. The results obtained have shown that the annual application of 80 kg.ha P₂Os and 60 kg.ha K₂O was enough to keep a good grouth and yield at about 90% of maximum productivity and to maintain the critical level phosphorus and potassium in the soil. Increases in potassium content resulted in drops of calcium and magnesium in content dry matter. Phosphorus and potassium contents in leaves were larger in Belterra than Paragominas. Nitrogen and potassium were the nutrients taken up in higher proportion, being followed, in decreasing order, by Ca, Mg, P and S in soybean dry matter. In soil Oxisol with low content of P and K after applied of 80 kg ha of P₂Os and 60 kg ha¹ of K₂O soybean cv. Sambaiba 3.600 kg ha", a good performance wher compared with Brazilian average.