Calibração visando à adubação potássica para as culturas da soja, trigo e milho sob sistema plantio direto no Paraguai Recommendation of potassium fertilization to the main grain crops under no-tillage in Paraguai

O alcance de elevados rendimentos e da maxima eficiencia economica e obtido com o auxilio das recomendacoes de fertilizacao, as quais influenciam a correta tomada de decisoes. O objetivo deste trabalho foi gerar recomendacoes de adubacao potassica para as culturas de trigo, milho e soja sob sistema plantio direto (SPD) a partir da conducao de experimentos de calibracao em cinco diferentes locais na Regiao Oriental do Paraguai. Os experimentos foram conduzidos de 2003 a 2006, em solos com diferentes texturas e niveis iniciais de fertilidade, em regioes representativas de producao de graos e em locais com historico de adocao do SPD. Foi conduzido um experimento nos departamentos de Misiones, Itapua e Amambay e dois experimentos no departamento do Alto Parana. O delineamento experimental foi de blocos ao acaso em parcelas subdivididas com tres repeticoes. Os tratamentos consistiram de cinco doses de K2O (0, 25, 50, 75 e 100 kg ha-1) e as aplicacoes foram efetuadas em superficie antes da semeadura das culturas. Foram coletadas dez amostras de solo por parcela na profundidade de 0 a 10 cm antes da semeadura e apos a colheita de cada cultura. O teor critico de potassio (K) no solo e de 75 mg dm-3 para as culturas de trigo, milho e soja. As classes de fertilidade de K no solo sao “muito baixo”, “baixo”, “medio”, “alto” e “muito alto”. Solos da classe de fertilidade “alto” a recomendacao de K e de manutencao, 7,5 kg ha-1 por tonelada de graos de trigo e milho exportados e 25 kg ha-1 por tonelada de soja exportada. Abstract The achievement of high yields and maximum economic efficiency is obtained with the aid of fertilization recommendations, which influence correct decisions. This research aimed to generate recommendations of potassium fertilization for crops of wheat, corn and soybean under direct plantation system through conducting calibration experiments in five different locations in the Eastern Region of Paraguay. The experiments were carried between 2003 and 2006, in soils with different texture and beginning levels of fertility, in representative areas of grains crops production and in areas with record of adoption of the direct plantation system. An experiment was conducted in the departments of Misiones, Itapua and Amambay and two experiments in the department of Alto Parana. The experimental design used was randomized blocks with subdivided plots and three replicates. And to the potassium the treatments were five doses of K2O (0, 25, 50, 75 and 100 kg ha-1) and the applications were done in the surface before the crops sowing. It was sampled ten soil samples by plot in the depth of 0 to 10 cm before the sowing and after the harvest of the crop. The critical level of potassium (K) is 75 mg dm-3 crops of wheat, corn and soybeans. Fertility classes of K in the soil are “very low”, “low”, “medium”, “high” and “very high”. Soil fertility class “high” K is the recommendation of maintenance, 7.5 kg ha-1 per ton of wheat and corn exported and 25 kg ha-1 per ton of soybeans exported.