Potassium rates on the cationic balance of an Oxisol and soybean nutritional status after 8 years of K deprivation

Highly weathered soils from humid tropics naturally have low contents of available potassium (K) to plants. Under these conditions, the K deprivation can change the equilibrium among cations in the soil and the nutritional status of some crops as soybean (Glycine max(L.) Merrill). A field experiment related to K fertilisation, spring soybean and diverse species of fall/winter crops, such as wheat (Triticum aestivumL.), corn (Zea maysL.), sunflower (Helianthus annuusL.) or oat (Avena strigosaL.), has been carried out in Southern Brazil since 1983. The K deprivation for 8 years reduced soybean grain yield and the K contents in soil and plant tissues. K extractants, such as Mehlich-1 and ion exchange resins, had diverse sensitivities with the variation in the K exchangeable contents induced by K rates. The increased soil K content after K fertilisation reduced calcium (Ca) and magnesium (Mg) contents in index leaves and altered its contents in the soil extracted by 1 mol L-1KCl and ion exchange resins. Among the micronutrients, only B contents changed due to increased K rates, and had significant correlations with K and Ca contents in index leaves. The calculated Diagnosis and Recommendation Integrated System (DRIS) indices were compatible with soybean yield and K contents in soybean index leaves.