Soybean and Soil Potassium Pools Responses to Long-Term Potassium Fertilization and Deprivation under No-Till

Lack of potassium (K) fertilization can affect the nutrient reserve in soil and soybean [Glycine max (L.) Merrill] yield in humid tropical agroecosystems. The effects of K management in soil K pools and soybean nutrition is best evaluated by taking samples from long-term experiments. The aim of this study was to investigate changes in K pools and soybean nutrition as a result of K fertilizer deprivation in an Oxisol under no-till. Soil samples were collected in a field experiment started in 1983 in southern Brazil. Non-exchangeable K contents were extracted by boiling HNO3, whereas exchangeable K contents were extracted by Mehlich-1 and ion exchange resin. Grain yields and K contents in grains and index leaves (trifoliolate leaflets) were also obtained. Potassium fertilizer deprivation restricted soybean yield and reduced exchangeable K in a nonlinear fitting. The total K contents were influenced by K rates, without effects by K application. The 1 M HNO3 solution had high positive correlations with exchangeable K extractants, K leaf, and K grain contents. However, the exchangeable K extractants were better for interpreting the results due to higher correlations with yield and superior sensitivity to K application after deprivation. Potassium applications influenced K removals by soybeans and K leaf contents. The results indicate that long-term K management has effects on stable K pools (non-exchangeable and total K) and highlights that the buffer capacity of exchangeable K was strongly related to these stable pools. This underscores the importance of avoiding a negative K balance in soybean production systems to conserve K reserves and exchangeable forms.