Rates and Residual Effects of Potassium Fertilization in a Brazilian Soil

Potassium (K) fertilizers are commonly an essential component for proper plant nutrition in acid soils of the tropics, including Brazil. In many areas farmers are tempted to cut back on fertilizer expenses, which could compromise good yields, profit and food safety. Studying the impacts of K fertilizer cut back on Brazilian soils is important to demonstrate the negative effects in the medium to long run and this was the main objective of this study. A field trial was carried out for six years in a clay Oxisol in Itiquira, Mato Grosso, growing soybeans every summer and maize second crop or Brachiaria grass during the fall/winter. The treatments involved were: (1) four application rates of K₂O (22.5, 45, 90, and 135 kg K₂O/ha), plus a control with no K₂O added, in interaction with suppression or not of K after the third year, (2) three levels of base saturation (40%, 55%, and 70%), (3) three rates of phosphogypsum application (0, 2, and 4 t/ha), (4) two times of K₂O application, and (5) two localities of application.

Yield responses to K₂O additions were frequent through the years, but relatively small. The yearly average negative effect on grain yield of both crops from the suppression of K₂O was of 385 kg/ha for soybeans and 617 kg/ha for maize second crop, compared to the recommended rate of 90 kg K₂O/ha and the control. Cutting back the K rate after the third year, did not have much effect on soybean and maize second crop. In certain years. Grain yields of both crops were positively affected by phosphogypsum application, while maize second crop yield was also positively affected by liming.

The experiment has shown that this soil has a high K buffering capacity. It is very possible that recycling of K from deeper soil layers is helping to maintain yields, even when applying relatively low rates of K₂O. It will be interesting to note how much longer the soil can provide such nutrients without dramatically impacting the yields.