Abstract: The use of ground natural rocks (in natura) containing K was evaluated for the cultivation of rice (NEPTUNE et al., 1980), corn (Siqueira et al., 1985) and more recently in corn, soybeans and mil- let (Resende et al., 2006), whose agronomic results were not satisfactory. However, when the potassic rock is fused at high temperatures and calcare- ous is added, you get the product known as fused magnesium potassium phosphate (TK) with K availability increased. In a study to evaluate the fused magnesi- um potassium phosphate in corn, there was an in- crease in the production of dry matter and higher K accumulation accumulation in plants (FAQUIN et al., 1987) mass. High agronomic efficiency of this source was also observed for Marandu-grass with equivalent or su- perior results when compared to KCI. No study, however, was con- ducted to assess the efficacy of fused magnesium potassium phosphate in providing K for soybean, which is recognized for its strategic importance in the Brazilian agribusiness. In light of the above, this study aimed to assess the efficacy of the fused magnesium potas- sium phosphate as a source of potassium for soy- bean
