| Summary: | Abstract The objective of this work was to evaluate the application of silicate ultramafic alkaline rock, with different granulometry, as an alternative potassium source on the increase of soil fertility and on the nutritional status and biomass yield of the alfalfa and centrosema crops. The used soil was a Typic Ultisol with 27.4 mg dm-3 K and 733 g kg-1 clay. The experimental design was completely randomized, in a 4 × 2 + 1 factorial arrangement. The treatments consisted of two granulometries (<0.3 and 0.3-2.0 mm) of alkaline ultramafic rock (4.0% K2O), four K rates (0, 100, 150, and 300 mg kg-1), and an additional treatment with the application of 150 mg kg-1 K in the form of KCl (58% K2O). Both legumes were grown in a greenhouse and subjected to five successive harvests. In comparison with KCl, the application of silicate rock has the potential to increase K uptake and root and shoot dry weight yield in alfalfa and centrosema. Granulometry does not affect rock use efficiency, and alfalfa presents a better K efficiency recovery in the soil after successive harvests compared with centrosema; the inverse is observed for K use and K uptake efficiencies.
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