Dissolution of K-silicate rocks with Acidithiobacillus thiooxidans biogenic acid: characterization and agronomic performance of the end-products

Solubilization of plant nutrients from silicate rocks, using acids produced by microorganisms has emerged as both an environmentally friendly and an economically viable process for fertilizer production. In this work, biogenic acid was produced using Acidithiobacillus thiooxidans and elemental sulfur, and used it to solubilize K from verdete (Ve) and phonolite (Fo) rocks, comparing its solubilization efficiency to that of commercial sulfuric acid at 1 : 2 and 1 : 5 rock : acid ratios. After acid solubilization, the Ve and Fo rocks were chemically and morphologically characterized using several techniques. Agronomic performances of biogenic K-fertilizers derived from Ve and Fo were evaluated in a sequential corn–soybean–millet cropping system. Acid type and rock : acid ratio influenced the solubilization of K species from the rocks. Overall, biogenic acid led to a higher solubilization of K species in water and in citric acid than commercial sulfuric acid. The amount of solubilized K was higher at a 1 : 5 rock : acid ratio. Total dry matter yield and total K uptake by the crops increased as K rates increased and varied among K sources. Ve and Fo biogenic K-fertilizers provided higher amounts of available K to the crops than their in natura forms (rocks). The effect of acid solubilization was more notorious for Ve rock, significantly increasing the value of its acidified product as a fertilizer. Thus, our data demonstrate that biogenic acid produced by A. thiooxidans using elemental sulfur is a promising alternative to leach out K from silicate rocks.