An environmentally-friendly alternative to phosphate fertilization is the supplementation of P from rock phosphates, however, these sources do not have P readily available to plants. In this sense, the reduction of particle size from grinding techniques have been studying to increase the solubility of the phosphate minerals. However, a difficulty presented during this process is the tendency of agglomeration of the particles, preventing them from maintaining nanometric dimension. Elemental sulfur (S°) comes up as a dispersant potential that through the acidity generated by the S° oxidation to sulfate (SO42-) can aid in the solubilization of mineral phosphates. The fungus Aspergillus niger has the ability to oxidize sulfur, besides to have a high production capacity of organic acids such as citric acid, oxalic acid, and gluconic acid that increased the solubilization of phosphorus. Thus, A. niger presents two mechanisms that may favour the solubilization of phosphates: first by natural acidification provide by oxidation of S° and the second by organic acids production. In this work, a composite was designed based on a matrix of S° prepared by low-temperature processing, reinforced by rock phosphate (P) particles acting as P fertilizer, and with encapsulation of Aspergillus niger as an oxidizing microorganism.
The effects of dispersion of the phosphate particles on the elemental sulfur matrix and starch were evaluated, as well as the natural acidification provided by the oxidation of S° and the production of organic acids in the solubilization of P from the natural phosphate. In addition, after the release of P and SO42-, from the composite-fertilizer incubated in the soil, the dynamics and interaction of P with the colloidal fraction of the soil were analyzed using P K-edge X-ray absorption near-edge structure (XANES). The inclusion of A. niger provided a means for improved S° oxidation and concomitant faster P release. The proposition of a granule fertilizer with a simultaneous dispersion of particles of phosphate rock, elemental sulfur, and A. niger spores can allow the reduction of pre-processing (e.g., for soluble fertilizer production) and reduces the indirect costs related to the conventional acid solubilization process and waste treatment. Soil incubation studies, probed by XANES, indicated that the composite structure played a role in nutrient fixation and immobilization, showing that nutrient dynamics was governed by the local pH. This fully integrated material (a smart fertilizer) is an innovative strategy for eco-friendly agronomic practices, providing high nutrient delivery with minimal source pre-processing.