Biochar and humic substances-based fertilizers: synthesis, characterization and agronomic use

Abstract

Organomineral fertilizers (OMFs) over mineral fertilizers may have higher agronomic value for growing plants in Oxisols. Several routes and matrices can be used to synthesize OMFs, including humic acid (HA), composts, and biochar (pyrolysis). Chemical and spectroscopic analyses can access nutrient forms and pools in OMFs, and spectroscopic techniques such as infrared analysis can predict the values quantified in chemical analyses. Thus, the present study aims to use different routes in the synthesis of OMFs, based on Zn-AH, NPK-compost produced by composting, and NPK-biochar with higher agronomic value over mineral fertilizers and use infrared spectroscopy to predict properties and nutrient pools in NPK-OMFs. In the first study, the mixture of Zn and HA promoted the complexation of Zn by the HA structure; when these molecules were applied via soil, compared to Zn sulfate, there was an increase of Zn in soil solution, and growth and nutrition of maize plants followed by the cultivation of brachiaria in Oxisols. In the second study, using different synthesis routes of OMFs, Zn-OMFs based on HA were synthesized with the aim to gradually release Zn that modified the dynamics of Zn released

in the whole soil and its solution, increasing the use efficiency of Zn in the sequence of maize- brachiaria cultivation. In the third study, different OMFs were produced by composting

different mixtures between sources of P, coffee husk, and chicken manure; among the OMFs produced, those that included monoammonium phosphate (MAP) in the composted mixtures had the highest agronomic value. Through infrared analysis, it was possible to predict the properties and nutrient pools in the OMFs. In a fourth study, the OMFs synthesized based on MAP in step 3 were tested in two contrasting Oxisols, and it was observed that the synthesized OMFs had a gradual release of P, which contributes to the reduction of the initial contents of P in soil solution, increasing the maize biomass and nutrition over the exclusive use of MAP. In the fifth study, OMFs were synthesized based on the acidulation of Araxá phosphate rock (APR) and in its mixture with coffee post-harvest residue before pyrolysis. Production of OMFs

based on biochar promoted a gradual release of P, and the production and nutrition in a maize- brachiaria crop sequence depended on the proportion of P soluble in neutral ammonium citrate

plus water in the OMFs. The sixth study was developed based on the outputs of the fifth study evaluating new routes involving the acidulation of APR, a subsequent mixture of biochar produced from coffee post-harvest residues. The new synthesized OMFs had high levels of NPK, and the P from the OMFs was released gradually, either in studies of kinetics release in water or citric acid or by evaluating the P contents in the soil solution during maize cultivation in a medium textured Oxisol. This gradual release of P in soil solution correlated positively

with P accumulated in the maize shoot, which increased the biomass over soluble mineral NPK- fertilizers.