Abstract Twelve preparations from peat humic acid were obtained through laboratory treatments modifying its structural parameters, oxygen (O)‐containing functional groups and nitrogen (N) content. The treatments included acetylation, amidation, ammonia fixation, methylation, nitration, oximation, sulphonation, and selective degradations. The transformed humic products were characterized by routine laboratory analyses. Then, plant yield and different properties of two soils treated with the corresponding potassium (K)‐humates were determined in greenhouse experiments. The main effects and interactions between parameters affecting the response of soil and plant to humic acid application were analyzed through factorial designs. Whereas the application rate (2 to 4 Mg/ha) had no great bearing on plant yield (accounting for between 84 to 176% of the untreated soils), the opposite occurred with aggregate stability and cation exchange capacity. Among the parameters examined in the humic acids, the hydrogen/carbo...
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