Effects of different chemical modifications on peat humic acid and their bearing on some agrobiological characteristics of soils

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/carbon (H/C) atomic ratio showed the most significant correlation with plant yield (inverse correlations with the optical density and relative fluorescence intensity were found), whereas the amounts of O or N were in general unrelated to the agrobiological properties of the amended soils. It is postulated that the differences observed in the edaphic role of the humic preparations were not due to a great straightforward influence of the chemical transformations applied, but mainly reflected their indirect effects on the polymer arrangement of the humic acid through changes in the surface properties of the whole macromolecular system.