Influence of compost on soil organic matter quality under tropical conditions

A field study was conducted in Puerto Rico to evaluate the impact of compost on soil quality and crop production in an ultisol. In particular, the effect of compost on the quality of soil organic mater (SOM) was investigated. Compost quantities of 0, 37, 74 and 148 Mg ha−1 were applied annually for 3 years to four different plots and a high single dose of compost was applied (444 Mg ha−1) to a separate plot in the first year of the experiment. Surface soil samples were collected from the five treatments 1, 2, and 3 years after compost application. Soil humic substances were extracted, fractionated and purified. The humic and fulvic acids (HA and FA) were studied to determine SOM quality. The following parameters were determined: (i) C-HA/C-FA ratio, (ii) E4/E6 ratio, (iii) Kumada classification of the humic acids, and (iv) fluorescence spectra. Higher compost application rates resulted in greater C-HA/C-FA and E4/E6 ratios (more humification and aromaticity) and a shift of fluorescence spectra to longer wavelength (with more functional groups). The results showed that composting produced statistically significant (α=0.05) changes in the C-HA/C-FA and E4/E6 ratios. The fluorescence spectra also showed considerable changes. Additions of compost to soil increased the quality of SOM by increasing the quantity of HA with more functional groups and aromacity. Our research demonstrated that addition of compost not only increased the quantity, but also the quality of SOM, thus improving soil quality and productivity.