Abstract
For many decades, it was commonly believed that humic acids were formed in soils by the microbial conversion of plant lignins. However, an experiment to test whether these humic acids were formed prior to plant matter reaching the soil was never reported until the late 1980s (and then only as a side issue), even though humic acids were first isolated and reported in 1786. This was a serious omission, and led to a poor understanding of how the humic acid content of soils could be maintained or increased for optimum fertility. In this study, commercial sugar cane mulch and kelp extracts were extracted with alkali and analyzed for humic acid content. Humic acids in the extracts were positively identified by fluorescence spectrophotometry, and this demonstrated that humic acids are formed in senescent plant and algal matter before they reach the soil, where they are then strongly bound to the soil and are also resistant to microbial metabolism. Humic acids are removed from soils by wind and water erosion, and by water leaching, which means that they must be regularly replenished. This study shows that soils can be replenished or fortified with humic acids simply by recycling plant and algal matter, or by adding outside sources of decomposed plant or algal matter such as composts, mulch, peat, and lignite coals.
Highlights
Humic acids are brown-black polymeric acids of plant origin that are ubiquitous at the Earth’s surface
In 1989 [9] and 1991 [2], we reported for the first time that humic acids are present in senescent plant and algal matter, which meant that they enter the soil prior to any supposed conversion of plant lignins by microbes
The analysis of humic acids from various sources has shown that they are very similar to those occurring in soils, which is consistent with more modern research [12]
Summary
Humic acids are brown-black polymeric acids of plant origin that are ubiquitous at the Earth’s surface. FS is a powerful and convenient technique for the analysis of humic acids because the excitation spectra and emission spectra each occur over an extremely wide range with highly characteristic shapes (ideal for 3-D fluorescence); even more importantly, the excitation and emission spectra overlap [15]—a phenomenon that does not appear to occur for other organic compounds. This fundamental technique formed the basis of the analytical analysis in this study. This study compares the fluorescence data of humic acids from various sources to that of soil humic acids to determine if such sources are useful for the replenishment of soil humic acids
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