Biochemical Nature of Humic Substance Sub-Fractions and Correlation with Plant Growth Responses.

Humic substances, ubiquitous in soils and natural waters, are very complex mixtures of organic compounds, where composition fluctuates depending on source.  Each component varies in its individual structure, function, polarity, and reactivity, but as a whole, they have long been regarded as beneficial to soil fertility and plant growth.  In a recent study, an aquatic humic substance was fractionated according to pH and polarity, to identify components responsible for plant growth stimulation.  Bioassay testing with corn (Zea mays) showed that larger root systems occurred when treated with lower carbon rates of hydrophobic material.  The opposite was true for hydrophilic material, where root systems were larger at higher carbon rates.  This suggests that plant growth stimulation is due to both the amount and the type of carbon present.

In another study, rice (Oryza sativa) was grown in a hydroponic system containing a humic substance biostimulant.  Over the course of 75 d, the hydroponic solution was characterized in order to understand which components were preferentially being taken up by the plants.  The highly oxygenated, aromatic compounds were primarily utilized by the plant, leaving behind the less oxygenated and more aliphatic constituents.

During the course of these studies, as well as many others, it has become readily apparent that multivariate statistical approaches are necessary in order to better understand how the composition of these products correlates to the observed enhanced plant growth (both elongation and weight).  Principal component analysis assists in grouping samples based on their characterization and determining the types of compounds enriched in each, while Spearman’s rank analysis correlates these specific characterization qualities with the change in plant biomass.  We found that different types of compounds are associated with increased root/shoot length and mass depending on specifically which treatment method (i.e., seed, soil, or foliar) was applied.  This indicates that plants utilize and select these compounds differently depending on how, and at what stage, they come into contact with them.  In general, if we can begin to elucidate the ways in which plants respond best to treatments, we can potentially design a better product that will fulfill the needs of that specific plant, also taking into account environmental conditions.