Vindication of humic substances as a key component of organic matter in soil and water

Soil organic matter is a complex mixture partly of recognizable, largely unaltered plant components plus a group of highly modified materials that bear no morphological resemblance to the original components. Their formation results from transformation and decomposition processes collectively known as “humification.” Humic substances, a family of closely related compounds, are considered to be a major product of this process. In recent years, several articles have questioned the role and even existence of soil humic substances as a distinct entity in soil organic matter. They regard soil organic matter as a continuum of degradation reactions by unspecified processes from the original biomass inputs to carboxylic acids, and eventually to carbon dioxide without the formation of new classes of compounds along the way. We disagree fundamentally with these views and highlight the errors, misconceptions, misinterpretations, inbuilt bias, lack of scientific rigor, and failure to consider works and evidence that run counter to their preconceived views. In response, we present an evidence-based rebuttal, detailing the decomposition processes of plant components and their transformation to a range of products, some of which, including humic substances, have a degree of resistance to microbial degradation. We provide an appraisal of information about the genesis and compositions of humic substances, and the importance of compositional knowledge to devise treatments to modify their rate of decomposition. We note that all reputable turnover models that accurately predict soil organic matter behavior include one or more persistent pools of carbon which is not compatible with the concepts in the soil continuum model.