The principles of humic substances

Two principles are presented that define the molecular nature and ecologica l role of humic substances (HS). The First Principle (i) accounts for and o rganizes an extensive body of apparently disparate data relating to the ina bility to purify and establish a molecular structure for HS; (ii) offers a conceptual framework for dealing with HS and for evaluating the applicabili ty and limitations of various experimental methods; and (iii) identifies mo lecular heterogeneity, in combination with pronounced chemical reactivity, as constituting the essence of HS. Five corollaries to the First Principle spell out its consequences in more specific detail. New definitions of HS t hat offer greater insight into the molecular nature of these materials aris e from the First Principle. The inapplicability of the molecular structure concept to HS is explained. The concept of hypothetical pseudostructures is introduced to help visualize the chemical reactions and interactions of HS without the unjustified assignment of specific structures to the material as a whole. Constraints in the design of experiments and in the interpretat ion of experimental data caused by the heterogeneous nature of HS are discu ssed. The Second Principle makes a connection between the molecularly heter ogeneous and chemically reactive nature of HS and the ecological need for a reactive and persistent medium for plant growth. Concepts presented herein have broad implications in many fields, including chemistry, geochemistry, environmental and soil sciences, and ecology.