ArticleTHE PRINCIPLES OF HUMIC SUBSTANCESMacCarthy, Patrick Author Information Department of Chemistry & Geochemistry, Colorado School of Mines, Golden, CO 80401. E-mail: [email protected] Received July 12, 2001; accepted Aug. 21, 2001. Soil Science 166(11):p 738-751, November 2001. Buy Abstract Two principles are presented that define the molecular nature and ecological role of humic substances (HS). The First Principle (i) accounts for and organizes an extensive body of apparently disparate data relating to the inability to purify and establish a molecular structure for HS; (ii) offers a conceptual framework for dealing with HS and for evaluating the applicability and limitations of various experimental methods; and (iii) identifies molecular 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 that offer greater insight into the molecular nature of these materials arise 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 interpretation of experimental data caused by the heterogeneous nature of HS are discussed. The Second Principle makes a connection between the molecularly heterogeneous 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. © 2001 Lippincott Williams & Wilkins, Inc.