Humic substances are the most stable fraction of organic matter in soils and may persist for thousands of years (Stevenson, 1994).

Humic substances are found everywhere in nature, arising from the chemical and biological degradation of plant and animals. They are found in soils, sediments, coals, fresh and salt waters, and other natural materials. Approximately 40-60% of dissolved organic carbon (DOC) in freshwaters & 60-70% of the total soil carbon (C) content occurs in humic materials, representing the largest reservoir of carbon on earth. Their dark coloration is related to quinone structures formed under oxidative soil conditions trapped in the humic hydrophobic condition.

Humic substances is a generic term referencing all natural organic matter found in nature outside the living cell, including humus and humate materials.

Humus is a brown to black organic substance composed of partially decomposed plant or animal material (i.e. natural organic carbon compounds) that provides soil nutrients and assist in water retention. Generally this material is considered recently degraded organic matter obtained from composted plants or manure material that is applied to soil to enhance micro-flora growth. Low levels of humic acid does exist in humus material.

Humate is a material containing high levels of humic acid. Humates was originally humus from decomposed organic materials (i.e. contains carbon units or compounds) that was entrapped by various rock formations thousands of years ago. Through time and pressure high levels of humic acid were formed. Humate geologic deposits are found throughout the world and within different mineral deposits. Humates in calcium carbonate (limestone) are considered more desirable for agricultural use than leornardites (e.g. coal) because of heavy metal contaminates.