Humus

Humus is the resulting substance from the optimal breakdown of plant material, usually referred to as composting. It has several properties of value to the grower of crops:

• It acts as a sponge for water, thus increasing the water holding capacity of the soil
• It promotes the formation of soil aggregates called crumbs in heavy soil that allows more rapid water penetration and the infiltration of air required for optimal plant root health
• It makes heavy soil more crumbly, so easier to till
• It is a colloid

The latter requires a little explanation. Colloids are materials that carry negative electrical charges on their surfaces. Many essential plant nutrients occur as positively charged ions: calcium, magnesium, molybdenum etc. The colloids in healthy soil hold on to these nutrients due to the opposit electrical charges, preventing them from leaching by water movement, but allowing the plant roots to access them by emitting hydrogen ions.