All organic matter, as it decomposes, forms smaller and smaller particles. When it breaks down as far as it can and yet still can be identified as organic matter, it is called humus. The process of “humification” takes place naturally in soil and substrates, or in the production of special compost, like the bakashi. While plants die-off and are broken down into simpler compounds by microbial fungi and bacteria they’re eventually transformed into humus. All humus is a carbon based, organic substance. It is made of extended, tough chains of carbon compounds with a sizable surface area. The surface areas hold electrical charges, that draw in and store nutrient particles called cations. Humates are notable for their humic acids, namely Humic, Fulvic, and Ulmic. Humus is known as a colloidal substance, and boosts the soil or substrates cation exchange capacity, CEC.

Humus is the “life-force” of living organic potting soil.

Humus | Decomposing Matter

Image copyright Dennis Kunkel Microscopy, Inc.

The difference between humus and compost is that decomposing compost matter is an inhomogeneous substance, with rough plant parts observable. An entirely humified organic substance, on the other hand, will be consistent in character, with specific shape and structure. Humus is the ultimate stage in the decomposition of organic matter. Humified organic matter, observed through an electron microscope, will show very small yet plainly recognizable plant remnants which can only be mechanically broken down once the decomposition has been completed. The image on the right is an electron microscope rendition of humus/brown, decaying compost/green, and mineral particles/purple.  Scientist and researchers have a very specific definition of humus but not so in our horticultural community.


Active and Passive Humus

Rich compost, ready to apply is generally known as active humus. It is applied as a top dressing or as a substrate component with organic compounds that will release more nutrients when they are decomposed further. But in scientific circles, if the organic compound is not totally decomposed, it is not humus at all. Researchers define humus as a stable, passive molecule  which would not change structures in the soil unless it is a simple mechanical breakage into two or more pieces. But for horticulturalist, humus comes in active and passive forms. Active humus in compost is still abundant in plant remains unbroken into it’s simpler stable form. Passive humus composed of humic acids and humins, will be so very insoluble they can’t be divided further by microorganisms. Therefore passive humus is considerably immune to additional decomposition and provides very few readily accessible nutrition to soils and/or substrates themselves.


Benefits of Humus

There are many benefits to plants which humus provides. Humus can hold up to 90% of its mass in water, and so enhances the soil’s ability to store water. The chemical composition of humus allows it to buffer abnormally high or low levels of pH in the soil. Harmful elements including heavy metals and even too much nutrients, will be chelated, which means bound to the organic compounds of humus and kept from moving into the system. Also, during the humification, fungi and bacteria always exude sticky gums that promote a beneficial assembly of the potting soil by maintaining particles together. This allows even better oxygenation of the soil substrate. Particle distribution size is a very important physical determinant to soil drainage and oxygen supply. Larger particles allow for more air and less saturation of water. The biggest benefit of humus however is its colloidal characteristics. Humus is THE number one ingredient that will maintain nutrients (cations) in substrates so they will not wash away when watered. At the same time the collation property buffers the addition of too much nutrients so as not to overdose the plant. So look for a high percentage of humus when buying a substrate for live organic growing.  But humus is an extremely intricate compound, the full nature of which is still not fully understood by researchers.