humus-colloidSoil colloids are the most dynamic part of all soils and substrates. It establishes their chemical and physical attributes. Two of the most important colloids in soils are clay and humus. However,  substrates for potting mixes humus  is king. So Heads Up! This is very important. In typical soil it is the inorganic colloids, mostly clays, that are the most abundant components. However, in a well made substrates for potting mixes it is the tiny organic humus, less than 0.001 mm across that will determine the availability of nutrients for the plant. Organic humus colloids are many times more active than inorganic clays because of their higher CEC. It is fairly easy to add enough humus in potting soil where as field soils are too extensive. For a good potting soil, what needs to be done is pack the substrate with the optimal quantity of humus. Younger plants need less nutrition. Too many colloids charged with nutrients will burn it.
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Characteristics of a Colloid

Humus is amorphous, meaning their physical and chemical characteristics are not very well defined. Clay particles are normally crystalline, making clay’s physical and chemical characteristics anamorphic. Both inorganic and organic colloids are  components of most soils and substrates. (When I speak of “soil” I am referring to soils in fields. When I us the term “substrate”, I am referring to potting mixes.) The majority of the substrate solids like coco fiber and peat moss, are inert leaving the organic humus to define the substrate’s physical and chemical properties. So, when looking for a well made, effective substrate, look toward the quantity and quality of the humus ingredients. The passive biochar humus and the active humus in the compost are chief concerns. Since organic humus does the same job as clay but better, don’t look for clay in a substrate.
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The most essential quality of a colloids is its capacity to adsorb, keep for a time, then discharge ions on its surface. Most colloids have an overall negative charge due to their chemical and physical make up. That negative charge will be balanced out through a large number of cations attracted to its outer surface. Therefore, colloids could be seen as large anions covered with a group of rather loosely stored cations. Typical water molecules can also be adsorbed onto colloid surfaces. They will be found in the hydrated arrangement on the cations. The volume of water around a specific cation is very important, since the effective radius of the cation goes up with more H2O or down with less.

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The Size of Colloids

Colloid’s characteristic of being very small, is important to the cation exchange. For example, take a clay particle, known for its high cation exchange capacity. The individual clay particles are so tiny, you can not distinguish one from the other under a microscope. These small particles, when mixed in water, do not dissolve but are suspended indefinitely. They do not settle as sediment at the bottom of the container nor are in solution. A substance with this suspension characteristic is called a colloid. As mentioned, the size of the particle is very important. If a substance is small enough, when there is a positive or negative charge close to them they are effected… attracted or repulsed. This would not happen to a larger particle because if its greater mass. All small partials are effected by electrostatic charges, even bacteria and fungus.

Humus is a colloid, which, like all colloids, does not float to the top nor settle out to the bottom. Colloids because of their miniscule size have a lot of surface area in relation to their volume. The surface area of a humus colloid is truly staggering. It could have as much surface area as a thousand square meters per gram weight. Humus is a very special commodity. While clays can have the same high surface area, they might not have the same high CEC as a fully developed, decomposed humus particle. Humus always has a huge amount of available cation sites, even more than clay.