Clay and humus are the two smallest particles in soil, and together they make up the colloidal fraction of soil. A colloid is a particle able to stay suspended in a solution for very long periods of time, without settling to the bottom —you can see this in the photo below as the murky liquid layer above the soil fractions. (This photo is from Soil Experiments You Can Do At Home). This photo was taken on Sunday 5th May, 2019, a week after setting the experiment up.

The photo below was taken today, Tuesday 11th June, 2019, five weeks and two days later (ie just over six weeks after setting the experiment up). Only now has that murky layer started to settle out! The liquid layer is still murky, but much clearer. In fact there were two distinct layers with the bottom half of the liquid layer slightly more opaque than the upper half, but these were disturbed when setting up the photo unfortunately.

This layer became less clear as the jar warmed in the sun without further disturbance, so it would appear that temperature has an effect on the solubility of these colloids. And here are the two photos side-by-side. You can see the new and darker organic matter layer above the clay layer in the second photo:

This organic matter layer is mostly humus. Humus is the organic component of the colloidal fraction and clay particles make up the mineral component. Clay particles have a diameter of less than 0.002 mm, with the smallest of these being about 0.000 000 5 mm in diameter. Humus particles range from 0.000 004 to 0.000 04 mm in diameter.

As we’ve seen here and here, soils with good structure and which are well-suited for plant growth are characterised by many crumb-like granular peds, which are composed of clay minerals and humus. Soils with these peds have good nutrient retention, nutrient availability, drainage, aeration, friability and tilth. The small size of granular peds contributes to a soil’s drainage, aeration, friability and tilth qualities, but it’s their clay and humus components specifically that are behind the nutrient retention and availability.

What is it about these two microscopic components that is so important to plant growth? Two things: their very large collective surface areas and their negatively charged surfaces.

Consider a cube 10 mm a side. Its total surface area is the area of each square side, times six. Or 100 mm² × 6 = 600 mm².

Now break that cube into cubes each 1 mm a side — there are now 1,000 small cubes with the same collective volume as the original cube, but with a much larger collective surface area.

Break each of those small cubes down further, and further, and you can visualise how the collective surface area increases the smaller and more numerous the cubes become, though their collective volume stays the same.

Clay minerals are actually plate-like in shape, and disks have a larger surface area than a cube of the same volume. A gram of clay has a surface area anywhere from 10 to 800 m² and a gram of humus has a surface area of 800 to 900 m². A gram of coarse sand, each particle of which has a diameter from 0.2 to 2 mm, has a surface area of only 0.01 m² in comparison.

Clay minerals form from rocks fragmenting into smaller and smaller pieces over time through weathering. There are four main groups: the kaolinite group, the montmorillonite/smectite group, the illite group, and the chlorite group. The common elements to all are silicon (Si), aluminium (Al), oxygen (O) and hydrogen (H), and some groups contain additional elements such as iron (Fe), magnesium (Mg), manganese (Mn), sodium (Na), potassium (K), nickel (Ni) and zinc (Zn).

A clay particle is made up of sheets of these atoms, stacked on each other, and it’s the particular atoms within the sheets and how the sheets are arranged that gives each group its particular properties. This is an entire field of study in itself, and far too detailed and involved for this blog, but it is how clay minerals are structured that is important in terms of nutrient retention in soil and availability to plants. One relevant thing to know about clay minerals is that this structure usually has an overall negative charge — the importance of this will become apparent later.

Humus is the highly decomposed, black, jelly-like remains from microbial breakdown of organic matter. It is extremely stable and resistant to further microbial attack — it may further decompose over several years, but may also persist in some soils for hundreds if not thousands of years! It is primarily composed of carbon (C ), hydrogen (H) and oxygen (O), with some sulfur (S) and nitrogen (N). As with clay, humus too usually has an overall negative charge.

Neither a large surface area nor a negatively-charged surface by themselves is enough to influence retention and availability of nutrients to plants, but the two in combination is a different matter. Negatively-charged surfaces are active, in that they attract positively-charged particles. And this activity, when spread across the large surface areas that clay and humus particles provide, means a lot more activity can occur than would otherwise for larger particles of the same volume.

But what is this activity and what does it do? The activity is called cation exchange, and what it is and how it works, we will get to. But before we do that, next week we’ll take a brief interlude with a crash course in chemistry to explain what cations are in the first place!