The many advantages of humus

The volume of organisms in any body of soil is dependent on the soil environment and supply of food for them; it is not a static amount.

The many advantages of humus
This dry cover crop mulch will feed soil organisms for months and be converted to humus. Photo: Bill Kerr
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Humus, which is formed by the decomposition of plant material in the soil, provides excellent living conditions for these organisms, but for food they require digestible organic matter. This ends up as humus.

The amount of humus lost naturally and the amount newly formed from additional undecomposed organic matter can form an equilibrium, keeping the soil humus level static.

Temperature, soil aeration and soil moisture all play a role here.

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Water retention
A hot climate and sandy soil are less conducive to high humus levels, while cool, moist climates generally result in higher humus levels. Few farmers realise how effective humus is at retaining soil moisture, rather like a huge sponge.

A humus level of 1% means that 15ℓ of water can be stored per square metre of soil (down to about 30cm deep); a 2% humus level can store 30ℓ water/m2, and so on.

My soil has a 5% humus level, enabling it to retain 75ℓ water/ m2. What this means struck me forcibly one day years ago when I was transplanting some vegetables.

My soil was bone dry, but without really thinking about it, I used the rule of thumb method of 1mm of water wetting the soil to a depth of 1cm, thinking that would be sufficient. After a while, though, I could see that the plants were drought-stressed.

I then discovered that the water had not penetrated deep enough. Using the water-holding capacity of the soil as a guide, I calculated that I actually needed 2,5 times more water to wet the ground 30cm deep than would be the case for most soils because of the quantity of humus in my soil.

Humus provides another substantial benefit: it enables the soil to retain minerals better.
In a soil lacking organic content, the clay minerals provide the cation exchange capacity to the soil. Clay has a large surface area, which allows for the attachment of positively charged elements on which plants feed.

Apart from this exchange capacity, various clay minerals also have a positive effect on soil structure. Montmorillonite, for example, expands and contracts as soil moisture varies and can be self-mulching. Kaolin is a dead clay that is used for making pottery.

Humus, however, has a far greater cation exchange capacity than clay and vastly improves the capacity of the soil to hold plant food.

In addition, it has an anion exchange capacity that clay does not possess. This enables the humus to retain negatively charged elements such as nitrate. The anion exchange capacity of humus is not as great as its cation exchange capacity, but certainly has an influence when the humus content is high.

Less crusting
A humus-rich soil has yet another advantage: it enables water to penetrate rapidly and hence reduce crusting. Crusts seal off the surface after a heavy downpour and cause run-off.

The crust can also interfere with seed emergence and provide an environment for damping off organisms. In addition, the crust can get very hot in the sun and transfer this heat to fine seedlings such as beet and carrots, burning their stems.
Bill Kerr is a vegetable specialist and breeder.

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