The Importance of Soil Humus

The Importance of Soil Humus

Peter Burton
13th May 2010


Humus  The more or less stable fraction of the soil organic matter remaining after the major portions of added plant and animal residues have decomposed.  Usually it is dark in colour.
From: The Nature and Properties of Soils 13^th edition Brady Weil

A simple field test comparing the colour of the topsoil under a long-term existing fence line with the same depth of soil from a representative sample of a grazed area indicates whether a soil is gaining or losing humus/carbon, or relatively neutral. 

This raises the question of the importance of humus in the soil.  Humus markedly increases water holding capacity as it is capable of adsorbing water equivalent to about 80 – 90% of its weight, over four times that of the holding capacity of clay.

Soils containing more humus or organic matter have better physical structures as they contain more granules.  A higher percentage of granular aggregates allows a more rapid interchange of soil gases and allows plant roots to penetrate further and more rapidly.

Cation exchange is also positively affected and may account for up to 90% of the adsorbing ability of a mineral soil.  The major cations are those of calcium, magnesium, potassium, and sodium.

The supply and availability of all nutrients including nitrogen, phosphorus, and sulphur also increases.

In short a soil that is steadily gaining organic matter, carbon, and humus will grow more total dry matter than a comparable soil that is not.  There is also likely to be a marked improvement in the ability of well-structured humus rich soil to provide vitamins and growth-promoting substances.

Both the feed quantity as well as quality increases improving animal production and as the purpose of pastoral farming is food production, human health is also enhanced.

This explains the difference in the performance, in all respects, of properties within a short physical distance particularly during periods of adverse climatic conditions.  Comparing farm performance this season, a difficult one by any standard, provides useful information.

Healthy carbon sequestering soils have a far lower incidence of pasture pulling, with markedly reduced grub and beetle infestation. 

Pasture pulling is primarily due to the inability of plants to anchor themselves.  This is nearly always a result of tight compacted soils that do not easily allow root penetration.

As soil health improves, all aspects improve resulting ultimately in little if any pest or disease pressure and which may also help explain facial eczema spore counts varying between neighbouring properties.

There are properties that have survived this season with little requirement for pasture renovation, a nil requirement for pest and disease control and pasture production little changed from the last.  With all animals close to ideal condition the affects of the adverse climatic conditions of the last few months are now confined to this season.

With warm winter temperatures predicted, next season can now be looked forward to with growing confidence.

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