A sense of humus

Humus is the end product of the breakdown of organic matter. Organic matter is anything that is now dead but was living once (the remains of plants and animals). All humus is organic matter but not all organic matter is humus.

Humus is dark in colour, smells like sweet earth, is gummy, colloidal and has very high water holding capacity. Humus is made by soil organisms, especially fungi and bacteria which consume the organic matter, and by plant roots which secrete waxes and by slow breakdown of plant parts containing lignin, which does not decompose readily. These products interact with each other to form the complex material called humus.

All humus is not identical. Depending on the origin and composition of the raw materials from which it develops, the particular bacteria and fungi present and the balance between activity of each type of organism, and on soil conditions such as moisture, temperature and aeration, the characteristics and quality of humus will vary. Although the exact processes in humus formation are not well understood, it is likely that there is a very long maturing process, that recently made humus is less stable and can be easily mineralised or broken down into less complex, useable nutrients. Older humus matures into a very stable, long lived product, such that the average age of humus particles in the soil is 1,000 years. In the end all humus is eventually mineralised back to simple constituents, so the aim is to cycle humus.

Humus provides the following functions:

• Water retention: Can hold up to 100 times its own weight in water.

• Nutrient holding: Holds nutrients in soil and protects them from leaching or erosion, but in a form that can be accessed by plants

• Improves soil structure: Humus has an open, lattice structure which holds air and water. It makes soils lighter to work and helps to gum individual soil particles together into aggregates or crumbs, which allows easier entry of water, air and plant roots. Improves sandy and clay soils equally well.

• Helps soil to warm: because it is dark in colour and holds air, which has insulating capacity, humus can modify soil temperatures at either extreme

• Balanced pH: Humus provides a buffering function which moderate both very acid and alkaline soils back to a balanced pH.

• Immobilising contaminants: Humus can hold in the soil contaminants such as lead, cadmium and pesticide residues, preventing them from entering plants and hence the food chain.

• Feeding the soil food web: Humus is made by soil life and is consumed and broken down by soil life too. It is continually being made and destroyed as part of the healthy functioning soil ecosystem.

What is a colloid

Colloids are small particles which can remain suspended is water. Small clay particles are also colloids. Colloids form a gel by binding with water. Gelatine is an example of a colloid. You know that there is water in a jelly, but you can’t squeeze it out. But plant roots can get the water.

Milk is a colloid, so is mayonnaise.

The outer layer of colloids contain particles with a negative charge. This means they can form bonds with positively charged particles, which happen to include important nutrients including calcium, magnesium and potassium.

Humus can therefore hold, and slowly release these nutrients, forming a store from which a small, steady supply of nutrients can become available.

Green manures

Green manures are crops grown not for harvesting and consumption by humans, but for the purpose of feeding the soil. Green manures may be slashed to the ground and left or turned into the soil. They provide humus to the soil; prevent excess nitrates leaching from soil; are an alternative to fallowing in order to maintain soil cover and prevent erosion; assist in the control of weeds by smothering; and help in rotations for the control of plant diseases.

Many species are suitable for using as a green manure. The right crop for each situation will depend on the environmental conditions and local knowledge about how they grow, whether they become weeds and at what stage of growth it is best to incorporate them into soil.

The green manure crop must be suited to local soils and climate to provide a large quantity of humus - forming material in the growing period. A large root system is just as important as a mass of above ground growth, so fibrous roots are highly desirable. Legumes and non legumes are suitable for use as a green manure. Many growers add rock minerals to the green manure crop so that they can be partly processed during the growing and humus forming stage..

All green manures should be turned in before maturity, preferably while still quite green and actively growing. If non legumes are used it may be necessary to supply additional nitrogen at the time of ploughing in. Avoid over-cultivation when digging green manures in, they only need to be slashed to the ground or lightly tilled into the soil.

If the temperature is not too low, moisture supply is adequate and the green manure crop is still succulent, succeeding vegetable crops may be planted about three to five weeks later.

Suitable species: legumes


Sow in Autumn. Leave 3-4 months before turning in.

Field Peas:

Sow May through to early spring. Leave 3-4 months. Suitable for summer sowing under irrigation.

Broad Beans: (Fava Bean):

A good winter crop which is frost tolerant and will stand some water logging. Turn in preceding a spring sown crop. Minimum 3 months.

Cow Peas:

Will tolerate hot weather. Sow October to February. Earlier sown crops produce more growth. Frost sensitive.

Sub Clover and White Clover:

Sow in Autumn. Does not produce large amounts of foliage but useful for nitrogen fixation.


Produces a large bulk and will compete well with weeds. Do not allow to seed. Minimum growing period two and a half months.

Suitable species - non-legumes:


Provides large amounts of organic matter. Good drought resistance. Much growth below the surface helps to stabilise fragile soils. Minimum growth period 2-3 months.


Cold tolerant. Sow in Autumn or in Winter in high rainfall areas. Leave 3 months before turning in.


Sow spring or summer, grows very quickly. Leave minimum 6 weeks. Not frost tolerant. May produce large quantities of seed if not turned in.


Sow March to Spring. Will host club root so is unsuitable for use in brassica cropping. Leave 3 months.

Annual ryegrass:

Provides a good bulk material for turning in. Needs to be dug in to 5 cm depth to control regrowth. Minimum growing period 4-5 weeks.

Deep loosening, shallow turning

Soils are organised and ordered according to an understandable and predictable pattern They are mot just a random set of events. One way soils are organised is vertically. Look at and road cutting or into a trench. Wherever it is, it is most likely that the colour and texture are organised vertically. Texture means the mixture of small and large particles (clay being the smallest, followed by silt, sand and gravel.

These patterns are so predictable that scientific classification systems use them extensively. Colour changes through the profile and texture contrasts are important information for anyone who studies soil, and often simple visual clues are enough for experienced people to understand much about how that particular soil was formed and how it will behave.

Soils are organised vertically all over the world - because that’s the way they want to be. They are usually darker near the surface (because humus is dark in colour) and the surface layers usually contain less clay, because the tiny clay particles are washed down between the larger particles. Calcium and other materials are often layered through soils too. Plant roots, detritus organisms, nitrogen foxing organisms and other soil life need to be at the surface, where they are protected from drying out by the soil environment, but still have access to oxygen from the atmosphere.

All soils are organised this way - but when we attempt to use them for cropping, we almost always try to turn them over and mix them up, using a spade or fork or power tool, such as a rotary hoe, which mixes and pulverises soil.

In organic growing, we try to work with nature as much as possible, Recognising the vertical organisation of soils, organic growers try to interfere with this organisation as little as possible. This gives rise to various forms of ‘no-dig’ gardening and the concept of no-inversion tillage.

We say, “deep loosening, shallow turning”.

Loosening soil means putting tines through the soil to cause slots or cracks, but does not imply turning over. Loosening helps to get air and water into soil and helps soil organisms and plant roots to penetrate into soil.

There are good reasons for cultivating soil, to control weeds or to create a seed bed. Organic growers need to do this too, but they turn soils only shallowly, going just deep enough to accomplish the task at hand, and leaving soil to its natural organisation.

Generally organic matter (compost and raw organic material) do not need to be dug in, but can be left on the surface and covered with a mulch layer, or just lightly scratched into the surface.

nature has created this thin soil layer all over the world. That shallow layer has to do a lot of work, to produce plants which become food and produce oxygen. Soils store and use carbon, oxygen, sulphur and many other nutrients and are a vital part of the cycling of these nutrients in the ecosystem. It is only a very thin, fragile layer to do all this work - best that we work with soils, and encourage them to remain organised the way they need to be.

Nature does know best.

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