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The Why’s and Wherefore’s of Compaction

Bruce Cockroft

Victorian Department of Agriculture, Kyabram, Vic. 3620

Compaction of soils is common in agricultural regions throughout the world and farmers are making progress in dealing with the problem. For example, regular chisel ploughing of many sandy loam soils in California is almost routine now, ploughing to 40cm is routine on many large Government farms in China, subsoil amelioration is practised in Britain, soil mixing to depth is common in Holland and of great interest is the ability of some of the volcanic ash soils of New Zealand to withstand any compaction by stock even in wet winters,

In south-eastern Australia we have several research groups studying soil compaction and amelioration, including those in the Rutherglen area, Wagga, Tatura, Canberra, Adelaide, Griffith and Kyabram. Will also have many manufacturers and farmers involved in ameliorating compacted soils.

When we read about compaction, amelioration and stabilisation of soils in the scientific journals, we find a great deal of agreement about the theory of what happens. When we go out into the field and try to do something about compaction, we find a lot of disagreement and conflicting results. We have a long way to go in field practice and much more understanding is needed. These problems are partly due to the wide variety of causes of compaction; partly due to the complexity of the effects of soil, plant, man, weather, machines and stock; partly due to re-compaction by machinery, stock or rain.

The Soil as an Environment for Roots

If we take the average production of crops and pastures in Australia and compare these with yields on the best soils overseas we find that the overseas production can be anywhere between twice and seven times the Australian. One of the reasons for this difference is that Australian soils are not productive compared to the best that exist. I suggest that it is very useful indeed for us to study the properties of the best soils in places like the United States mid west, California, the lower Rhine delta, eastern England, the Yangse delta and New Zealand. Highly productive soils have features common to them all, and many of these features are missing from Australian soils, mainly because our soils are old.

The most productive soils have the following features. The deep subsoil is permeable to water so that excess water has somewhere to drain to -gravel, volcanic ash, porous limestone are examples. The subsoil is porous, stable, soft, well-drained, permeable to water and chemically fertile; the main visible features to look for are softness and abundant visible pores. The surface soil is medium textured - (not very sandy or clayey), soft, aggregated when cultivated, stable in water, resistant to compaction, permeable with a high infiltration rate, high in organic matter, high in biological activity and high in plant nutrients; the main visible features to look for are depth to subsoil, stability of aggregates, porosity, softness, texture (loam), fertiliser history and biological activity (earthworms).

To assess the productivity of a soil in the field we must keep these needs in mind and identify soil features that will affect root activity. The previous paragraph emphasises the physical features of our soils. In Australia we have discovered that our soils are poor and in the past we have concentrated on overcoming chemical deficiencies leading to the use of superphosphate, nitrogen fertilisers, potassium fertilisers, trace elements and lime. We are now in a phase of development where we need to concentrate on the physical properties of our soils.

We must think of the soil as the environment in which roots live and an active root system is essential to high productivity from the plant. Roots need in their environment, water, nutrients, aeration, soil that is soft and an optimum temperature. Surface soil provides these needs to a better extent than subsoil, so the deeper the surface soil the better, and the shallower the cultivation the better. Visible pores are important in water flow and drainage so the more of these that we can count on an undisturbed face from a shovel of surface soil, the better. Hard soil material cannot be penetrated by roots so we look for soft surface soil. Earthworms are a good indication of biological activity and biological activity is important in making soils porous, stable and soft (earthworms, roots). The amount of crusting caused by rain and the degree of compaction caused by machinery or stock also need to be assessed.

The subsoil needs to provide drainage of excess water, storage of water, and opportunity for root penetration. A count of visible pores on undisturbed faces, using a shovel or large auger, will indicate whether water will penetrate properly and drain sufficiently; at least 15 pores per square inch is needed. The amount of water that the subsoil can store depends on the amount of fine pores; sand and compacted clay have few of these, so the ideal is a soft, open clay to clay-loam material. The degree of root penetration that can occur depends on the aeration of the subsoil and the hardness. A subsoil with plenty of visible pores and good underdrainage in the deep subsoil will be well-drained; hardness can be felt, Soil hardness should be estimated only when the soil is moist (field capacity).

Types of Compaction

(i) The Compacted A2

Soil scientists call the surface soil the A horizon and the subsoil the S. Within the A horizon we often find two layers, one darker due to organic matter that has accumulated, and below this A1 we find a white, bleached A2 layer which is light coloured because of low organic matter.

Because of the low organic matter this material is unstable, collapsing when wet and compacting under traffic. It is usually hard and roots find penetration difficult. Because it contains quite a lot of sand compared to the subsoil, it often remains permeable to water (we can see many pores in it) even though it is hard. If the layer is both hard and non-porous then neither roots nor water will penetrate.

The obvious remedy is to break up the A2 with some kind of deep tillage. Water, roots and aeration then penetrate readily. However, the material will readily collapse again when wet and with traffic. The key to stabilising this material is organic matter. If the farmer can build up the organic matter in the A2 it will become stable like the A1. In addition, the addition of judicious amounts of clay will assist by “lubricating” the sharp sand grains. In one experiment tile addition of 15% of clay was enough to keep the soil stable and allow roots to penetrate; the clay was chiselled up from the subsoil. Clearly these methods are long-term and in any case we need a lot more information on amelioration of A2 layers

(ii) Compaction by traffic

Traffic, including either machinery or stock or both, will compact many surface soils. These agents exert very high bearing pressures and if the load is “shearing” as under the driving wheel of a tractor, rather than “static”, the compaction effect can be doubled. On cropping land the cultivator breaks up the top of the compacted A horizon but the soil beneath remains dense. The remedy is to break up the whole compacted layer at regular intervals as necessary or to avoid traffic compaction as much as possible. This is difficult and impossible to eliminate entirely; however, farmers have used “tram lines”, others graze susceptible soils only when they are dry, others use machinery with a large bearing area. Again a lot more information and research is needed here.

(iii) Compaction by dispersion and slaking

Soils will collapse to a dense mass when they disperse or slake. The farmer cultivates and with subsequent rain the whole cultivated layer runs to mud which then sets hard on drying.

If the problem is due to low calcium and high sodium and/or magnesium in the soil then we say that the soil, disperses. We cure dispersion by adding more calcium, usually as gypsum. A laboratory soil test indicates how much gypsum to use, In the field we can identify dispersion when we drop a small clod into a jar of rain water and a cloud of floating clay appears in the water within 12 hours.

If the problem is due to low organic matter then we say that the soil slakes. We can distinguish slaking from other forms of compaction by taking pieces of the soil, drying in the sun, then dropping them into a jar of rain water. If the clods collapse into very small pieces within an hour or two we say that it slakes. Many soils slake to pieces the size of sand and this is not usually a problem. But if the soil slakes to a mud then amelioration is needed. We do this by building up soil organic matter and encouraging a lot of biological activity.

(iv) Compacted clay pan subsoils

Many Australian soils have clay pans that are a poor environment for roots. They usually occur at 10 to 30cm, can be red or yellow and are quite dense. They have formed naturally under the influence of low calcium and high sodium and/or magnesium - that is, they are dispersive. They usually extend to 60 to 80cm and the material deeper than this is usually more permeable and softer than the B horizon above.

Improving this material is difficult but we use the same technique as with the other compacted materials - break up the dense material and stabilise it. We find that a ripper, paraplow or other machine is needed to break up these claypans and we stabilise the broken up layer with gypsum. Again, a lot more research and experimentation is needed here. I am recommending this type of treatment for orchards only at this stage until we develop our knowledge and practice in this type of soil amelioration.

Biological Activity in Soils

In Australia we have developed our knowledge and farming practice in soil chemistry - fertiliser use. We are now developing our knowledge and farming practice in soil physics - the main subjects in today’s seminar. But we have not been paying enough attention to that third important aspect in the management of our soils - soil biology.

Not only man manages soil; so do plants and animals. Roots push channels/pores through the soil, they rearrange the soil particles to desirable placings, they exude organic compounds to stabilise the soil and eventually they themselves die and decay to leave more organic matter. Fungi interlace and bind soil particles into stable aggregates. Bacteria and other microbes break down coarse organic matter, produce bonding agents to form ideal aggregates and take part in the cycles of nutrients in the soil. Earthworms and other animals produce channels in the soil, incorporate organic matter from the ground surface, digest coarse organic matter in the soil and produce casts of very stable material. The best soils in other countries have high biological activity compared to Australian and we must take steps to encourage biological activity and hence reduce the rate of development of soil compaction problems and even ameliorate compacted soils.

We increase biological activity in soils if we improve drainage, ensure an adequate supply of organic matter on which the soil population lives, try to keep the soil moist, maintain a reasonable level of plant nutrients and protect the upper part of the surface soil from heat, raindrop impact, ultra violet radiation and extreme drying by using mulches of organic residues.

In Victorian irrigation areas for example we have developed high biological activity to considerable depth in the profile and as a result increased crop yields greatly on difficult soils by such practices. We first broke up the subsoil by ripping to improve drainage and allow deeper rooting, land formed for surface drainage, irrigated lightly but frequently to keep the soil moist, controlled weeds with weedicides, applied the proper amounts of artificial fertilisers, eliminated cultivation and spread organic matter mulches. Improvements of this nature take a long time; frequently 5 years.

If our soil and plant scientists remain determined to develop an understanding of the theory of soil compaction and soil productivity, if our agricultural engineers keep designing, testing and developing better machinery and if the farmer perseveres and uses his own ability, scientific understanding and new agricultural machinery to increase his productivity, then we shall indeed make progress. We people who are interested in productivity from our soils will then continue to learn how to manage our soils better, how to recognise problems, how to ameliorate those problems, how to build our soils deeper and how to put these things into practice on the farm. If we do this, we can then say to the conservationists, to the decision-makers, to the public and to future generations of farmers, that far from destroying our soils as we increase our agricultural productivity, we improve them.

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