Soil structure decline

Good soil has a definite structure. It contains water, air, mineral particles and organic matter. The proportions of these will vary, depending on the soil type. A well-structured soil allows water and plant roots to penetrate. Usually such a soil is called friable (lumpy). Certain agricultural practices cause the decline of soil structure.

Agriculture WA estimates that some 3.5 million ha of the southern agricultural area of Western Australia is affected by structural decline of some sort. In terms of lost production, this costs the community $70 million per year.

Good soil structure depends on uninterrupted cycles

Soil is more than just dirt; it is an ecosystem teeming with life - or it should be. Worms, fungi, bacteria, insects and other small animals make soil their home. The cycle that produces a soil with an open, crumbly structure relies on the activities of these organisms. Burrowing animals, such as earthworms, mix the soil and produce tunnels that keep soil open, while bacteria and fungi break down organic matter, releasing nutrients for plants to use and forming organic matter (humus). Organic matter and good soil structure go hand in hand; it cements sand, clay and silt particles together into lumps called aggregates, thus creating many pores and spaces for air and water to move in and out of the soil. Such a soil offers little resistance to growing roots, shoots and germinating seeds and affords plenty of room (habitat) for the soil's inhabitants.

What causes soil structure decline?

The main cause of soil structure decline is over cultivation. In over-worked soils, populations of beneficial organisms dwindle and the humus formation cycle is broken.

Cultivation of soil is done to kill weeds and prepare a seedbed for a crop. Modern farming machinery is capable of covering larger areas and allows faster and more aggressive cultivation, resulting in more soil disturbance. Pressure and vibrations from vehicles used to work soil and harvest crops also cause damage resulting in the formation of compacted layers. This so-called 'hard pan' restricts water movement and plant growth. In pastoral areas, soil structure declines because of the pressure of grazing by introduced stock. The pressure of their feet crumbles the soil, destroying its structure.

What are farmers doing about it?

Reducing the number of cultivations (or minimum tillage) goes a long way to recovering soil structure. Some farmers now seed their crops without any cultivation beforehand; a herbicide rather than cultivation is used to kill weeds. Retaining cereal stubble and growing legume,(clover medic), pastures in rotation can help to improve and maintain soil structure by adding organic matter to the soil. On some soils, adding gypsum (calcium sulphate) can help farmers to adopt minimum tillage practices. Adding gypsum to certain poorly structured soils helps to open up the soil. The improved soil structure then allows the farmer to adopt a new tillage system that is less damaging to the soil.

No tillage farming systems aim to reduce the level of cultivation to a minimum essential for high yields, soil conservation and reduced weed emergence. The current systems, (tillage), rely heavily on herbicides for re-seeding and post seeding weed control. These seeding practices result in less weed germination, primarily in the undisturbed soil between rows. This reduces the cost of the crop and increases the reliability of yields, making cropping more profitable.

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