Things that happen when your wheels turn – Is soil compaction flattening your profit and sustainability?

As published in ‘Grower’, September 2009.

The invention of the wheel, is often raised as a measure of human progress. It has evolved from wood and stone, to steel and rubber. Once roads were formed and compacted for traffic, commerce and agriculture changed forever. Those with heavy loads to move had an option other than water, sled or legs. In agricultural cultivation, draught animals have been replaced by machinery which revolutionised crop production to keep pace with the food demand of a growing population.

The diameter, width and the softness of the tyre or surface, determine the footprint of any wheel.  That zone is where the pressure of load and vehicle is spread on soil or pavement – ‘where the rubber meets the road’. This footprint is an interesting place. It determines the quality of traction, wear and tear, and how well a vehicle or tractor can carry or pull a load. Here upward resistance equals the downward pressure of the wheel. If the surface is already compacted, little or no compaction will occur. If the surface is soft or loose, compaction is the result. Also rolling resistance and friction will increase on a soft surface. This increases the energy, and fuel consumption required to move, or reduces the load able to be carried.

Soil has some natural compaction which occurs with time, gravity, water movement and the passage of animals. Some consolidation of soil is desirable to prevent wind erosion and provide seed contact with soil for germination. Compaction can be an asset where repeated traffic is intended. (A lot of energy goes into compacting the base of our roads for instance). It is when the level of compaction of the soil affects the structural aggregates or peds in the soil, that structure and pore spaces are adversely affected.

In the growing zones of a crop, this is a problem. In a paddock or crop, the rolling wheel is riding on soil and vegetation. If the soil is soft the wheel is climbing onto loose material and rolling it down. In the process it is squeezing gas and water from the profile, closing pore spaces and making changes which can be longterm, and detrimental. Depending on the soil type and its state of dryness and existing compaction, around 75% of the undesirable effects of compaction can occur in the first pass.

In our cropping soil, when soil compaction is excessive, it can lead to the loss of structure, erosion, biological degradation and loss of water holding capacity and nutrients. In some regions of the world, compaction has led to the loss of productive capacity of soils and the departure of agriculture, probably forever. More commonly, compaction is a factor in reduced yields and deterioration of structure in cultivated soils. As land prices rise and our population increases, these soil quality trends will need to be reversed.

In next month’s Grower we will discuss how the use of GPS to guide machinery is containing compaction to traffic bearing zones, which are separate from the garden areas for growing the crop. This is allowing growers to reduce fuel consumption and other inputs and improve the profitability and sustainability of their cropping.

Controlled traffic farming (CTF) is all about managing soil compaction – confining it to narrow strips across the land and maximizing the remaining undamaged soil area for cropping. In practice CTF means matching machinery tracks so they take up the least possible area. Farm conversion to CTF is about adopting a CTF “mindset” – the belief that separating wheels and crops is a key method of reducing costs and increasing returns.

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