*Note - this report may contain independently supported projects, which complement the work in this GRDC research program.

Research has shown that on average every single area of a paddock has wheel compression on it every year under a normal cropping system. Even in a minimum tillage and direct drill situation the paddock still has a lot of pressure from tractors, boom sprays, windrowers and headers.

One method of preventing soil compaction is to enter a controlled traffic system where all implement wheels are forced to follow permanent tracks or tramlines. Crop yield is sacrificed under these tramlines but the improved yields in between should more than compensate for this. In essence raised bed farming forces you into a controlled traffic system.

For some growers, raised beds do not suit their soil type, paddock slopes and/or management system. However, controlled traffic cropping can still be used quite successfully as long as tramlines are established and machinery modified so that wheel widths of tractors, combines and boom sprays are compatible. Boom sprays also need to be modified to spray either a combine width or three combine widths.

Markers then need to be established on fence lines so that consistent paths can be followed every time. With improvements in GPS technology it is feasible that tramlines could soon be cost efficiently established using this method.

Benefits of controlled traffic

Benefits of controlled traffic cropping include reductions in seeding and spraying overlaps. These reductions can be quite large and can result in significant savings. Some work from WA has also shown a significant decrease in screening levels due to controlled traffic.

However controlled traffic cropping is not the only answer, as soil compaction problems are not the only soil structural issue.

Surface crusting is one form of soil structural decline that can have a marked effect on crop establishment and hence yield.

Many modern crop varieties have short coleoptiles and can experience problems emerging if surface crusting occurs. If you have poor emergence then signs of crusts on the surface can be a give away. If you dig down to where the seed is sown you can usually find germinated seeds with twisted coleoptiles that just can't find their way to the surface. However, be careful interpreting the results as sowing too deep can also cause similar problems. As well, some systemic fungicides and the herbicide trifluralin shorten coleoptile lengths.

Role of gypsum

Gypsum can assist in preventing surface crusting as long as the soil is in fact gypsum responsive. A dispersion test is simply done by carefully putting a small piece of soil (about the size of a ten cent piece) in a container of distilled water. If slaking occurs (the aggregate falls to bits) or if dispersion occurs (a cloudy or milky appearance occurs in the water - may take up to 20 hours), then gypsum will probably assist. This test is routinely done in most soil testing laboratories.

Gypsum however is only really effective in stabilising chemically unstable soils for about two to three years. After that more gypsum would need to be applied. A longer-term approach is to increase the soil's organic matter levels by having good healthy pastures in the rotation.

Organic matter helps bind soil particles together with a type of organic glue. The resulting stable aggregates are less likely to crust, increase water infiltration and are generally more fertile. However over cultivation will quickly destroy these aggregates resulting in surface crusting problems reoccurring.