It is widely understood that wheel traffic, particularly in moist conditions, produces an increase in soil strength, with adverse consequences for soil physical properties and fertility and the resulting crop growth.

Research conducted by Ward Voorhees in Minnesota has found that axle loads above 5 tonne are more likely to cause lasting damage to the soil at depths greater than 15 cm.  Any increase in soil strength caused by livestock is limited to the top 10 cm of the soil profile and is disturbed during seeding.  In general, increases in soil strength deeper than 15 cm is a concern as it is not readily disturbed during normal seeding operations and moisture and temperature are more stable at depth giving less chance for natural repair.

Voorhees also investigated the persistence of the deeper compaction caused by farming operations.  He detected increased soil strength three seasons after a vehicle pass.  Australian investigations have shown that in random traffic farming over 85 per cent of the paddock is subject to at least one wheel pass, even in no-till.  Research in Queensland has also been able to measure the effect of imposed wheel traffic 5 years after the initial pass.

A review of research findings related to the surface soil (down to 15 cm) shows the consistent theme that higher tyre pressures lead to more soil compaction and a degradation in the soil physical health.  This reduces factors such as infiltration rate, soil porosity and the number of beneficial soil organisms.

By using ‘high volume’ tall, wide tyres farmers are able to carry heavy axle loads at lower inflation pressures and reduce the amount of damage done to the soil profile.  Selecting radial tyres can also reduce the amount of damage as they can carry the same weight at a lower inflation pressure as the same size bias tyres.  This is because they offer a flat, stable and wider footprint and a better ground pressure distribution.  In some cases, there has been a 55 per cent reduction in ground bearing pressure when going from a standard bias tyre to a large volume radial tyre with flexible side walls.

On-going research funded by SAGIT across the Eyre Peninsula is aimed to investigate if soil compaction is economically limiting yield on a range of soil types and how long any beneficial deep ripping effects last.

AUTHOR: Cathy Paterson, Research Officer - Soil Compaction, SARDI
CONTACT: Cathy Paterson, Minnipa Agriculture Centre, Telephone: 08 8680 6205 or Mobile: 0428 116 231