THE optimal moisture level needed to avoid soil compaction in cotton fields is well below previous recommendations, according to the latest research from CSIRO and the University of Southern Queensland.

The research, funded by the Cotton Research and Development Corporation (CRDC), looked into soil compaction from John Deere 7760 round bale pickers.

It found the optimal soil moisture level to avoid compaction was below the previous recommendations of being near the soil plastic limit (the point at which the soil goes from just starting to crack, to one where it behaves like plasticine).

What do growers need to know?

• Increases in machine weight and large traffic footprint can substantially impact yields and tillage energy costs by compaction impacts through at least 80 centimetres depth.
• Flotation is not avoidance of compaction: Just because you don’t see substantial wheel ruts at the surface does not mean compaction is not happening below. The stress on the soil is transferred to depth, and if the soil is close to the plastic limit then compaction will occur.
• The simplest way to determine plasticity is to take samples to around the depth of the major rooting zone (up to 40-50cm, from top of hill) and squeeze it between thumb and forefinger. If it does not feel like plasticine then it is lower than the plastic limit.
• The best approach to managing soil compaction is to avoid it, which is why Controlled Traffic Farming (CTF) is best practice for limiting compaction, increasing yield potential and decreasing environmental cost.
• Where CTF is not yet available, compaction can be minimised by ensuring in-crop traffic only occurs well below the plastic limit, and as close to the permanent wilting point as possible. Traffic when soil is near or above the soil ‘plastic limit’ should be avoided with the JD7760 if a CTF system is not used.
• The soil profile should be dried down prior to defoliation to minimise compaction: once the crop is defoliated no soil moisture is extracted by the crop.

What if it rains during picking?

The closer the soil’s wilting point to the plastic limit, the greater the risk of compaction. For many of the industry’s cracking clay soils, the plastic limit will be very close to being the same as the wilting point limit. For compaction to be minimised the soil needs to dry out to a point below the plastic limit.

For example, if a cracking clay soil (0-10cm) has a field capacity (FC) of 57.7pc, a wilting point (WP) of 30.9pc and plastic limit (PL) of 29.1pc, the PL and WP are very similar which makes the soil susceptible to compaction.

This soil would need to undergo 28.6mm of drying to go from FC to PL.

Depending on daily evaporation rates it could take a considerable period of time for the soil to become dry enough to fall below the plastic limit.

At 6-8mm per day it would take 4-6 days to dry out the top 10cm of soil.

After significant rainfall events the time taken to dry to lower depths will be much longer.

Source: CRDC/CottonInfo

For more information, see the soil compaction section within chapter 6 of the CRDC/CottonInfo Australian Cotton Production Manual.

Video 1: ‘Measuring compaction’. John Bennett describes recent trial work where soil compaction was compared between a conventional JD7760 (with standard and low tyre pressures), and a JD7760 modified for controlled traffic farming.

Video 2: ‘Consider controlled traffic farming to minimise soil compaction’. With the advent of large machinery, soil compaction has become a major issue in agriculture. John Bennett discusses the impact of compaction and the conversion of machinery to controlled traffic farming.