By Felicity Harrop, Soil CRC Adoption Manager
During the most recent Soil CRC webinar, Professor Terry Rose (Southern Cross University) shared regionally specific insights from our ‘Building soil resilience and carbon through plant diversity’ (4.1.007) project and Dr Alice Melland (University of Southern Queensland) presented the impact on crop water availability and yield from those treatments in the ‘Assessing soil water storage’ (4.2.005) project.
Six years of Soil CRC research across NSW and Victoria has delivered a clear message for growers: diverse plantings in water-limited environments can negatively affect subsequent winter crop yields. The research compared summer cover crops, temporary wheat-vetch intercrops, and diverse crop rotations to understand their impact on soil health, water use and crop performance.
Summer cover crops always used water
- Every cover crop reduced stored soil moisture ahead of the next crop.
- Typical deficits were 20–50 mm less soil water and 40–80 kg/ha less available nitrogen at sowing.
- Even small cover crops (500–1,000 kg/ha biomass) still drew water from the profile.
Yield impacts were generally small but tended negative
- Most cover crop treatments produced slightly lower yields than the control.
- In wetter seasons, crops often recovered and yield penalties were minimal.
- In drier environments where stored summer moisture is critical, the risk is much higher.
Soil health benefits were limited
- Researchers found no consistent improvements in soil carbon, microbial activity or other soil health indicators.
- The likely reason: most cover crops simply didn’t produce enough biomass often enough to drive change.
Temporary wheat-vetch intercropping was low risk
- Terminating vetch at 12–13 weeks avoided yield penalties in wheat.
- However, there were no significant gains in yield or grain protein.
A positive sign
Researchers found evidence that repeated cover cropping may improve subsoil water-holding capacity and encourage deeper rooting, potentially increasing plant available water over the longer term. However, this benefit needs to be weighed against the immediate cost of water use.
Key take-home messages
- Cover crops can provide ground cover and increase plant diversity.
- In southern dryland systems, they will almost always use valuable soil water.
- Biomass production is often too low to deliver measurable soil health benefits.
- Temporary intercropping with vetch appears low risk when terminated early.
- Cover crops are best used strategically, not routinely.
Actions for growers
- Use cover crops where ground cover is sparse or erosion risk is high.
- Consider seasonal outlooks carefully before sowing a summer cover crop.
- Factor in the water use, especially where stored summer moisture drives yield.
- Terminate companion legumes early (12–13 weeks) to minimise competition for water and nutrients.
- Explore other diversity options, including pulse crops, and strategic brown-manure phases.