Soil Tests: Digging deeper for constraints to plant growth

Gainsborough Farming is located near the eastern edge of eastern Moree Plains, not far from the bedrock hills which are the source material for the relatively young deep and productive alluvial soils located in the region.

When the Hub’s Regional Soil Coordinator, Cameron Leckie and Communications Manager Erin Byles visited Gainsborough Farming in February, Justine and James were busy preparing for winter planting. The remnants of the previous crops’ residue, protecting the soil surface, was widespread. Justine and James use this crop trash to protect their soil from the potentially damaging impact of intense rainfall and improving the infiltration capacity of the soil. The carbon rich material also provides a food source for the soil food web.

The difficulty of inserting (and removing) a soil core down to a metre with a trusty post hole rammer can tell you a lot about soil condition, especially soil structure. The difference being largely a measure of soil compaction and moisture content. As renowned soil scientist Dr David McKenzie recently pointed out, the benefits of improved soil structure vastly outweigh the benefits of improved soil organic carbon levels from a soil water holding capacity perspective. The water holding capacity afforded by good soil structure is of critical importance to maximising soil moisture especially in dryland cropping systems. Coring at all four sites at Gainsborough Farming was a clear indicator that the soil is not compacted and testament to the Controlled Traffic Farming system.

Three of the sampling sites were heavy clay Vertosols (colloquially known as cracking clays or shrink-swell soils) on the flood plain whilst the fourth was a Dermosol (having a structured subsoil) on a slope. Compared to the other sites the crop residue was sparser at this site.

Whilst this maybe explained by the lighter soil texture (less clay) and associated lower Cation Exchange Capacity (the ability for the clay particles to hold and exchange positively charged plant nutrients such as calcium, magnesium and potassium) leading to lower biomass production, the soil chemistry results highlighted a potentially different cause.

Soil pH is a vitally important property impacting upon all aspects of soil chemistry. The laboratory results for the standard 0-10 cm depth highlighted a strongly acid pH. Further testing in smaller depth increments as part of a soil profile description identified even higher acidity in the top 8 cm of the soil compared to the slightly acid conditions immediately below that depth, with the pH becoming alkaline in the subsoil.

These results point to surface acidity. The impacts of acidity include lower crop productivity, key plant nutrients (phosphorus, magnesium, calcium and molybdenum) becoming less available and toxic amounts of some elements becoming available (aluminium and manganese). The latest advice, where an acidity problem exists/potentially exists, is to sample for pH in five centimetre increments, as a standard 0-10 cm soil test can hide these problematic layers (view more information on pH stratification from the Grains Research and Development Corporation). Given the potential costs associated with ameliorating soil acidity, it is critically important to diagnose exactly what, and where the problem is located before determining a management response such as liming rate, product and application method.

This example provides a case study as to how soil data can be used to diagnose soil related issues and help to implement a management plan to address the issue to maximise productivity for both the benefit of the farmer (in the form of yield) and the landscape (in the form of increased biomass production).

If you are looking for more information on soil, Soil Science Australia’s Resource Library is a fantastic resource jam packed with authoritative and reliable information.  

As part of the SQNNSW Hub’s commitment to encouraging farming practices that improve soil health, the Hub will be running a World Soils Day Competition once again in 2024.

View competition details

Watch the Case Study Video