The Grains Research & Development Corporation (GRDC) provide some excellent and informative podcasts that are great listening when sitting on a tractor. We came across a recent podcast that provided some very interesting research that has implications all farmers and growers and relates closely to our own research.
Over the past five years CSIRO soil scientist Clive Kirkby and his associates have been conducting trials looking at conventional cultivation techniques where crop stubble is cultivated into the soil.
The control plots of Kirkby’s trials had stubble cultivated into the soil while the test plots had nutrients (nitrogen, phosphorous and sulphur) applied when cultivating the stubble.
The trials have resulted in increases in soil organic matter when the stubble has been supplemented with added nutrients.
Kirkby says that “while conservation agriculture has been aiming at adding the minimum amount of nutrients to grow a crop, it has inadvertently led to a decrease in soil organic matter. The organic matter in plant material is actually nutrient poor. The organic matter in the dead microbe material is nutrient rich.”
Kirkby suggests that the benefits of the stubble and its contribution to soil organic matter are not the dead stubble itself; rather it’s the dead microbes that have spent their lives digesting the stubbles.
Mulching the stubble helps to incorporate it into the soil, and that the nutrient quantity in the stubble will degrade over time so the sooner it is incorporated into the soil the more nutrient benefit the soil will receive from the stubble.
“We have essentially got two crops” says Kirkby. “The crop that we grow above the ground and the crop of soil organic matter (below the ground). The two crops need their own set of nutrients. If you pull the nutrients out of soil organic matter…the carbon levels will go down”.
The amount and type of nutrients that should be added to the stubble will vary from year-to-year and on the type of crop the stubble is derived from. Canola is usually high in sulphur so canola stubble may only need nitrogen and phosphorous added, while stubble from a crop that had a particularly good yield may need more nutrients added since so much of the previously applied nutrients were utilised in achieving that yield.
Kirkby’s comments on farmers who leave their stubble standing were that they are doing the right thing if erosion were a major problem for that farmer, but “they’re not doing the optimum thing to treat soil organic matter levels”.
Typically BioAg cropping soil fertility programs contain a combination of quick and slow release P by way of MAP as a starter P and BioAgPhos as a source of capital P, along with a sulphur source, elemental gypsum or SOA. It is common that these programs contain more P and S than conventional fertiliser programs.
When stubbles are to be incorporated generally we prescribe 10 litres of UAN or 10 kg of urea per tonne of stubble to be incorporated per hectare, in addition to Digest-it which aids in the digestion process.
The important point is that once the nutrients, N, P and S are bound up in this organic material, they become non leaching and non locking. As the organic matter grows so too does the soil’s natural fertility. BioAgPhos is a great source of phosphate when it comes to building soil organic matter, health and fertility.
Since inception, a key aim of the range of products and nutrition programs BioAg has developed has been to look after the health of not only the plant, but the soil as well.
BioAg’s own trial work, as well as independent trials and studies such as this one by Clive Kirkby and his associates at the CSIRO mentioned above help to justify the benefits of healthy plants and soils.

Michael Douglass