GRDC stubble management guideline No. 16
What is stubble nutrition? What can I do to utilise it?
Nutrients remain in stubble after a crop has been harvested. These nutrients are made available to subsequent crops by soil microbes through break down of the stubble when moisture is available. These nutrients then become available for plant uptake through the soil. Nitrogen applications can assist in speeding up stubble break down along with rainfall events. An initial mechanical breakdown of stubble in to smaller pieces through means of chopping, mulching or spreading can also increase the speed of stubble break down and availability of nutrients in the soil.
Two key processes of stubble breakdown involve mineralisation and nitrification, vitally dependent on the role of soil biota. Mineralisation occurs when soil microbes breakdown organic nitrogen to ammonium. Nitrification is the process by which soil microbes convert ammonium to nitrate. Nitrate is the most plant available form of nitrogen. The retention of stubble can assist these processes as stubble provides carbon and nitrogen (energy) for microbial activity.
The role that stubble plays in the tie-up (conversion of inorganic compounds to organic compounds by microorganisms) and release of nutrients is critical to all farming systems. In a dry summer, stubble breakdown can be slow as rainfall aids stubble decomposition. A lack of summer rain is expected to delay the availability of nutrients from previous crop stubble to the following crop.
The retention of stubble also enhances the input of carbon in the soil. The carbon to nitrogen ratio (C:N) of residues is an important factor in determining the contribution those residues will make to carbon availability as it determines the speed of breakdown. Pulse residues (C:N 20:1 to 41:1) are more decomposable than wheat residues (C:N 45:1 to 178:1). Faster decomposition may improve nutrient availability for the following crop.
As part of the ‘maintaining profitable farming system with retained stubble in Victoria and Tasmania’ project, Birchip Cropping Group (BCG), Southern farming Systems (SFS), Irrigated Cropping Council (ICC) and Victorian No-Till Farmers Association (VNTFA) have investigated stubble nutrition and its influence on stubble retained systems.
Field trials and observations from the grower groups have been conducted as part of this project which has been supplemented by previous research and anecdotal evidence from industry experts and experienced growers across Victoria. This output has been primarily focused in the Wimmera and Mallee region and the Southern high rainfall region including Tasmania .