Research shows that nutrients released from stubble, including phosphorus, need to be factored in when planning fertiliser requirements
Sarah Noack taking soil and stubble samples from her father's property near Truro.
PHOTO: Casey Doolette
University of Adelaide PhD student Sarah Noack received a GRDC scholarship in 2010 to research the diversity of phosphorus (P) forms in crop stubbles and how stubble management affects the breakdown and release of stubble P in soil.
Working with Professor Mike McLaughlin and Dr Ron Smernik, both from the University of Adelaide, Dr Therese McBeath, CSIRO, and Dr Roger Armstrong, from the Victorian Department of Primary Industries, Sarah has conducted research in the laboratory, glasshouse and field.
Key questions include how much phosphorus will the previous crop’s stubble supply, and when will this phosphorus be available to plants during the coming growing season.
Crop stubbles sampled in the summers of 2010–11 and 2011–12 contained amounts of phosphorus equivalent to one to five kilograms of phosphorus per hectare.
This phosphorus can be released directly to soil as soluble phosphorus (where it can be used immediately by the crop or chemically fixed in the soil) or it can be absorbed by microorganisms and subsequently released back into the soil in the future.
The type, size, placement and overall stubble load, as well as rainfall and soil moisture, can all influence the timing and amount of phosphorus released from stubbles to soil.
A recent glasshouse experiment conducted by Sarah examined how the size and placement of stubble affected the amount of stubble phosphorus returned to the soil, and how much of this phosphorus was subsequently used by wheat plants.
Pea stubble (collected from a grower’s paddock) was added at a rate of 10 tonnes/ha. Stubble treatments included simulated ‘conventional tillage’ (stubble incorporated into the soil), no-till (stubble left on the surface), and treatments simulating the use of the Harrington Seed Destructor, in which stubble was ground before addition to the soil surface.
Using 32P and 33P radioactive isotopic labelling techniques, Sarah measured what happened to residue phosphorus in the soil and in the plant, microbial and soil-extractable phosphorus (resin) pools. There were four main findings to come out of this work.
- Stubble treatment had little effect on plant dry matter but did increase plant phosphorus uptake in the conventional tillage and no-till systems compared with the control (no residue).
- Within 80 days of application to the soil, a significant amount of stubble phosphorus was released and taken up by the subsequent wheat crop for all treatments.
- The percentage of wheat phosphorus derived from stubble ranged from nine to 16 per cent across the three residue management treatments. The remaining wheat phosphorus came from fertiliser (19 to 23 per cent) and soil (63 to 72 per cent) sources.
- On average, 50 per cent of the added stubble phosphorus in the conventional tillage treatments was detected in plant, microbial and resin phosphorus pools compared with 20 per cent for the two surface stubble treatments. This is most likely due to the lower moisture contents of surface stubble in no-till systems resulting in less microbial activity and hence lower rates of stubble phosphorus release.
After harvest, stubble will start to decompose and release phosphorus via leaching or microbial decomposition. Depending on how quickly this phosphorus is released, phosphorus from this stubble may be taken up by summer weeds or become chemically fixed in the soil before crops can access it.
One advantage of using conventional tillage to manage stubble would be increased decomposition rates and greater stubble phosphorus release in the short term. In no-till systems the breakdown of stubble is slower, but still has the potential to contribute to crop phosphorus uptake and improved soil structure and moisture retention.
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