The research team measured soil phosphorus before sowing, at peak vegetative biomass (growth stage 65) and at maturity of the break crop, and then before sowing and at maturity of the following wheat crop. The aim was to understand the effect of different break crops on microbial and available phosphorus in the soil as well as crop phosphorus uptake in the subsequent wheat crop.
There were three findings from the study: the effects of break crops on in-season P mobilisation, the lack of influence of break crops on pre-sowing available phosphorous in the subsequent season, and the greater P uptake and yield of the wheat after break crops.
“We found that during the break crop phase, there was an increase in available P at some sites, but not others, while microbial P increased at one site,” Dr Doolette said.
The in-season mobilisation of P by break crops ranged from 0 to 30 kg P/ha, with no clear trend in P mobilisation with crop type. However, large increases in mobilisation were observed in crops which were green or brown manured (12-17kg/ha) compared to harvested (1-7kg/ha).
“However, when we sowed the wheat in the following season, we didn’t see any difference in pre-sowing soil phosphorus, available or microbial, compared to the previous year, or between the different break crops,” Dr Doolette said.
This showed that the increase in mobilisation observed in some crops in the previous year had not persisted in the soil, even following the high mobilisation seen from manuring.
Despite the pre-sowing measurements showing no P increase after the break crops, the third set of results – the performance of the wheat crop – provided some interesting findings.
- There are two major pools of phosphorus in soil – inorganic and organic
- Not all inorganic P is plant available in soil solution, some P may become available but is temporarily stuck to the surface of soil particles, whereas some P is more permanently unavailable (‘lost’) as it is fixed to other minerals in the soil.
- Unavailable inorganic P may be ‘released’ and become available P by the action of exudates from crop plant roots.
- The amount of plant available P (as represented by the hatched box in the diagram) relative to the total inorganic P pool is influenced by soil type, especially clay content and mineralogy.
- Soil organic phosphorus is not directly accessed by plants, it is readily converted to plant-available forms of P by soil microbes during a process called P mineralisation. P in the microbial pool is one stage closer to being available to plants.
“We found that if you had a break crop followed by wheat, the wheat crop had a greater uptake of phosphorus and significantly higher yield than if you had sown a continuous wheat crop.”
Phosphorus uptake in wheat was 0-2.5kg P/ha greater after canola than for wheat on wheat and 2.6 -6.6kg P/ha greater after legumes (peas, lentil, lupin and vetch). Yield increases after break crops were 0-2.6t/ha compared to wheat on wheat. Legumes provided a greater wheat yield benefit compared to canola.
There was no significant difference in wheat P uptake or grain yield between the different legume break crops, even when the crops had been green or brown manured.
The reason behind this outcome is not yet understood. There could be other factors which may have contributed to a higher P uptake by wheat after break crops. Some theories include increased root length due to improved N availability and less root disease.
The next stage of the university’s research will focus on better understanding what is happening beneath the soil surface. Glasshouse trials labelling break crops with phosphorus isotopes has so far found that wheat takes up about 19-26 percent of the P input from canola or lupin roots. Further work with dual-label break crops with nitrogen and phosphorus isotopes to distinguish the relative contribution of these two nutrients to wheat growth and yield.
A/Prof Ann McNeill, 08 8313 8108, firstname.lastname@example.org
Dr Ashlea Doolette, 08 8313 8107, email@example.com
View the GRDC Phosphorus Fact Sheet.