Break crops could hold key to tapping soil phosphorus reserves

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Researchers are looking beneath the soil surface to understand how break crops affect phosphorus reserves

Photo of three people in a field

Dr Ashlea Doolette (pictured right soil sampling Kaspa field peas at Karoonda in 2011 with PhD students Foyjunnessa and Yulin Zhang) has found break crops might open up reserves of soil phosphorus for subsequent crops.

PHOTO: Tim Heath

University of Adelaide research, as part of the GRDC Soil Biology Initiative, has demonstrated break crops have positive effects on phosphorus uptake and yield in subsequent wheat crops.

Only 10 to 30 per cent of applied phosphorus (P) fertiliser is taken up by crops, so a large proportion of fertiliser costs are lost.

The project investigated whether break crops can be used to ‘unlock’ some of the lost P in order to improve P uptake and grain yield of subsequent wheat crops.

Crops of lupins, field peas, vetch, lentils, canola, cereal rye and wheat were sown in 2011 at Hopetoun and Longerenong (Victoria), Karoonda (South Australia) and Junee (New South Wales), in different combinations at each site. Crops were either harvested or green or brown manured, then followed by a wheat crop.

Researchers measured plant-available soil P before sowing, at peak vegetative biomass (GS65) and at maturity of the break crop, and then before sowing and at maturity of the following wheat crop.


During the break-crop phase, in-season mobilisation ranged from 0 to 30 kilograms of P per hectare, with no clear trend in P mobilisation with crop type. Inputs of mobilised P to soil in plant residues were larger (12 to 17kg/ha) if crops were green/brown manured compared with harvested (1 to 7kg/ha).

There was no long-term effect on plant-available soil P from the increased P mobilisation observed in break crops.

There are two major pools of phosphorus in soil – inorganic and organic.

Plants take up inorganic P from soil but not all of it is readily available to plants. Some inorganic P may temporarily be stuck (adsorbed) to the surface of soil particles and becomes plant-available over time. Other inorganic P is more permanently unavailable (lost) by being fixed to minerals in soil. The amount of plant-available P is influenced by soil type, especially clay content and mineralogy.

Yield increases after break crops were up to 2.6t/ha compared with wheat on wheat.

Soil organic P is not directly accessed by plants, but is converted into a plant-available form by soil microbes during a process called P mineralisation.

After a break crop, wheat had a greater uptake of P and higher yield than a continuous wheat crop.

P uptake in wheat was 0 to 2.5kg P/ha greater after canola than for wheat on wheat and 2.6 to 6.6kg P/ha greater after legumes. Yields of wheat after break crops were from 0.7 to 1.7t/ha greater than after continuous wheat. The yield benefit from legumes was 0.1 to 0.6t/ha greater than that from canola.

There was no significant difference in wheat P uptake or wheat grain yield following the different legume break crops.

Researchers are now focusing beneath the soil surface. Glasshouse trials using tracer showed that 12 to 20 per cent of the P in break crop root residues was accessed during early growth of following wheat.

More information:

Associate Professor Ann McNeill,

08 8313 8108,;

Dr Ashlea Doolette,
08 8313 8107,

Associate Professor Ann McNeill from the University of Adelaide talks about the challenges in understanding what happens to phosphorus returned to the soil in crop and break crop residues:


Agronomy packages take on northern pulse challenge


N higher after legumes than wheat

GRDC Project Code UA00119

Region Overseas