Nutrient balancing could cut fertilliser costs

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Soils with high levels of both phosphorus and potassium availability have the potential to boost cereal plant nitrogen and water use efficiency (NUE and WUE), leading to cost reductions and improved grain yields.

That is the finding from glasshouse trials conducted by CSIRO Agriculture in Floreat Park, Western Australia, in 2015.

If these results can be replicated in the field, this could improve the NUE of wheat cropping on WA soils, cut nitrogen fertiliser requirements and minimise nitrogen losses in low and variable-rainfall areas.

Dr Elliott Duncan, a CSIRO Agriculture postdoctoral fellow, is leading the research into nitrogen, phosphorus and potassium interactions through the GRDC’s More Profit from Crop Nutrition program.

He says improving the efficiency of cereal plants in recovering and using nitrogen is one of the biggest steps to reducing the economic and environmental risks of applying excess nitrogen fertiliser.

“Typically, a wheat crop in a paddock will accumulate 30 to 50 per cent of applied nitrogen in the grain,” he says.

“What we wanted to know was whether this conversion could be improved if other key nutrients, such as phosphorus and potassium  were made more available to the plant.”

Dr Duncan says availability of phosphorus has potential to lift NUE because it can increase total root length, root proliferation and below and above-ground biomass, which directly affects the plant’s ability to capture nitrogen and store it in above-ground tissues.

He says having more potassium in the soil can improve NUE by assisting plants in the uptake of nitrogen, helping to redistribute nitrogen from leaves to grain and improving the formation of grain protein.

“Although there has been extensive research into the importance of nitrogen, phosphorus and potassium reserves in soil on crop performance, research quantifying how different combinations of all three alter root production and development are comparatively rare,” he says.

“Consequently, it is unknown whether these below-ground changes affect the ability of the crop to capture and utilise nutrients to increase grain production.”

Wheat growing in glasshouse

GRDC-funded glasshouse trials of nitrogen, phosphorus and potassium interactions at CSIRO Floreat.

PHOTO: CSIRO 

To investigate this further, CSIRO scientists used ‘rhizo-boxes’ that allowed them to assess (to 60 centimetres in undisturbed soil) the root growth and architecture of the modern wheat variety Mace (2010 release) compared to the historic variety Heron (1958 release).

These boxes were used because the study of root production in situ is inherently difficult and results in large errors that often mask ‘real’ trends or effects.

Dr Duncan says they found Mace accumulated about 11 times more nitrogen when the availability of phosphorus and potassium increased in soils and Heron accumulated about five times as much nitrogen under the same scenario.

He says when the availability of either phosphorus and/or potassium decreased to concentrations approaching critical levels, nitrogen uptake decreased considerably.

The next step was to assess NUE when Mace, Wyalkatchem and Westonia were grown to maturity under different combinations of nitrogen, phosphorus and potassium in soil.

“We found all of these varieties generally produced more grain and recovered more nitrogen when the availability of both phosphorus and potassium increased even beyond ‘critical’ levels, compared with situations in which soil phosphorus or potassium availability were increased individually,” Dr Duncan says.

“Assuming there are no other soil constraints to limit root growth and development, these results indicate that higher soil phosphorus and potassium levels have the potential to reduce the amount of nitrogen needed for crops to reach their rainfall-limited yield potential.

“This is likely to be due to crops having a more prolific root system in terms of depth, thickness and biomass, which becomes even more vital in drought-prone and low-rainfall areas where subsoil moisture is further down the profile.”

Dr Duncan says, in theory, increasing soil phosphorus and potassium has the potential to boost NUE, reduce the need for extra nitrogen fertiliser, enhance crop WUE and provide major benefits in environments with drying climates.

“The next step will be to validate these results under commercial paddock conditions,” he says.

More information:

Dr Elliott Duncan,
elliott.duncan@unisa.edu.au;

Dr Margaret Roper, CSIRO Agriculture,
08 9333 6668,
margaret.roper@csiro.au

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