Crop residue management determines whether the nutrients they contain are returned to the soil or removed and replaced via fertiliser.
Grain crop residues contain varying amounts of plant nutrients such as nitrogen, phosphorus, potassium, sulphur, and magnesium (see Table 1).
Table 1: Nutrient content of crop residues (Source FAR Arable Extra, Issue 103, December 2013)
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Approximate content (kg per tonne of residue)
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Wheat straw
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Barley straw
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Oat straw
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Ryegrass straw
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Nitrogen
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5.9
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4.6
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5.3
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10.1
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Phosphorus
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0.7
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0.4
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0.5
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1.0
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Potassium
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12.8
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12.9
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21.2
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13.8
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Sulphur
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1.2
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1.3
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1.0
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1.4
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Magnesium
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0.7
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0.7
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0.4
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1.2
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Weighing up the options
“The various options for managing crop residues – whether it be removing, burning, or incorporating them back into the soil – all have their own challenges and opportunities,” says Ballance Agri-Nutrients Science Extension Officer Jessica Hall.
“Residue that is baled and removed off farm has an economic value, but you also lose almost all of the nutrient content of the residue. So, it’s wise to compare the value of the straw to the cost of replacing the nutrients via fertiliser.”
The economic cost of straw nutrient losses can be calculated using the Foundation for Arable Research (FAR) spreadsheet. Results can be tailored by changing the bale weights and fertiliser prices.
Burning residue results in losing most of the nitrogen, sulphur, and carbon content of the residue as gas. But about 80 per cent of the potassium, 60 per cent of the phosphorus and most of the calcium and magnesium content of the residue are retained, and are returned to the soil as ash.
“By letting the crop residues decompose, nutrients will slowly be released. The rate of decomposition and release depends on the residue type, size, if and how it’s incorporated back into the soil, and the nutrient itself,” she says.
“For example, barley straw decomposes faster than oat straw, which in turn is faster than wheat straw. Smaller pieces of residue decompose faster than larger pieces. Incorporating residues back into the soil by using discs instead of ploughing mixes them more thoroughly into the soil, so they break down faster.”
Residues can also be left on the surface and the next crop can be direct drilled into them.
Both incorporated and surface-retained crop residues have a high carbon to nitrogen ratio, which can affect the availability of soil nitrogen. As the soil organisms decompose residues, they take nitrogen from the soil. Using fertiliser containing nitrogen to support decomposition is not always reliable, but drilling nitrogen fertiliser, such as DAP, with seed provides sufficient nitrogen (and phosphorus) to support early crop development.
“When plant material breaks down, the rate of nutrient release varies. Potassium’s generally released pretty quickly as it’s not ‘built in’ to plant cells; after five weeks about 50 to 90 per cent of the potassium content will be released. Nitrogen, phosphorus and sulphur are integrated into plant cells so are released more slowly.”
FAR research has concluded different residue management systems are unlikely to result in major differences to soil organic matter, but recent research suggests removing crop residues limits earthworm abundance and biomass.
“Whichever residue management option you choose, ensure you consider the nutrient removal or retention when selecting the right fertiliser for your next crop,” says Jessica.
THIS PROMOTIONAL FEATURE WAS SUPPLIED BY BALLANCE AGRI-NUTRIENTS