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When do retained stubbles increase the need for nitrogen?

Microbes use nitrogen (N) from the soil to decompose stubbles. When stubbles have a high carbon: nitrogen (C: N) ratio, soil mineral N is tied-up. Cereal stubbles have a high C: N ratio of around 80:1. Canola stubble C:N ratios are higher and legume stubbles are lower.

As stubbles break down soil mineral N is immobilised until crop residues decompose to have a C: N ratio of around 15:1. The question is if immobilisation of N will coincide with the early growth of the following crop.

If it does, extra N fertiliser may be needed to get the following crop off to a good start. Approximately 10 kg of additional N is required to break down one tonne of cereal stubble. A 6 t/ha wheat crop, leaving about 8t/ha of stubble may require up to an extra 80 kg N/ha at sowing to ensure the young crop can access enough N.

In time, the N becomes available as the stubble decomposes. Rootzones with good microbial activity can hold up to 40 kg/N/ha in the root zone for later release. Healthy root zones also reduce N leaching, keeping it where the plants can use it.

Burning stubbles makes some N available immediately, however up to 80% of the stubble N content is lost during the burn.

You may need to apply a similar amount of N fertiliser to compensate for immobilisation of N as to replace N lost through burning. Applying N to compensate for temporary immobilisation builds up paddock nitrogen stocks. Replacing N lost through burning maintains nitrogen levels.

Retaining stubbles also improves soil carbon levels and helps soil biological fertility.

 

More Information

What happens to nutrients when stubbles burn

What happens to urea with high residue loads

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