NVZs – Arable farms: The NMAX limit and Crop N Requirement

This term has become synonymous with the new NVZ rules, but isn’t as hazardous as many farmers fear, says Andersons’ consultant Oliver Lee.

“NMAX effectively means the maximum permitted amount of nitrogen from any source that a crop may receive. However, it doesn’t apply to all crops – for example, oats aren’t subject to this.”

For instance, our example farm, Meadow Farm, has five crop types, including grass. Based on average yields, the following NMAX values apply.

Winter Wheat (8.5t/ha yield) =220kg/N/ha

Winter Barley (6.5t/ha) =180kg/N/ha

Winter OSR (3.5t/ha) =250kg/N/ha

Winter beans =0kg/N/ha

Grass =330kg/N/ha 

“However, there are variations. For instance, there will be different limits given for milling wheats, or for grass depending on the number of silage cuts. You’ll also need to have record to show you comply with NMAX to meet inspections.”

Crop N requirement

“This is a separate calculation,” says Mr Lee. Everyone in an NVZ needs to work out the nitrogen requirement for each crop in each field. It’s possible that several fields with the same crop will be the same, but you do still need to make the calculation and show that you’ve done it. The results will show if you comply with NMAX (above).

Essentially there are four steps. Having identified the field, its crop, and any references like field numbers, the first step is to assess the available nitrogen already in the soil. This is called the Soil Nitrogen Supply (SNS). “This can be done in two ways, either by having the soil tested, or by using RB209, which assesses it based on rainfall, cropping history and soil type.”

You can buy a hard copy of the RB209 fertiliser handbook here

The next step is to determine the requirement for the specific crop you have planted. RB209 can provide the answer, or an agronomist or FACTS-qualified adviser.

Step three is to assess the manure applications, and their frequency, given to the crop, and, of those, the crop-available N in that manure. “You can either use the default values given in the DEFRA booklets or have manure samples analysed.” says Mr Lee.

The final step is to establish the total crop requirement, less any nitrogen from manures. This give you a balance, which is the quantity to be applied in the form of manufactured fertiliser.

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