Nitrogen management decisions remain tricky

Nutrient management decisions remain tricky this season, despite low winter rainfall leaving soil mineral nitrogen plus crop N levels above average in many parts of England, ADAS says.

In its annual report on winter rainfall and its impact on nutrient management decisions, ADAS highlights that the area with less than 150mm excess winter rainfall this winter extends west into parts of Dorset and Somerset and the Welsh Borders, and from Staffordshire up the east coast of England to Northumberland.

Growers use excess winter rainfall amounts to the end of March to decide which rainfall table to use in the Fertiliser Manual’s field assessment method for determining soil N supply.

Usually only East Anglia, Lincolnshire and parts of Yorkshire enter the spring with less than 150mm of excess winter rainfall.

“Unless there is heavy rain over the next few weeks, most farms in large parts of England may be justified in using the low rainfall table in the Fertiliser Manual,” ADAS suggests.

The table tends to reduce the recommended N rate, as it assumes N leaching losses over winter have been low, and soil mineral N plus crop N levels are higher than average.

“But growers must make sure other factors support this approach before making any decisions to reduce the total amount of N to the crop,” ADAS stresses.

These include considering last year’s crop performance, the weather since harvest and the number of tillers and green area index in autumn-sown crops, it says.

For example, the mild autumn has promoted crop growth in many regions, meaning crops could have taken up much of the soil mineral N left in the soil last autumn.

In parts of the west and north-west where winter rainfall has been close to or above average, it is still important to evaluate previous crop performance and winter weather, the report says.

“Some later drilled crops are short of tillers and growth. Is it because of drilling date or due to low soil N supply? The latter is more likely to be true on light/shallow soils where the previous crop yielded well and winter rainfall is at or above average.”

Assessing soil N supply in each field is important to making the correct decision on N use, and within a Nitrate Vulnerable Zone mandatory part of nitrogen planning that must be carried out and recorded before any nitrogen as manufactured fertiliser or organic manure is applied.

Hutchinsons testing results

Soil mineral N testing by Hutchinsons has shown more variability than normal, the firm’s Matt Ward says.

“We’ve seen a much greater discrepancy between the highs and lows from the more than 600 samples we’ve taken on farm.

“The commonly aired consensus this spring is that over-wintered crops are forward, lush and frothy and that nitrogen levels are higher than average. I think that overall this is the case and I share the view that the total dose will be down this spring, but I think it will be in the region of 20% rather than the 30-40% reduction that has been suggested.”

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