RESIDUAL SOIL nitrogen levels are extremely variable this season and inaccurate assumptions could be costly to yields, Kemira GrowHow has warned.

While some patterns have emerged, such as heavier soils appearing to have lower residual N levels, generalisations are dangerous, explained the firm‘s agronomist, Allison Grundy.

“Over applications will cost more, let alone implications for NVZs and the environment, but assumptions about residual N levels being higher than they actually are, will be potentially damaging to yield.”

One example from 130 core samples showed the amount of residual N in silty clay loams in the East and SE ranged from 15kg N/ha to 75kg N/ha and similar variation was found elsewhere, she said.

The reason for the variability is because relatively low winter rainfall means the amount of residual N remaining in soils has been complicated by several factors, she said.

Mild temperatures have increased N mineralization in many soils, making more available for crops to use and thus reducing residual levels.

In other parts, lying water has caused denitrification and loss of N to the atmosphere, Ms Grundy said.

The amount of organic matter in soils will also influence the ‘Potentially Available Nitrogen‘, she added.

“The only really safe option when developing this year‘s fertiliser plan is to measure the actual quantity, real time.”

Further nitrogen management advice based on the company‘s research is available from www.kemira-growhow.co.uk.