Soil nitrogen use might not be used 100% efficiently

The current assumption by government’s fertiliser recommendation bible, RB209, that soil nitrogen was used 100% efficiently was a gross over-simplification, Stuart Knight of The Arable Group said.

His assessment was made based on a one-year HGCA-funded project he led, along with Rothamsted Research‘s Andy Macdonald, aimed at improving understanding of the soil nitrogen supply.

“As a national average the measurement of soil mineral nitrogen is a reasonable guide to the amount of N the soil might supply, but for each individual situation this shouldn’t be taken too literally.”

It was one of the reasons why it was difficult to find a consistent relationship between measured soil mineral nitrogen levels, particularly between 0 and 100kg/ha N, and optimum fertiliser dose, he explained.

Crops grown on sandy soils and/or where lots of rain fell were likely to use soil N less efficiently than on clay soils or in dry conditions, where the same amounts of soil N were present, because of higher leaching, the project found.

Mineralisation of N in the soil was another key factor, as that also supplied N to crops. “One way of improving accuracy of fertiliser dose would be to accurately estimate the amount of N that might be mineralised,” he believed.

A new three-year HGCA project would be looking at a range of test kits to improve these estimates, he said.

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