“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.