High levels of rainfall and the continued bad weather in the first months of 2013 have left many farmers concerned about the level of nitrogen losses that may have occurred.
Some advisers are recommending that more nitrogen be applied, as a matter of course, but with ammonium nitrate at around £300/t the cost of applying extra nitrogen cannot be ignored.
Targeting additional nitrogen applications to those fields which actually need it makes good economic sense.
Nitrogen losses will mainly come from leaching but where soils have been waterlogged or even underwater for long periods denitrification may also occur.
According to Sean Stevenson from analytical services specialists, NRM, "The exact extent of nitrogen losses through leaching and/or denitrification following heavy or sustained rainfall can be hard to quantify."
"It is the nitrate form of nitrogen that is susceptible to loss though leaching and denitrification so the extent of any loss is governed by the amount of the crop nitrogen supply that was in the nitrate form when the rainfall occurred."
Other factors also influence the extent of nitrate losses, application timing, soil characteristics the type of nitrogen fertiliser and so on.
In general leaching losses are more likely on sandy soils and denitrification more apparent on medium and fine textured soils that have poor drainage.
However the only way to be sure of how much nitrogen is in the soil is to conduct a soil mineral nitrogen test.
Stevenson explains further: "Soil Mineral Nitrogen is the nitrate-N plus ammonium-N content of the soil within the potential rooting depth of the crop."
"When estimating the total amount of N that will come from the soil – that's the Soil Nitrogen Supply (SNS) – you also need to include an estimate of the N already in the crop and an estimate of the mineralisable soil N. A test is also available from NRM to determine this mineralisable value.
"Once you have your SNS index you can use The Fertiliser Manual (RB209) to determine the correct fertiliser nitrogen recommendation."
