How much nitrogen do I need?

As winter reaches the half-way point, many regions in the UK have seen high rainfall accompanied by mild weather conditions. These factors may have had an impact on ‘over-winter’ nitrate loss and the level of crop nitrogen uptake. It is the balance of these two factors that will determine whether this season’s nitrogen inputs from fertiliser will be higher or lower than normal. Farmers should consider local conditions to decide which factor has been the most important.

Stuart Staples, Bunn Fertiliser Crop Nutritionist, talks about how you can calculate the amount of nitrogen you need.

To understand crop requirement you need to know:

     a)Total crop nitrogen requirement, based on previous experience of yield and crop quality.

     b)The nitrogen available to the crop from the soil (soil nitrogen supply) or, the nitrogen already taken up by the plant.

The difference between values a. and b. is then fulfilled by fertiliser application or organic inputs.

The Soil Nitrogen Supply (SNS), i.e. soil mineral N + estimate of N in the crop + estimate of soil mineralisable N, can be assessed using the ‘Field Assessment Method’ or by the ‘Measurement Method’, involving soil sampling and analysis.

  • The “Field Assessment Method” takes into account soil type, previous crop and rainfall. These values determine the SNS.
  • The “Measurement Method” uses a direct measurement of nitrogen in the soil to assess how much nitrogen is already in the crop  and an estimate of mineralisable nitrogen.

The SNS can be influenced by a number of factors, but in principle:

Field Information 
Soil Nitrogen Supply (SNS)
Low SNS
High SNS
Soil Type
Sand & shallow soils
Medium or heavy soils
Organic inputs
No organic manures
Regular organic inputs
Past management
Cereals, OSR, Potatoes,  Sugar Beet, Low N veg, Forage (cut)
Break crops, combinable crops receiving high N rates, Peas, Beans, High N veg
Autumn/Winter Crop growth
Poor growth & limited N uptake
Active growth & high N uptake
Rainfall
High rainfall
Low rainfall

With recent exceptionally high rainfall in many parts of the country, it is likely there will be a strong demand for nitrogen this season. Soils are likely to have been depleted of nitrogen, and there will be demand to get early nitrogen on to many crops provided they are not too far forward in their growth.