It also calculates crop nitrogen and nitrate contents, the distributions of water and nitrate down the soil profile and the amounts of nitrate leached below different depths from the soil surface. Weather files have been prepared to represent the daily mean, temperature, rainfall, and potential evaporation for different parts of the world. To run the model it is first necessary to select the most appropriate weather file and make any adjustments to monthly rainfall. Although it should therefore be possible to simulate N-response of crops grown in many countries, it must be emphasised that the validity of the model has only been tested in West Europe. In the model the soil is visualized as consisting of 20 consecutive 5 cm thick layers.
Roots develop laterally and vertically in the soil.
The volume of soil from which they can extract mineral-N increases with plant mass until a stage is reached at which further root development ceases or until the roots reach a hard pan or other barrier to root penetration. Fertilizer and crop debris are incorporated in the uppermost layers of soil. It can be taken up by plant roots, can be leached downwards during rain or can move upwards during evaporation from the soil surface. Crucial to the estimation of plant growth are two crop-nitrogen parameters, the critical N concentration and the maximum possible N concentration, and the way they decline with increasing plant growth. This information is provided by a file ancillary to the program.