The root model consists of three main parts:

• the physical extension of the root system
• the total root length of the crop
• the distribution of the root system depending on depth and distance from the crop row

The development of the root system depth is calculated using accumulated temperature from the day the crop was planted. After a lag period (ddglag) the rooting depth increases linearly with accumulated temperature. The length of the lag period and the rate of rooting depth development are controlled with crop specific parameter values. A crop specific base temperature (Tmin) for the calculation of root growth is also used and Tmax is then set to Tmin+20 °C. Horizontal root extension is calculated in the same way, but for each soil layer the calculation starts when the roots reach this layer rather than when the crop is planted. In this way horizontal root growth starts progressively later at larger depths.

Crop root length is then calculated as a function of

• crop biomass
• crop growth stage
• the parameter value of root class “Rtclss”.

The root biomass is calculated as a function of aboveground crop biomass, a fraction which declines with crop size, but increases with Rtclss (1<2<3), to allow for crops with different root/shoot ratios. Total root length is then calculated from the simulated root biomass and a fixed specific root length which is used for all crops.

Root length is distributed spatially into a 2D array of soil units. Root distribution is calculated to a maximum depth of 2 m, and to a maximum width of half crop row distance. The soil units used in this array are 0.05 by 0.05 m. The root length declines by a logarithmic function from the topsoil downwards and from the crop row to the interrow soil. The steepness of this decline is controlled by one parameter for the vertical distribution (az) and another parameter for the vertical distribution (ax). At the early growth stages where root width is less that plant to plant distance within the row, an arbitrary function is used to reduce N uptake capacity accordingly.

Nitrogen uptake is calculated as a function of crop nitrogen demand and potential root nitrogen uptake on a specific day. The simulated crop nitrogen demand is received from the crop growth part of the model. The potential supply from the soil is calculated as a function of the root length in each soil unit, the content of ammonium-N and nitrate-N in each soil unit and the value kN read from the Croptable, to control root nitrogen uptake efficiency.

A function is then used to balance actual nitrogen uptake according to crop nitrogen demand and potential root nitrogen uptake. When crop nitrogen demand and potential root nitrogen uptake are close to each other, the simulated nitrogen uptake will be below either value, but at very high or low nitrogen supply relative to demand, the uptake will be fully controlled by crop nitrogen demand and potential root nitrogen supply respectively.

Often the calculated actual nitrogen uptake will be lower than the potential root nitrogen supply. When this is the case, the actual depletion of soil nitrogen will reduced proportionally from the potential value in all soil units. At last, a specific calculation is made of nitrogen taken up from below 0.9 m in the soil. This is made as nitrogen leaching loss and other nitrogen balance figures are shown mainly from the 0-0.9 m soil layer in much of the model output, and it is therefore necessary also to have an output showing how much nitrogen is taken up from below this zone.