Vertical and horizontal development of the root system of carrots following green manure

Cover crops grown as green manure or for other purposes will affect nitroge n (N) distribution in the soil, and may thereby alter root growth of a succ eeding crop. During two years, experiments were performed to study effects of nitrogen supply by green manure on root development of carrots (Daucus c arota L). Total root intensity (roots cm(-2) on minirhizotrons) was signifi cantly affected by the green manures, and was highest in the control plots where no green manure had been grown. Spread of the root system into the in terrow soil was also affected by green manure treatments, as the spread was reduced where spring topsoil N-min was high. Although N supply and distrib ution in the soil profile differed strongly among the treatments, no effect was observed on the rooting depth of the carrot crops. Across all treatmen ts the rooting front penetrated at a rate of 0.82 and 0.68 mm day(-1) degre es C-1 beneath the crop rows and in the interrow soil, respectively. The mi nirhizotrons only allowed measurements down to 1 m, and the roots reached t his depth before harvest. Extrapolating the linear relationship between tem perature sum and rooting depth until harvest would lead to rooting depths o f 1.59 and 1.18 m under the crop rows and in the interrow soil respectively . Soil analysis showed that the carrot crop was able to reduce N-min to ver y low levels even in the 0.75 to 1.0 m soil layer, which is in accordance w ith the root measurements. Still, where well supplied, the carrots left up 90 kg N ha(-1) in the soil at harvest. This seemed to be related to a limit ed N uptake capacity of the carrots rather than to insufficient root growth in the top metre of the soil.