Denmark is an intensively farmed country and is surrounded by shallow coastal waters, sensitive to nitrate pollution. Due to this, nitrogen (N) regulation in agriculture has been earlier and stricter than in most other countries. Catch crops have been an important part of this regulation for the last 30 years, leading to substantial research, testing and experience in catch crops.
This paper deals with research directed into understanding and improving catch crop effects. I will focus on three aspects of catch crop optimisation:
1) catch crop root growth,
2) the catch crop N effect on the succeeding crop through N mineralisation and pre-emptive competition, and
3) the effect of catch crop placement in crop rotations.
The challenge with N is that it leaches down the soil profile, and therefore it is obvious that fast deep rooting will be beneficial. Early sowing promotes deep rooting, but the choice of catch crop species is even more important. Among common catch crops, the rate of root depth penetration by oilseed radish is almost three times higher than that of ryegrass, and such differences have been shown to have a great effect on the ability of catch crops to catch N and reduce nitrate leaching loss.
Much work has gone into understanding N mineralisation from catch crop residues, in order to predict their N effect for succeeding crops. However, mineralisation is only “half the story”. When catch crops take up N, which would otherwise have remained as available N in the soil, catch crop N uptake occurs in a pre-emptive competition with the succeeding crop. The total N effect of a catch crop on the succeeding crop is the combined effect of mineralisation and pre-emptive competition. Especially in retentive environments, with loamy soils and low precipitation surplus, the N effect of catch crops often becomes negative, due to pre-emptive competition.
Placement of catch crops within crop rotations is important, taking into account both what was grown previously and what is to be grown after the catch crop. It is obviously advantageous to grow them where the pre-crop is harvested early, and where it leaves much N in the soil, but this is also a good rotational position, where farmers often prefer to establish e.g. winter wheat. Catch crops lift N in the soil, so more is available in the topsoil and less in the subsoil after growing a catch crop. Due to this effect, catch crops should preferably be grown before shallow rooted crops, as this will optimise their overall effects on N leaching and N supply for succeeding crops.