Influence of green manuring in wheat rotations under different cultivation systems

Abstract

Winter wheat was grown in each of two rotations (i) continuous wheat, and (ii) rotational wheat in a sequence alternating with natural regeneration set-aside. For each rotation, three cultivation techniques were used to establish the wheat crop: (i) plough, (ii) minimum tillage (discing, one pass) and (iii) direct drill. Each treatment combination was then divided into two, one half being sown with white mustard as a green manure crop, prior to sowing the wheat crop. For the continuous wheat blocks, this involved sowing the mustard immediately after the harvest of the previous wheat crop, and for the rotational wheat, sowing mustard in spring in the set-aside stubble. In both cases the mustard was sprayed off prior to sowing the next wheat crop. The trial was run at Cirencester in Gloucestershire (medium soil over gravel) and Andover in Hampshire (light, shallow chalky soil). The four years of the project allowed four continuous wheat crops and two first wheats (after set-aside) to be examined, in terms of the influence of cultivation method on soil properties and microbiology, and crop yield, and the interaction of these with the growing of a green manure.

Soil analyses of microbial biomass (assessed as biomass carbon), and of microbial respiration (assessed as carbon dioxide release) indicated higher levels of these where non-inversion cultivations were employed (discing and direct drilling, compared to ploughing). Subsequently, higher levels of microbial biomass and respiration were also recorded where green manure had been incorporated, assumed to be a result of soil microbes feeding on the newly-introduced substrate. These changes in soil biochemistry were always greater in the deeper, heavier-textured soil at Cirencester than in the shallow chalk soil at Andover.

Yield increases in the wheat crops, as a result of green manuring, were not consistently evident until year 4, and then only in the rotational wheat with the relatively heavier soil. The soil assays which showed improvements in soil microbiology are known to be sensitive to early changes: it is to be expected therefore that translating these into increased yield would take time (increases in soil mineral N in early spring, following green manure incorporation, were noted, but only in the order of 20kg/ha or less).

Growing mustard as a catch crop between continuous wheats was not successful, due to the unpredictability of suitable rainfall in the short establishment window, and green manuring in general was not compatible with direct drilling, due to the lack of cultivation which would incorporate the mustard and thus benefit its breakdown and the wheat crop's establishment.