It is well-known that the inclusion of a legume can improve the overall health of a crop rotation.
But how these benefits translate in different management systems is being explored further by Catriona Willoughby, a PhD student at Scotland’s Rural College (SRUC) and NIAB.
Her work has included field trials assessing three management systems across the UK: a conventional site in Norfolk with low legume usage over the last nine years; an organic farm in north east Scotland with a mixed organic rotation which incorporated livestock and had three-year grass and white clover leys; and an organic rotation which was livestock free and included a one-year grass and red clover ley, plus faba beans one year in every six, and cereal crops undersown with clover.
After carrying out topsoil sampling and a combination of chemical and biological assessments, some big differences between sites were identified.
Ms Willoughby said: “This was not entirely unanticipated, given the differences in geography and management.
Soil health indicators “But what was really interesting was in this divide, our biological soil health indicators very clearly seemed to favour the organically managed site, while most of the chemical indicators were very much the other way towards the conventionally managed rotations with low legume inclusion.”
Delving deeper into the soil biology, the organic, legume driven rotations with three-year grass and white clover leys and FYM additions were better performing than stockless rotations with short red clover leys, grain legumes and undersown cereals.
When worm counts and organic matter were assessed, the mixed organic rotation with the three-yearley and grazing livestock with manure additions, was the highest performing of the rotations considered, followed by the stockless organic rotation with its shorter red clover ley.
All of the conventional rotations performed similarly, regardless of changes in organic amendments across them, and had lower soil organic matter and worm counts than either of the organic rotations.
“The story does change somewhat in the case of potentially mineralisable nitrogen [PMN],” Ms Willoughby said.
“In conventional rotations, those with no organic amendments and those treated with green waste compost have the highest amounts of PMN in their soils.
“As a general rule, PMN was lower in the organic rotations which we believe is the lack of nitrogen fertiliser.
We did notice the mixed organic rotation had a higher PMN than the stockless rotation.
“In terms of soil nutrient content other than nitrogen, we were expecting to see the conventionally managed rotations with their fertiliser additions and their other external inputs would have a higher nutrient content, particularly in the case of P and K, but that wasn’t what we found.
Some of this is likely down to the nutrient status of the underlying soils of these very different sites.
“Given they had all been under these systems for at least 10 years, it was interesting to see there was no big differences in terms of the soil phosphorus content across any of the rotations.
“For potassium, there was almost double in the convention al green waste and paper crumble amended rotations as in organic stockless rotation, which had the lowest levels of potassium.
For calcium, we saw this huge number from the paper crumble.
All the others were about the same, but we did see lower levels in the organic rotations as well.”
A visual evaluation of soil structure (VESS) was also carried out at the time of sampling, with a higher VESS score indicating a poorer soil structure.
Structure “What we found wasn’t necessarily what I was expecting because I thought that generally a long grass and white clover ley may drive the score down.
“Although it was generally the organic rotation with longer ley period that had slightly lower scores than the stockless rotation, we found that conventional rotations with low legume inclusion appeared to have a better structure and this was quite uniform across the site, regardless of organic amendment.”
Ms Willoughby added that the higher rainfall received by the site in north east Scotland may have led to increased VESS scores in organic rotations compared with the conventional rotations.
Work to assess the structural attributes of the soils at both sites in greater detail is ongoing.