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28 Sep 2021

How different systems stack up for soil health

As featured in Arable Farming Magazine

How different systems stack up for soil health

by Arable Farming

The power of pulses was discussed at an Association of Applied Biologists conference, which explored advances in legume science and practice from intercropping to soil health. Alice Dyer reports.

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 Scotlands 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 , 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 wasnt 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 wasnt 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.

What is influencing the bean yield gap?

There are a number of factors that influence bean crop yield, from weed pressure and seasonality, to pollinators and farmer experience.

As part of the LegumeGap project, three factors associated with yield were explored irrigation, inoculation and tillage.

Christine Watson, professor of agricultural systems at SRUC, said: We know beans dont like much water, or too little.

We looked at relative yield of irrigated beans versus a rain-fed system.

In both cases, irrigation increases yields.

The benefits from irrigation in moisture-deficit situations are so clear that farm-scale investment in irrigation infrastructure is probably more widely warranted.

Although inoculation of field beans with bacteria to promote nodulation is often seen as unnecessary in European situations, the data showed that it provides benefits, but having the right bacterial race is important.

Beans were grown in four ways: alongside no rhizobium inoculum, with rhizobium inoculum targeted at the bean, other inoculum given to the plant, and inoculated with local soil.

Prof Watson said: The wrong inoculum can be worse than no inoculum.

In the field where beans have been grown before, you see a very positive impact of that.

That reflects in the nodulation on the roots of these beans.

The bean inoculated with local soil has the best root system, perhaps showing the adaptation of the soil microflora to the bean crop.

Inoculation The biggest increase in relative yield in inoculated over uninoculated was where there was no history of beans having been grown in that soil.

Theres a big impact from the soil history.

We saw three years of successive crop failure in our rotational trials in Aberdeen.

We almost convinced ourselves it was a micronutrient effect, but as soon as we used some commercial inoculum, weve never had a bean failure since.

Meanwhile, zero-tillage provided a clear benefit to faba bean yield over ploughing, but min-tillage did not (see graph).

For carbon conscious farmers, growing a grain legume one year in every five can save 30kg of N/ha.

Prof Watson said: From a Scottish perspective, this is around 1.4% of Scotlands agriculture emissions.

Bees found to favour certain bean lines

Significant differences have been observed in bee visitation rates in different field bean lines, showing some varieties are more attractive to bees than others.

Field beans benefit greatly from bee pollination, with studies showing it can enhance crop yield by more than 50% compared with plants grown in the absence of pollinators.

Jake Moscrop, a PhD student at the University of Cambridge, is exploring strategies to enhance pollination of field beans by developing crops which are attractive and beneficial to pollinators.

In the project, which he hopes will help inform breeders on which floral traits may help to attract pollinators, he is looking to determine how much floral variation exists between current field bean lines and which traits influence pollinator behaviour.

Characteristics He said: Variation has been observed in the floral traits for field bean lines and bees are able to use some of these characteristics as learning cues in controlled conditions.

One trait that varies considerably is sugar concentration in the nectar.

On average, the lowest sugar concentration I have observed was seen in BPL10 and the largest in the line LG Cartouche.

In field trials this year, differences were also seen in the visitation rates of bees to different field bean lines with contrasting characteristics.

Mr Moscrop said: When we break this down, we can see most of these visits were made by white-tailed bees and carder bees.

Is crop diversity always beneficial?

Around 10% of France and Switzerlands winter oilseed rape area is now grown alongside a companion crop, but as these crop mixtures become more complex, containing several species, a team of researchers is exploring how this benefits plant biomass, N fixation and soil health.

PhD student Xavier Bousselin, of Swiss research organisation Agroscope, said: Sowing oilseed rape with a service plant, which is usually a legume, fixes nitrogen and contributes to better cover of the soil which will outcompete weeds to stop or reduce the use of herbicides.

Then during winter, the cold temperatures will kill off the service plants.

In spring the dead mulch will mineralise and contribute to the nitrogen nutrition of winter oilseed rape.

This can reduce nitrogen use by 20-30kg per hectare without yield loss.

Although increasing the number of species grown could improve production and resource use efficiency, the relationship is not linear and sometimes crops will have already reached their peak, even when there are low levels of diversity, he said.

The glasshouse experiments found there was a slight decrease in plant biomass when OSR was intercropped with one or two legumes, and an even greater decrease when niger was added to the mix.

Mr Bousselin said: This decrease remains relatively small and faba bean, grass pea and niger together are still less competitive than the one OSR plant.

However, there were clear differences in nitrogen accumulation, with all mixtures that included faba bean having significantly higher amounts of fixed nitrogen.

The inclusion of grass pea gave much more variable results.

Mr Bousselin added: We also saw that nitrogen fixation tended to be increased when a legume was intercropped with rapeseed, but when we added niger it was not further increased, so the fixation was already at its maximum.

Contrary to literature, it was also found that the presence of faba bean did not increase soil microbial activity, which Mr Bousselin said would be further explored.

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