Organic matter benefits might prove over-optimistic – Agronomist & Arable Farmer

Is organic matter so important when it comes to optimising crop yields?

This was one of the questions raised at the recent Hutchinsons Agroecology Conference. Rothamstead Researcher Professor Andrew Neal suggested that the industry is preoccupied with organic matter.

Despite plenty of discussion around the benefits of increasing organic matter in the soil, especially as part of a regenerative system, Prof. Neal explained that there is a lot about soils that we don’t really know. He advised that growers should not boost organic matter for the sole reason of increasing yields.

Data from long-term experiments in Europe highlighted that straw and green residue, straw, and slurries added little to final yield, and farmyard manures only increased yield by around 2.5%. Increases also depended on the crop itself, with potatoes seeing a +5% yield boost, but most saw very little change. Interestingly, autumn sown cereals could actually see a decrease in yield due to organic matter, while spring cereals did see a small increase.

“So, there is not a lot of evidence to suggest that increasing organic matter can lead to an uplift in yield,” he said.“ Therefore, this should not be at the forefront of our minds when moving towards agroecology.”

He also questioned the value of carbon sequestration schemes. Referring to a report from the Institute for Agriculture & Trade Policy (IATP) from November 2020 he highlighted that it suggested offsetting schemes can be damaging to farmers, with some endangering food security and limiting autonomy. The same report said that the impact of methods to sequester carbon-and market this as carbon credits-have dubious benefits for the climate and can be difficult to quantify, with extreme weather events such as drought causing organic matter levels in the soil to drop.

“A key problem is that these schemes place the responsibility at the feet of the farmers, when the majority of agricultural emissions take place outside of the farm and are driven by large corporations in the production of fertilisers and pesticides,” he noted.

“When you look at evidence showing that total soil sequestration potential is only equivalent to between two and seven years of fossil-derived emissions, we have to accept that we are not adding organic matter to our soils, nor should we, for the carbon market opportunities.”

However, after what started on a slightly negative footing Prof. Neal then took a more positive tone. Despite the small yield gains from increased organic matter levels, it shouldn’t deter growers from striving to boost soil organic matter.

Prof. Neal explained that the benefits of organic matter are seen at a micron level within the soil. Higher levels of organic matter increased microbial activity and formed more porous soil structure, creating pathways for root structures, and allowing greater water absorption, though strangely this was only in clay soils.

“Texture determines the response to organic inputs,” he said. “Studies show that just 1% increased organic matter can improve the water holding capacity of silty clay loams by 354,000 litres per hectare.”

More porous soils can also improve nutrient use efficiency and increase oxygen levels throughout the soil profile, minimising the risk of anaerobic patches. “These all serve to build resilience in the soil,” he concluded. “You may not notice the difference in a normal year, but during extreme weather events your soils will be able to maintain yields.”

Yield was one part of the presentation by Shropshire farmer John Bubb and McCain vice president of agriculture, James Young. McCain Foods has committed to sourcing 100% of its potatoes globally from regenerative farming practices by 2030. Potatoes are not a crop talked about frequently when it comes to reducing tillage, but Mr Bubb – who grows 186ha of potatoes for McCain – explained how he has brought the crop into a regenerative system.

Since 2019, the business has integrated cover and catch crops into the rotation, using a mix of linseed, buckwheat, phacelia, vetch, clover, and oats, which is grazed off or sprayed off if necessary. The business has also minimised its chemical use, avoiding soil sterilant since 2020 and cutting its nematicide use to just 8%,as well as adapting its nutrient plan to include more organic manures and trace minerals.

“Comparisons between our conventional crops and the crops established with regenerative practices had the same marketable yield, but with a £345/ha saving,” he said. “We’re now involved with trials to limit our herbicide use with re-ridging and are planting companion beans and peas crops alongside the potatoes.”

To assist growers like Mr Bubb transition to a more regenerative system, Hutchinsons will publish an Agroecology Charter later this year – the first of its kind, according to the company.

The company sees agriculture at a tipping point when it comes to agroecological methods, many of which can be interchanged with regenerative agriculture.

Hutchinsons expects more conventional farmers to take up regenerative methods, either to reduce the cost of inputs, to capitalise on options within the SFI, or due to pressure from the government, retailers and the public.

This is why Hutchinsons see the need for the charter. The company hopes it will provide a clear definition of agroecology, as well as outlining key management processes, the risks associated with them and a useful aid in demonstrating agroecology benefits.

It was a focus that Mr Foyle picked upon, saying that quantitative and qualitative evidence would help growers to backup agroecology claims. “We’re already collecting a lot of data that help to quantify our claims around agroecology,” he said. “We have yield data, input data – even if this is just invoices from suppliers – and details of fixed costs such as labour, fuel and machinery expenditure. This is enough to get a base-level cost of production and start comparing different systems and rotations.”

When this is combined with qualitative data, such as soil nutrition levels, pH and organic matter, worm counts, Visual Evaluation of Soil Structure (VESS) scores, water infiltration and more, it cannot inform management decisions but will give an indication of how changes are impacting the soil health. “This grasp of figures will become increasingly important as growers balance their margins with outside pressures,” he concluded.