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Soil-health focus pays yield dividends (Sept 23)

The soil biome is key to grassland productivity gains. So what is it and how can producers work with their soils to improve grassland yields and forage quality, and reduce input costs?


The drive to optimise grass quality and productivity has traditionally focused on two or three aspects of soil health – the physical and chemical. Both are well understood and, typically, well managed. But the third aspect, soil biology, has received less attention.

Independent plant and soil health specialist Joel Williams says it’s an area with huge potential. “The assumption is that soil organic matter is predominantly made up of decaying plant material, but research now shows that between 60% and 70% of grassland soil organic matter is microbial remains or ‘necromass’, and two thirds of this is fungal.”

“While the ‘optimum’ balance of living bacteria and fungi is still widely debated, the contribution of microbial necromass to soil organic matter indicates that it is critical in grassland productivity, and there’s a strong argument for encouraging it to thrive within our farming systems.”

Science shows that fungi and bacteria have important roles in soil aggregation, nutrient cycling and defending plants against disease and pests. All have a significant impact on grass quality and yield.

Soil structure

“These are aspects of pasture management that producers have spent significant amounts of time and money maintaining,” says Sylgen Animal Health’s Romney Jackson. “So consider the cost savings when microbes are doing these jobs instead.”

“Building and maintaining soil structure is a good example,” he says. “It is mainly about aggregation, and the spaces between them, allowing water, air and plant roots to infiltrate soil, giving grass access to the resources it requires.

“Historically we’ve used cultivations ahead of re-seeding, mole-drains and other interventions to optimise the physical aspects of soil health. Going forward we could save time and costs by focusing on supporting the microbes that live in the soil.”

Mr Williams describes soil aggregation as a two-step process: “Fungi play a more dominant role, but bacteria are also required,” he says.

“Digestion by soil microbes happens externally. They excrete various enzymes and metabolites – substances that break down their food and function similarly to stomach acid. And many of these are also sticky so they chemically pull and hold extremely small soil particles together.

“Fungi produce long strands and threads, known as hyphae. As the hyphae penetrate the soil, they physically pull those micro aggregates together into bigger, or macro, aggregates.”

Extreme weather impacted grazing in summer 2023, and improved soil structure, aided by microbial activity, could go some way to help here.

Increased yields: slurry inoculant offers a good return on investment

Good aggregation

“Rather than water being lost through runoff and then sitting on the soil surface and evaporating, good aggregation means that heavy rainfall can filter down deep into the soil profile and is retained for grass to use during periods of drought,” says Mr Williams. Improving soil structure is just one useful function of soil microbes. Arguably their greatest benefit to dairy production is through their cycling of nutrients.

“Microbes take the nutrients they need from two sources – organic matter and soil particles/parent material. The microbes’ metabolites contain a range of substances, including enzymes, organic acids and siderophores, for breaking down compounds,” says Mr Williams.

Enzymes and siderophores – molecules that bind and transport iron in micro-organisms – have different modes of action, but both are specific in the nutrients they unlock, unlike organic acids, which solubilise a range of nutrients.

“Some microbes are ‘specialised’ and are adept at scavenging for certain nutrients. It’s fungi more than bacteria, for example, that produce ligninases – enzymes that break down lignin, which is the compound that gives plant cell walls strength and rigidity.” While plants are also capable of producing similar substances, they are much less efficient at unlocking nutrients, and this is the reason why they work closely with soil microbes.

“Plants primarily focus their resources on feeding microbes with exudates – a carbon-rich food source for microbes excreted through plant roots – rather than breaking down soil minerals and organic matter themselves,” explains Mr Williams.

“With financial and environmental drivers discouraging the use of synthetic fertiliser, microbial cycling of nutrients could be increasingly important within dairy production,” adds Mr Jackson. “As well as unlocking major nutrients, including nitrogen, phosphorus and potassium, bacteria and fungi can supply grass with trace elements, such as iron, zinc and copper. Microbes have the unique ability to do this as and when plants require it, ensuring optimal growth.”

Mr Williams explains that plants can change the composition of root exudates to encourage the microbes with the abilities they need and discourage those they don’t. “Sometimes plants will produce more generalised root exudates, but at other times it will be more specific and depends on what they need.”

Unintended consequences

Unfortunately, applying fertilisers can have unintended consequences. With an abundance of nutrients in the soil, plants no longer need microbes and, after successive generations, can lose the ability to recruit them.

“Nitrogen fertiliser, for example, is simply too much for the rhizobia,” says Mr Williams. “Similarly, raw slurry could be good for the soil biome but not in the quantities it’s typically applied in.”

He adds that there are three ways producers can better utilise slurry for the soil biome. Applying less but more often is ideal, but often not practical on-farm. But using additives, like humic or fulvic acids, can help. “These materials, which can be added to the slurry tanker at the time of application, act like a carbon sponge. With their high exchange capacity they bind some of the nutrients up, lowering the immediate availability but creating a slow-release fertilising effect,” he says.

“Slurry inoculants are well worth investigating. The idea here is that the microbes will consume the nutrients, stabilising them within their bodies and lowering the initial high nutrient load.”

Mr Jackson agrees: “Slurry inoculants can be an extremely useful tool but they aren’t all the same. SlurryForSoil, for example, has been designed specifically for plant and soil health, and contains 18 different plant-growth promoting rhizobacteria and fungi. Each have been chosen for their proven abilities to contribute to grass quality and productivity.”

In an independent trial by Duchy College in 2022, the additive was shown to retain major and minor nutrients in slurry and, when applied to pasture, increased forage yield and quality.

“Interestingly, it was impossible for the application of nutrients to have accounted for all the gains. So the researchers concluded that it was due to wider beneficial effects of the soil-borne microbes.”

“The results mirror those of producers who have been trialling SlurryForSoil on-farm. With increased yields and reduced need for synthetic fertiliser, they’ve seen an average return on investment of £136 per hectare.”

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