What does soil health look like for grassland, and how it can be improved to help support slurry utilisation and forage production? We spoke to two specialists to find out more.
TEXT NATALIE REED
Improved arable soil management, to enhance its health and biology, is increasingly seen as a way to lower costs by decreasing fertiliser requirements and the use of crop-protection products while increasing productivity and lowering farming’s impact on the environment. But could the same be true for grassland?
“There’s definitely scope for improvement,” says ADAS soil scientist Anne Bhogal. “A deeper understanding of soils and how they function, and thinking about what that means for grass productivity, will allow gains to be made.” Slygen Animal Health’s Geoff Hooper agrees. “Soil is producers’ greatest asset. It works hard and, on many units, soils could produce more and better quality grass without needing to apply more synthetic fertilisers.” Defining soil is fairly straightforward. It is a combination of minerals, such as sand, silt and clay, mixed with air, water and organic matter. But a ‘healthy’ soil is harder to pinpoint and differs, according to Dr Bhogal, upon context and function.
“And questions need to be asked and answered. What are the soil’s inherent properties? What is the ratio of sand, silt and clay?” she asks. “How deep is it? Where is it and what is the climate? Most importantly perhaps, what do we want the soil to do?”
Soil types
There are more than 700 soil types in the UK, each with different properties. Upland peats, for example, tend to have a low pH – an undesirable characteristic for grass or arable production.
“Increasingly, we’re demanding more from our soils,” adds Mr Hooper. “As well optimising grass productivity, soils are expected to mitigate flooding and store water during times of drought, as well as capture atmospheric carbon and support biodiversity.” Having established its context and functionality, the next step is to drill down into the three aspects of soil health – the physics, chemistry and biology. They are interdependent and need to work together.
“Soil physics is the arrangement of particles and how they are stuck together,” explains Dr Bhogal. “Soil physics determines how easily roots and water can get down into the soil profile.”
“Soil chemistry includes pH, cation exchange capacity and nutrient status. These tell us what nutrients are in a soil and how available they are for plants, and if they are at concentrations that could pose a risk of nutrient enrichment in the wider environment. This is important on dairy units, where producers may be regularly applying organic manures and slurry.”
Soil biology is the area producers have historically shied away from simply because it’s hard to measure, according to Dr Bhogal. “There are more organisms in a teaspoon of soil, for instance, than there are people on the planet. But soil biology is the critical link because it ‘sticks soil particles together’ – the physics. And it plays a key role in releasing nutrients – the chemistry.”
Soil biology: there are more organisms in a teaspoon of soil then there
are people on the planet
Symbiotic relationships
Soil biology includes macro-organisms, like earthworms, and mesofauna, such as the nematodes, mites, and springtails, through to the microbiology – the bacteria and fungi. All contribute to the healthy functioning of soil.
It tends to be the microbiology (bacteria and fungi) that work with plants and aggregate soils. “They produce ‘sticky’ substances that helps to bind soil particles together,” says Dr Bhogal.
Fungi also play an important role in aggregation, or holding soil particles together. Some fungi (mycorrhiza) form symbiotic relationships with plants, exchanging sugar excreted from roots for water that contains dissolved nutrients. It’s a relationship thought to date back some 400 million years, predating the dinosaurs, and is extensive. One gramme of grassland soil may contain as much as 100 million mycorrhizal fungi.
“Mycorrhizal fungi are vital in extending root systems so plants can access more nutrients and water,” says Dr Bhogal.
“The relationship is particularly important when it comes to the uptake of phosphate. By contrast, nitrate is soluble in water and moves through the soil profile with rainfall. That’s why it's prone to leaching.
“Phosphate is not so mobile and tends to stick closely to soil particles. The fungi extend the area of the plants’ roots systems in contact with soil particles and, therefore, their access to phosphates.”
Access nutrients
As well as helping plants physically access nutrients, soil organisms, including microbes, make them chemically accessible by breaking down the complex compounds found in organic matter into simpler ones. “Research has also shown that some microbes are effective at removing toxins in soils,” adds Mr Hooper. “Others have been shown to trigger host plants’ own defences against pests and pathogens in a process known as ‘induced systemic resistance’. Some are directly involved in defending plants from disease.”
“It is perhaps the most exciting area in terms of increasing productivity without incurring huge costs,” he says. “Microbiology is fundamental to soil health and play a huge role in delivering the functions we’re looking for – from building soil structure which helps with drainage and air flow, through to capturing and releasing nutrients which provides the building blocks for strong and healthy grass growth. They are nature’s taskforce.” How soil works is complex, but assessing its health can be straightforward.
“Fortunately, we don’t have to measure ‘all of the biology’ to get an idea of a soil’s ability to function,” explains Dr Bhogal. “If you measure organic matter and pH, as well as check for structural damage, you can get a good idea of a soil’s ability to support biological populations.”
“The interdependent nature of the physical, chemical and biological aspects of soil do, however, mean that it’s important to take a holistic view.”