Building Biology – Crop Production Magazine

Soil biology is fundamental healthy soil yet is hard to assess. In a bid to help farmers understand more, Hutchinsons’ soils specialist, Jade Prince, points out that different soil organisms can be broadly grouped into four categories or ‘trophic levels’, based on what they feed on.

She says first is the ‘basal’ trophic level, or the primary producers, which includes any living plant and organic matter (crop residues, roots etc). This is the energy source for all other biology beneath the surface so is a crucial starting point.

“There’s a symbiotic relationship between plants and soil biology, where almost half of organic carbon produced by plants going straight into the soil as root exudates, comprising sugars, amino acids and other compounds. These exudates, alongside organic matter, feed the second layer – the primary consumers – made up of bacteria and fungi.”

Jade adds that the third layer comprises larger organisms including protozoa, nematodes, soil mites, and springtails, which feed on the bacteria and fungi, and are key to nutrient cycling. “Then the highest trophic level contains predatory nematodes and mites, and higher-level predators, including ground beetles, and birds.”

She says earthworms – which don’t fit into any of these biological groups – are often a go-to indicator as a measure of soil health given their role in soil structure and drainage, but can’t be solely relied on for assessing soil biology.

While lab tests can measure soil biology, they take time, and microbial communities can change during the sampling and sending of soil samples, adds Jade. This is where Hutchinsons’ new ‘Microbe Ready Sessions’ can help, giving individuals or groups of farmers an opportunity to check their own soil microbe populations through a microscope.

“The aim isn’t to measure exactly how many organisms are in a sample of soil, but to provide an insight into the types of microbes that may or may not be there. Particularly those unseen by the naked eye,” she explains.

One of the greatest areas of concern is in the third trophic layer of larger predatory organisms, where many agricultural soils show a notable lack of numbers, says Jade. “A lot of soils have become bacterial-dominant because cultivations, and the application of synthetic fertilisers and pesticides, has, over time, suppressed numbers of fungi, protozoa and nematodes.

“Plants can change the exudates they’re excreting depending on what they want from soil organisms. So if plants are getting sufficient nutrients from artificial sources, they don’t have to give soil biology as much sugar in exudates and the microbe populations will drop.

“That’s when systems become reliant on synthetic inputs and we’ve effectively bypassed the microbial loop,” she explains.

Jade says the aim is to bring soil biology back by recognising its role in nutrient cycling. “There’s no silver bullet, so farmers have to take a holistic approach which considers various regenerative practices, allowing time for soil biology to recover.”

Composts and manures are particularly high in ‘trophic level three organisms’ such as protozoa and nematodes, so adding these to soil can help, although there are logistical challenges over large areas, she adds.

However, Hutchinsons reports promising results from using Johnson-Su compost extract with seed at drilling, which allows larger areas to be treated with a relatively small volume of compost material. In on-farm trials at the company’s Helix site, 75% increases in rooting have been seen in barley. This could be a result of nutrient cycling and availability providing the germinating plant with a better kick start, suggests Jade.

“Some farmers are using Johnson-Su on cover crops in the hope it’ll enhance the benefits they deliver to soil health,” she says.

“But how long the benefits of adding soil biology last is still unclear. We don’t know whether the predators we’re applying are surviving in the soil, or whether the benefits only last for that season. It seems as if Johnson-Su has to be applied every season; in a mono-crop system there’s simply not the variety of food required to sustain the higher levels of build.”

Feeding soil biology with a constant and varied supply of organic matter, and energy from root exudates, will help to support populations, she suggests.

“Organic matter can be in the form of composts, manures, crop residues, or other biomass such as from catch, cover, or companion crops. A varied rotation that maintains green, living material in the soil for as long as possible through the year is key. while bringing livestock into the rotation also helps.

“We do see slightly more protozoa in soils where farmers are starting to incorporate some of these regenerative practices,” confirms Jade.