The science of soil health

Soil at work

Grapevines obtain most of their food and water from the soil. They can do this more efficiently in soils rich in organic matter, which enhances soil chemistry, physics, and biology. Soil organic matter and soil carbon are technically different but practically the same –organic material is carbon-based, so boosting soil organic matter also lifts carbon levels. Organic matter improves soil chemistry by increasing nutrient recycling and availability. It improves soil physics by creating a better soil structure that facilitates root growth and water drainage. Soil organic matter also increases water-holding capacity, while surface organic matter limits runoff, crusting, erosion, and evaporation. These benefits are mediated by soil biology, the complex ecosystem of mostly microscopic organisms that drives nutrient recycling in the soil. Healthy soils contain a diverse mix of bacteria, fungi, protozoa, nematodes, arthropods, and earthworms. “But the point of soil health isn’t biodiversity,” clarifies Storey. “It’s about having the right biology – biodiversity that leads to functionality and keeps pests and diseases in check.” She lists five factors for thriving soil biology: soil cover, plant diversity, living roots, animal inputs, and minimal disturbance. Cover crops are great because they tick the first three boxes and can serve as animal grazing. “Remember that minimal disturbance isn’t only about tractors,” says Storey. “It’s also chemical and biological. These days, people think you can add biology from a bottle – soil health doesn’t come in a bottle. Using those products can disturb the soil balance.”

Test to know

“The principles of soil health are the same regardless of crop type,” says Storey, “but the details are context-specific.” She stresses that there isn’t a recipe for universal success. However, growers can obtain objective test results to measure their progress. She suggests that growers start with the microbial respiration or CO2 burst test. This test estimates how much microbial life is present in a unit of soil by measuring the CO2 production of the organism. However, it doesn’t tell you which microorganisms are present. “The microbial respiration test is like a starter pack,” explains Storey. “If it shows that there isn’t life in your soil, then you shouldn’t do the other tests until you’ve made some corrections.” But, once growers have established that their soils are alive, they can dive deeper using volumetric aggregate stability and soil nutrient tests. Soil aggregates are clusters of soil particles held together by organic matter and minerals. Root exudates and substances produced by soil organisms, such as root-associated fungi, are important in binding soil aggregates. Stable aggregates promote a stable soil structure, allowing air and water movement while resisting compaction and erosion. Storey recommends Haney soil nutrient tests that use different extraction methods than conventional tests. Haney extractions rely on water and organic acids that mimic natural root exudates. The results indicate how much nitrogen, phosphorous, and potassium is readily available to microbes and plants and how much is locked up in organic matter. Microbial activity will gradually release and recycle the nutrients in organic matter.

Under the microscope

Besides assessing physical and chemical soil characteristics, growers can also have their soil’s biology evaluated. The rise of molecular biology offers the intriguing potential of learning exactly which microbes are present in a soil sample, but Storey questions the value of this information. “Many growers want a list of names,” she says. “But a list of names doesn’t help if you don’t know what the organisms do – their function.” This is where the nematode community profile comes in. Nematodes are excellent bio-indicators because they occur in all soils and occupy diverse feeding niches. Different families specialise in feeding on bacteria or fungi or single-celled yeasts and algae, while others attack plants or insects. Some nematodes even hunt and kill other nematodes. Tallying all the nematodes in each feeding group in a soil sample allows the calculation of an enrichment index and a soil food-web structure index. These indices can then be plotted on a two-by-two nematode matrix (Figure 1). Perennial crops such as grapevines ideally want soils in the top right quadrant of the matrix. Other tests can report on the mycorrhizal fungi associated with plant roots or provide a breakdown of the different microbial groups in the soil. Interpreting the results of these tests is harder than for the nematode matrix, which is supported by a large body of international research.

Success in soil health

Storey has seen the interest in soil health and regenerative agriculture slowly increase over the past three decades. What is her advice to growers who want to improve their soils? “Start small, but not with the orphan block that’s the worst performer on the farm,” she says. “Choose a good block and plant a cover crop. Establishing cover crops is hard in the Western Cape because they are sowed in March and April when there’s very little rain – if you can get through the first two years, some cover crops will start seeding themselves.” Growers will have to experiment to find the right cover crops. Reputable seed companies can provide guidance, and Storey encourages growers to form support groups. “Even if it’s only five or six people. Get together monthly to talk about what you’re doing and what works.” And take a long view. Storey has observed that many growers give up when they don’t see results after the first year. Others worry about short-term yield reductions during the transition period or struggle to develop new solutions to old problems, such as weed control. The growers who succeed eventually reap the benefits of healthier grapevines and lower fertiliser and water use. Many also report better fruit quality and shelf life, although Storey points out that these postharvest effects haven’t been scientifically validated. Ultimately, she thinks the secret of success is a genuine love of understanding how soil functions. “It’s strange to say this, but people develop a passion for soil health,” she reflects. “I’ve never seen anyone return to conventional practices once that happens.”