By Simon Parfey, SoilBioLab
Understanding the intricate relationships between soil management, moisture content, and the soil microbiome is paramount to cultivating healthy and resilient vines that express the unique character of their terroir. By embracing the principles of regenerative viticulture we can directly influence these factors, fostering a thriving underground ecosystem that benefits the vines and the resulting wine.
Let us consider the impact of keeping the soil covered, a key tenet of regenerative viticulture.
Sandy soils
Consider a sandy soil type.
Sandy soils, characterised by their large particle size, generally exhibit good drainage but have a low capacity for water retention. This inherent characteristic means they can dry out rapidly, especially during periods of low rainfall and high evapotranspiration. This rapid fluctuation in moisture can pose a challenge to the soil microbiome and can potentially impair the natural function and long-term benefits of a more natural approach to farming.
However, by keeping the soil covered, whether through cover crops or mulches, it is possible to significantly modify the moisture dynamics of sandy soils.
A soil cover acts as a barrier, reducing direct evaporation from the soil surface. Conserving moisture helps to maintain more consistent water levels within the soil profile, mitigating the rapid drying that can stress, kill and reduce the number and diversity of soil microorganisms. Furthermore, the decomposition of organic matter from cover crops or mulches increases the organic matter content of the soil. Organic matter acts like a sponge, enhancing the soil’s ability to absorb and retain water (rain). This improved water-holding capacity, even in a naturally free-draining sandy soil, provides a more stable and favourable environment for the soil microbiome.
Clay soils
But near where I live, close to the South Downs, much of the soil is clay (over chalk) – like many of the vineyards that I visit in the south of England. This type of soil is characterised by its fine particles that pack together tightly, resulting in a dense and sticky texture when wet. These small particles create a vast network of very small pores (micropores).
A more clay soil structure can lead to a high total water-holding capacity due to strong capillary forces. However, the low permeability of clay soil means that water infiltration and drainage are generally slow, potentially leading to waterlogging. While clay soils hold a lot of water at field capacity (40% to 55% Volumetric Water Content VWC), they also hold a significant amount of water very tightly at the wilting point (15% to 24% VWC), which can result in a moderate available water capacity.
Mulches and cover crops can significantly affect the moisture content of clay soil in several ways:
• Reduced evaporation: A soil cover acts as a physical barrier, reducing direct evaporation from the soil surface. This would help retain the moisture already present in the clay soil for longer, particularly during dry periods.
• Improved infiltration: While clay soil has low permeability, a long-term soil cover, especially when combined with living roots (another regenerative principle), can improve soil structure over time. The addition of organic matter from decomposing cover crop residues or mulch can lead to the formation of aggregates, creating a crumblier structure with a better balance of pore sizes, potentially enhancing water infiltration into the clay.
• Moderated temperature fluctuations: A soil cover can buffer the soil temperature, keeping it cooler in hot weather and warmer in cold weather. This moderation can reduce evaporative losses during hot periods, helping to maintain moisture in the clay soil.
• Increased organic matter: Keeping the soil covered often involves the addition of organic materials or the growth and decomposition of cover crops. As organic matter increases, the water-holding capacity of the clay soil can be further enhanced. Organic matter acts like a sponge, improving both the amount of water the soil can hold and the availability of that water to soil organisms and plant roots.
But why, when we’re concerned about the soil microbiome, have I put such as high emphasis on soil water?
Put simply, through the work that we have done over the years, each day, for different types of growers, at SoilBioLab we know there is a direct correlation between moisture content and both active and total biomass (the total amount of bacteria and fungi and the proportion that is alive and functioning).
Graph 1 is taken from our monitor site in Wiltshire, England. It is a sandy silt loam soil that started its regenerative journey with a change to minimum tillage over seven years ago. We have been testing the soil there, every month, ever since. This data suggests that increased moisture levels generally lead to higher levels of microbial activity within the soil samples. By keeping soils covered and increasing their organic matter content, we create conditions for increased microbial activity and abundance compared to bare, dry soils.
Soil is a dynamic and complex ecosystem harbouring a vast and diverse community of microorganisms. The activity of these microbes is crucial for nutrient cycling, transforming organic and inorganic nutrients into forms readily available to the grapevine. Adequate soil moisture is essential for the metabolic activities of these microorganisms. By maintaining more stable moisture levels in the soil with a cover, we can support the consistent provision of essential nutrients to the vines, contributing to their health and vigour.
Graph 2, again from our monitor site, highlights the change in soil moisture through the year. Fluctuations in soil moisture, often seasonally orientated, can disrupt the delicate microbial balance, potentially making vines more susceptible to disease – after all, a healthy and diverse soil microbiome, plays a significant role in bolstering grapevine disease resistance. Beneficial microbes can compete with pathogens, induce systemic resistance in the plant, and produce antimicrobial compounds.
Protozoa are key microorganisms for nutrient cycling. They often congregate around soil organic matter and can consume thousands of bacteria every day
Consistent moisture, promoted by soil cover, helps maintain a stable and resilient microbial community capable of protecting the vines throughout more of the year, in the face of increasingly extreme climatic shifts, and throughout unpredictable, unseasonal changes in the weather.
A key priority when working with any type of soil should be to always keep the soil covered. This simple act directly influences soil moisture content by reducing evaporation and increasing organic matter retention. Having a more stable and improved moisture environment directly supports a more active and abundant soil microbiome. This thriving microbial community is fundamental for nutrient cycling, disease suppression, and ultimately, the health, resilience, and potentially the distinctive taste characteristics of the vines and the wine they produce.
Simon Parfey owns and manages SoilBioLab. The laboratory is based in Hampshire, UK and specialises in soil microbiology testing for soil health. Together with associate Nick Cooper, the team help viticulturists, farmers and growers to better understand their soil and implement tools and techniques to improve soil health and biological activity info@soilbiolab.co.uk