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Home , Soil organic matter

The importance of By Marcel Essling

What is soil organic matter and influence yield, vegetative growth and changing management practices (e.g. is it different from soil organic fruit composition. Before embarking on a increasing plant groundcover and carbon? program to increase SOC it is important ). Alternatively, if tests indicate to assess current vineyard performance that a soil already has adequate levels of Soil organic matter (SOM) is the portion (yield, fruit and wine quality) and SOC for the , temperature and of the soil made up of decomposed plant consider the impact that a change in soil rainfall, it may take a large amount of and animal materials as well as microbial fertility may have on vine balance and effort and resources to achieve increases organisms. It is composed mainly of fruit quality. It is also important to accept in SOC and it is less likely that the gains carbon, hydrogen and oxygen as well that Australia has a naturally ‘weathered’ will be stable. as small amounts of other elements landscape and Australian are including , phosphorous, , typically low in organic matter by world What are some of the options potassium, calcium and magnesium. standards. While there are management to increase soil organic As carbon is the main ‘building block’ practices that are detrimental to SOC carbon? (58%) of SOM, soil organic carbon (SOC) and alternatives to these should be The SOC in a vineyard is the difference is commonly measured as a proxy for found, the pursuit of increasing SOC between the inputs (plant material) and SOM. To estimate SOM from SOC, the should consider the limitations of the losses (from microbial , formula SOM = SOC x 1.72 can be used. Australian soil and climate, so time and erosion and light or heat damage) (Carson What is soil organic carbon? resources are not wasted. 2020). On the input side, maintaining volunteer swards, planting cover crops Carbon exists in two forms in soils – What is the right amount of and applying compost will increase SOC. inorganic and organic. Inorganic carbon soil organic carbon for my Applications of mulch will help protect is mineral–based and is relatively stable. vineyard? the soil surface from heat and erosion Organic carbon includes decaying The amount of SOC in a soil right now is a and if the mulch is an organic substance plant matter, soil organisms and function of the inherent characteristics of (see Proffitt 2014 for differences in mulch microbes. Organic carbon is of interest the soil, climatic factors and how the land types) it will add to the because it is often associated with soil has been managed. Soil-specific factors as it breaks down. Actions to reduce health and can be influenced by land including content, bulk density compaction and minimise cultivation management practices. and depth are inherent characteristics will improve and avoid the and difficult to change. Climatic factors Should I be trying to increase carbon losses that can occur when soil such as rainfall, temperature and my ? is disturbed. solar radiation will have site-specific Soil organic matter fulfils many impacts on SOC. Cover crops or volunteer sward important functions and higher levels decisions that influence SOC levels in Allowing plant growth or sowing the of SOM/SOC are associated with vineyards include: midrow with a can improve benefits including: • cultivation SOC. As a groundcover grows and • improved availability to senesces, leafy tissue, plant roots and • the amount and weight of trac plants and soil microbes root exudates provide an important • the presence or absence of midrow • better stabilised soil structure less source of carbon to the soil system. and under-vine swards prone to erosion This living cover protects the soil from • the types of groundcover planted erosion (water and wind) and light and • increased water-holding capacity heat damage. Reductions in surface • the application of compost or mulch. • increased water penetrability. crusting, improvements to soil structure White and Krstic (2019) provide an and higher levels of soil As climate is predicted to become indication of optimal values for SOC by are among the benefits observed from hotter and drier in many wine-growing soil type: > 1%, >1.6% and clay growing cover crops. areas of Australia, higher levels of SOC > 2%. These optimal SOC targets show the in vineyard soils would be beneficial Compost application difference that might be expected based to improve the buffering capacity purely on clay content and highlight Compost is an organic material that of soil by increasing its potential the importance of considering SOC has undergone controlled biological water-holding capacity. targets on a vineyard by vineyard basis. and chemical decomposition resulting Wine-grapes do not require high rates If a vineyard’s soil is badly degraded and in a stable substance resistant to of nutrient inputs. Significant changes tests indicate SOC lower than optimal further decay (Proffitt 2014). When to nutrient and water availability can levels, improvements can be made by environmental conditions are suitable,

82 Grapegrower & Winemaker www.winetitles.com.au September 2020 – Issue 680 a surface application of compost will contact with organic matter when it has References and further reading increase OM levels. This results been adsorbed to clay surfaces, coated Carson, J., 2020. How much carbon can in benefits to soil structure, water with clay particles, or buried inside small and aeration, water- and soil store? Wheatbelt NRM Caring for Our pores or aggregates. Cultivation of clay Country Fact Sheet. http://www.soilquality. nutrient-holding capacity and enhanced soils breaks up the soil aggregates and org.au/factsheets/how-much-carbon-can- microbial and abundance. gives microorganisms access to the OM. soil-store Mulch application If OM is brought to the soil surface Proffitt, T. 2014. Mulch v compost: What Mulch is any material (not necessarily it will also be subjected to climatic works best in your vineyard? Australian and an organic material) placed on the soil factors such as heat and light that lead to New Zealand Grapegrower and Winemaker surface. In vineyards it is typically applied (604): 40 -44. its decomposition. under vines and is used to improve White, R.E., Krstic M. P.2019. Healthy soils retention, reduce topsoil Recommendations for healthy vines. CSIRO Publishing. temperature variation, reduce erosion Soil organic carbon is important to and suppress weeds. The protection Penfold, C., Howie, J. 2019. Under- of topsoil from erosion and the soil the physical, chemical and biological vine cover cropping, Wine Australia moisture retention achieved through the processes in a healthy soil. Actions that Fact Sheet https://www.wineaustralia. use of mulch are beneficial to SOC. allow a soil to reach its SOC potential com/getmedia/384c2ac3-b0b1-4c7b- 9c7c-a8904090a69b/CORD_Factsheets_ given the constraints of a site should Cultivation CoverCropsUndervine_V2.pdf be encouraged. The key opportunities Clay particles and aggregates in White, R.E. 2019. What are the realistic in viticulture involve allowing growth soil physically protect organic expectations for making money out of matter from decomposition. This in midrows and undervine; minimising carbon credits in vineyards? Australian and is because it is more difficult soil disturbance and heavy traffic; and New Zealand Grapegrower and Winemaker for microorganisms to come into applications of mulch and compost. (666): 38–40.

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September 2020 – Issue 680 www.winetitles.com.au Grapegrower & Winemaker 83