South Africa Wine Technical Yearbook 2025

Vine performance and wine quality – compost’s impact Because compost improves soil health, it often leads to stronger vines and bigger yields. In some cases, it can also affect juice analysis. For example, Turkish researchers found that a combination of manure and pruning cuttings significantly increased grape yields, bunch weight, and improved several juice quality metrics after just two years. But more isn’t always better. Some compost treatments led to higher vine growth and increased yields, but a reduction in desirable wine compounds like anthocyanins and flavonoids. This suggests the extra nutrients were diverted to roots instead of grapes. Similarly, overuse of nitrogen-rich amendments can lead to overly vigorous vines, while having a negative impact on juice quality parameters such as increased malic acid, fewer anthocyanins, or too much potassium in musts, which is detrimental to wine quality. Careful monitoring is essential. The effects of compost depend on a number of factors, such as compost composition, climate, soil type and soil properties. Growers should track soil and vine nutrient levels throughout the season and adjust practices to avoid negative effects on grape quality or the environment. Soil carbon sequestration and greenhouse gas trade-offs A major promise of regenerative viticulture is its potential to sequester carbon and help combat climate change. Compared to annual crops, vineyards are already better at locking in carbon, and RV practices like no tillage, composting and cover cropping that build soil organic matter enhance this ability further. However, applying organic materials also releases greenhouse gases – especially nitrogen-based ones like nitrous oxide. One Spanish study found an increase in CO 2 and a 400% spike in N₂O emissions shortly after compost application, though levels returned to normal within weeks. Other research in California found no significant impact on emissions, grape yield or cover crop biomass. However, the study only lasted two years and may have lacked the sensitivity to detect longer-term effects. The jury is still out on the precise climate trade-offs of these practices. The road ahead – what we still need to know There is still a lot to learn about the application of organic soil amendments. Future research should include long-term studies that focus on: • Interactions between compost type, soil, rootstock and scion, in various climates. • Application rates and timing thereof. • Nutrient dynamics, grape composition, and greenhouse gas emissions. • Impact on carbohydrates (starch and soluble sugars) in vines during dormancy. • Whether organic amendments to soil can reduce the need for other inputs like foliar sprays.

Organic amendments like compost and biochar, on the other hand, offer a more sustainable alternative. These materials do more than feed the vines – they help suppress weeds, improve soil structure, retain moisture, and boost soil microbial life. All of these effects are critical for creating vineyards that are resilient to climate extremes and capable of long-term productivity. Figure 2 illustrates the soil carbon cycle and the potential impacts of vineyard management practices. The heart of the matter – soil health and the microbiome Healthy soil is at the heart of regenerative farming. Scientists define “soil health” as the ability of soil to support ecosystem functions – everything from nutrient cycling and carbon storage to water regulation and plant health. Microbes play a starring role in all this. They help build soil organic matter and cycle nutrients, and they also form part of the grapevine’s own microbiome. In fact, studies show that the microbial communities in vineyard soil can influence grape chemistry, and even the aroma profile of the wine itself, including the compound rotundone in Shiraz grapes. RV practices such as composting, biochar application and cover cropping all impact the soil microbiome, but results can vary depending on soil type and local climate. More research is needed to fully understand how these microbial changes translate into wine quality. Composting done right – what the research shows Organic matter often used in vineyards includes biochar, compost, manure, vine pruning cuttings and winery waste. The intention of using these materials is to improve soil nutrient status, organic matter, carbon content and water management. Repeated applications of compost or manure – especially at rates above 4 tons per hectare per year over five or more years – have been shown to significantly increase soil nutrients (especially nitrogen, phosphorus and potassium), organic matter and microbial biomass. But how you compost matters. For instance, simply spreading unprocessed vine pruning cuttings had minimal effect on soil carbon or nutrients in one long-term, 13-year Australian study. However, turning those pruning cuttings into compost or biochar made a big difference. A study in Spain showed that the application of composted manure enhances aggregate stability and soil structure, resulting in better soil water infiltration and water holding capacity. One Italian study found that applying biochar boosted the vines’ ability to retain water – an increasingly vital trait in the face of heat or drought events. Still, not all results are positive. A trial in Turkey found that a mix of pumice, straw and manure actually reduced soil moisture. The result could be due to the short duration of the trial (two years), specific properties of the soil, vineyard, or climatic reasons. This highlights the need to adapt practices to local conditions.

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TECHNICAL YEARBOOK 2025