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Vine Talk: Grape Growing Sustainability
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Vine Talk: Grape Growing Sustainability

Kerri Steenwerth

Dr. Kerri Steenwerth is a Soil Scientist with the Crops Pathology and Genetics Research Unit in Davis, CA. Her research focuses on the development of sustainable grape production systems and practices.

Welcome Dr. Steenwerth to Under the Microscope.

UMCalifornia is one of the most productive agricultural regions in the world, yielding two-thirds of the U.S.'s nuts and fruits. As the epicenter of the American grape, raisin, and wine industries, what makes California so suited for grape production?

KS – California's Mediterranean climate is ideal for grape production and other specialty crops, with its dry, warm summers and wet, cool winters. Additionally, the state hosts a diversity of soil types and microclimates that are well-suited to grape production. In fact, all but one soil order, the Gelisols, which are the frozen tundra-like soils, are present in California.

As a soil scientist, I find that the diversity of soils in California provides a great laboratory for understanding how vineyard management practices in the alleys and under the vines improve soil health.

UMFor the past few years, California has been experiencing unprecedented heat, drought, and wildfire conditions. How has soil science research helped grape growers deal with these natural factors?

KS – We have been looking into the use of cover crops and no-till practices as ways to increase soil organic matter storage capacity. Increasing organic matter in soil is crucial because it improves the soil's ability to hold water and its resistance to slaking (disintegration when in contact with moisture). In other words, higher levels of organic matter in soil lead to better soil structure and erosion resistance. Techniques like cover cropping and no-tilling practices also enhance water infiltration and can provide additional environmental benefits such as carbon offsets (reduction of carbon dioxide) and improved soil nutrient retention.

Currently, ARS is also working on investigating the impacts of heat and wildfire on wine grapes.

Handful of soilThis handful of soil was collected from underneath resident vegetation, or the plants that typically grow in a particular area. It's common for growers to develop a cover crop from annual grasses and other herbaceous plants to protect the soil below. (Photo by Kerri Steenwerth)

UMHow are grape growers currently dealing with the consequences of climate change and increasingly extreme weather events and how can they prepare for future environmental challenges?

KS: Droughts and long periods of dry weather in California often cause water shortages. To address this problem, growers are carefully managing their irrigation water applications using highly technical, automated approaches involved in precision agriculture. For example, they are integrating sensors that detect soil and vine water status and assess vineyard water use more holistically. Systems like these can help growers fine-tune drip irrigation applications so that they avoid using more water than necessary.

We have also surveyed wastewater sources from various wineries across California for their salt content and potential for degrading soil. Our findings suggest the wastewater sources we measured were sufficient for irrigation use.  Some vineyard operations also have started using their winery wastewater for irrigation of landscaping and vines, although this isn't always possible due to infrastructure limitations. Our challenge is still to figure out how to best recycle wastewater for long-term irrigation without damaging the soil or negatively impacting the grape chemical composition.

Soil core This soil core was collected to measure soil bulk density. Soils with high bulk density can suffer from compaction and poor drainage. Cover crops can mitigate this by adding organic matter to the soil, building soil aggregates. (Photo by Rachel Greenhut, University of California )

UMFor most people, the words 'fungus,' 'bacterium,' and 'microbe' all have relatively negative connotations. But your research indicates that these organisms may be crucial to soil quality and thus grape production. Please explain.

KS – We can think of microbes as the engines that return nutrients to the grapes. They provide various services like soil organic matter stabilization, which leads to carbon offsets that help mitigate climate change. Using DNA-based techniques, we can also identify and evaluate what exactly is in the soil providing these services.    

Field peas Field pea (Pisum sativum) serves as a cover crop to fix atmospheric nitrogen in the soil in the alleys between the vine rows. It is usually planted in a mix with other 'soil-building' cover crop species. (Photo by Rachel Greenhut, University of California )

UMIt seems like there's a connection between the general health of grapes and the soil they're planted in. Are there microbes that are unique to grape-producing soils?

KS – In one study, we went on a microbial safari and sampled soil microorganisms across the Napa Valley in California. We found that they formed fingerprints, or communities, that mirrored the region's American Viticulture Areas.  These areas are geographically limited American wine grape-growing regions, defined for their distinguishing features of climate, geology, soils, physical features (topography and water), and elevation.

We are investigating if vineyard microorganisms that participate in unique, beneficial interactions could be exploited to improve or modify grapevine performance and enhance wine properties in specific viticultural zones. There's so much excitement in the field of microbial ecology right now as we figure out how to leverage the natural processes that soil microorganisms perform for enhanced agricultural production.

UMCan other types of crops benefit from a healthier or enhanced soil microbiome? Can the concept be applied outside of commercial farming?

KS – In my research unit, several scientists are looking at non-chemical control of soil-borne pathogens that affect orchard tree crops in California. These practices involve adding organic amendments or creating anaerobic (low oxygen) conditions to decrease the population of target pathogens, like Agrobacterium tumefaciens. Organic amendments could provide other benefits, such as higher soil organic matter levels and improvements in soil aggregate stability. These co-benefits are being examined in various field trials in orchard crops. Control practices like these are exciting because they provide multiple benefits to improve soil attributes.

Grape clusters Grape clusters are harvested in picking bins like these in preparation for the evaluation of vineyard floor management practices like cover cropping and tillage on yields, cluster weight, berry size, and grape chemistry.(Photo by Kaan Kurtural, University of California).

UM What tips can you provide for home gardeners looking to grow grapes on their land?

KS – Many universities have Master Gardener programs that teach home gardeners about sustainable practices and pest management approaches suitable for their region. If I were growing grapes in a sunny, dry backyard near Davis, CA, I would choose a variety suitable to my climate. I might try a southern Rhône variety from regions with fairly hot, dry summers, setting it up with drip irrigation. Out of curiosity, I would set up a mini experiment, treating soil around the vines in groups of three with different amounts of compost or mulch from my local gardening store.

My interest would be to build soil organic matter and see if grapes could be supported with nutrients released from the decomposing amendments. Take notes to record management and appearance of the plants. Track the weather and associated dates when leaves emerge, bloom happens, grapes develop and mature and pictures over time. Follow your interests and explore what works as the grapes mature. – by Georgia Jiang, ARS Office of Communications.