Issue link: http://winesandvines.uberflip.com/i/853816
August 2017 WINES&VINES 27 GROUNDED GRAPEGROWING times, winemakers have relied on various yeasts and bacteria that live on the fruit to initiate and convert fruit sugar and other chem- ical components into wine. My artisanal wine- making friends still remind me that "the best fruit is the kind that you can crush and have it turn into good wine with no interventions." (This is not a view universally subscribed to!) Recent research by the David Mills lab at the University of California, Davis, suggests that microbes on the fruit may be more important than previously thought in terms of wine flavor and terroir. Microbial populations on fruit are definitely different between winegrowing re- gions, and metabolites from the microbes on the fruit can be traced to the wine. Grape varieties are also different in the composition and number of microbes on their fruit. Thinner skinned varieties such as Zinfan- del and Petite Sirah have tight clusters and berries prone to breakage, which may allow larger populations of yeasts, fungi and bacteria to grow. Thicker skinned varieties with looser clusters such as Cabernet Sauvignon have quite different microbe diversity and population numbers. Environmental factors such as rain- fall and humidity also affect populations. Vin- tage does not appear to be as important as the vineyard location itself. This may be a big com- ponent of the elusive "stamp of terroir" that so many have sought. In terms of population den- sity, there are more microbes in this part of a grapevine than the foliage. The rhizosphere is the immediate area of soil and associated microbes that interacts with plant roots. The microbes feed off of root exu- dates including proteins, sugars and sloughed- off plant cells. This symbiosis leads to more complex interactions, influencing plant growth and competition for resources. This ecosystem is very important in the cycling of nutrients and disease suppression of plant pathogens. The complexity of the soil microbial commu- nity is still rather hard to fathom—one tea- spoon of soil contains more microbes than people on our planet. These organisms are amazingly dynamic and resourceful. When conditions are not favorable for growth, they are quite dormant, waiting for resources to begin their life cycle. Moisture, favorable tem- peratures and nutrients in the form of root exudates from living plants and incorporated plant material such as cover crops or compost bring the microbial part to life. Here, microbial numbers and diversity are the largest in the plant microbiomes. So how does soil health differ from soil quality? As mentioned earlier, past discussions about soil quality focused on what was good for the vine. New discussions about soil health take a broader view of agricultural land that in- cludes enhanced soil conservation and build- ing soil carbon. This may require changes in soil-management practices that include add- ing more carbon to the soil via cover crops and/or using compost and minimizing tillage by trying to maintain vegetative cover year- round. If tillage is necessary, implements should cultivate the soil vertically rather than horizontally, as it doesn't invert the soil as much, protecting organic matter, aggregate stability and biological activity. By inverting the soil and breaking down aggregates, the soil loses organic matter and becomes less permeable to water, which in turn can cause lateral runoff and siltation into waterways. Since sediment is considered the No. 1 pol- lutant in California's North Coast, reducing any soil movement from vineyards is impera- tive for water. Maintaining cover is very im- portant on sloping ground. In dryland farmed vineyards where tillage is needed to conserve moisture, planting self-reseeding cover crops that germinate rapidly to protect the soil from crusting due to the impact of rain drops— along with straw mats to slow any runoff in critical slopes or water conveyances—is an important strategy to minimize runoff and water pollution. Mustard is an example of such a cover crop. Water infiltration improves significantly when plant cover is in place. Since the trend in many regions is to optimize irrigation and reduce the need for it, having a full soil profile following winter rains is a definite benefit. Soil scientists have for years noted the importance of maintaining cover to improve water infiltra- tion during rains and irrigation. This also helps during periods of drought. Building soil carbon is a long-term strategy to address climate change. Increased carbon dioxide levels in the atmosphere are causing more heat and drought. By converting CO 2 into plant matter that eventually digests to form stable humus in the soil (the complex amor- phous chemical end product that is dark in color and coats soil particles), this can assist in reducing greenhouse gases and be part of a strategy to stabilize climate change on our planet. Remember that soil has the potential to hold more than five times the volume of CO 2 than is in the atmosphere. Also important is the ability of leguminous cover crops to fix atmospheric nitrogen into forms useable by grapevines. If we assume that most vineyards need about 20 to 30 units of nitrogen per acre per year, that is a very easy target to reach with cover crops and 1 ton of well-made compost applied per acre. This greatly reduces the need for chemical fertilizer that is very energy intensive to manufacture. It is estimated that 31% of the energy used by American agriculture is for the manufacturing of fertilizer. Eighty-two per- cent of the fertilizer is imported from other coun- tries; 73% is manufactured from natural gas and 27% from coal. Also added to this is energy for transportation and application. Fixing nitrogen through legumes in the soil and recycling organic matter via composting (such as grape pomace and animal manures) uses less than 10% of the energy required to produce and apply chemical fertilizers. This is another benefit of harnessing the power of soil microbes as a component of healthy soil. Soil health metrics Measuring soil health is still open to much debate about useful metrics that can be readily applied for all situations. Obviously all of the issues associated with our older concept of soil quality apply with its focus on the needs of vines for good growth (well-structured soil, adequate fertility, low soil strength to allow easy rooting and cultivation, good infiltration and gas exchange). When we consider ecologi- cal services, we also can include soil moisture storage and reduction of sediment leaving the vineyard. Good metrics for measuring biologi- cal activity and the type of organisms that are active in the soil still is not clearly defined. Soil texture, soil depth, location of the soil, tem- perature and soil moisture conditions, vegeta- tion in the soil and organic matter content all affect what measurement of species, respira- tion, life forms and population numbers you are likely to measure. So clearly, this is a com- plex set of parameters to correlate with all of the functions that we might want our vineyard soils to accomplish including healthy vine growth, building soil carbon, improving infil- tration and preventing soil runoff. The Cornell Soil Health Assessment is one of the most organized systems in the United States to assess soil health. There are 12 tests utilized in assessing soil health factors includ- ing physical, biological and chemical proper- ties. The cost is $110 per sample for the basic testing, and there are optional add-ons for other soil properties and chemicals. The results score each soil property tested from worst to best based on a large number of soil samples analyzed in the northeastern United States and report on a percentile based on a normalized distribution. For instance, if we are considering organic matter content, a score in the range of zero to 30% would be considered poor and in New discussions about soil health take a broader view of agricultural land that includes enhanced soil conservation and building soil carbon.