Improving Soil Health in the Vineyard

By: Becky Garrison

Losses in soil structure, erosion, and overall soil health continue to be a hot topic amongst farmers, including grape growers. A virtual session on soil health was held at the Oregon Wine Symposium in February 2022 and moderated by Patty Skinkis, a viticultural specialist and professor at OSU.

Defining Soil Health

  Dr. Shannon Cappellazzi, the director of research at Grassland Oregon, defined soil health as “the continued capacity of the soil to function as a vital living ecosystem that sustains life.” Soil is needed to move, filter and store the water needed to sustain plants, as well as serve as the medium for nutrients such as dead and decaying animal material, manure and plant matter to get recycled and taken up by plants and other soil biota.

  In addition, soil is the modifier of the atmosphere. According to Cappellazzi, there’s about three times as much carbon in the soil as there is in the atmosphere. “The balance of the amount of carbon that is coming out of the soil on a daily basis naturally as it’s supposed to be, and the amount of carbon that’s going back into the soil is really critical to those atmospheric levels of carbon dioxide.” Also, soil is a habitat for organisms, such as insects and other kinds of arthropods, microorganisms, fungi and a host of other organisms.

Role of Plants in Soil Health

  Cappellazzi observed how there’s a growing realization that biology controls so much of what’s going on underground and about plants’ role in maintaining soil health. First, plants help prevent erosion from the elements by holding the soil in place, regulating temperature and moisture and helping moderate pests and diseases. In particular, the roots create pathways for the water, invertebrates, some mammals and gases to move in and around the soil. Along those lines, when the mycorrhizae fungi grow with plants, they increase the root surface area and create enzymes that break nutrients down, thus enabling the soil to get more nutrients.

  Also, microbes help form the glue that holds the soil together. When sand, silt and clay-sized particles get stuck together in these aggregates, they start to create pore spaces that allow for water and air movement in the soil. But if it just flows overland or goes downhill, the soil picks up whatever kind of contaminants are on the surface and brings that to the waterway.

  While one of the best ways to keep the soil covered is with living plants, Cappellazzi suggests that mulches can help armor the soil so that rain does not hit directly onto a mineral surface. Also, she recommends minimizing soil disturbance by reducing tillage.

How to Conduct a Qualitative Evaluation of Soil Health

  Dr. Jennifer Moore, a research soil scientist for the USDA’s Forage Seed and Cereal Research Unit in Corvallis, Oregon, shared information about the in-field soil health assessment utilized by the Natural Resources Conservation Service (NRCS). This assessment aims to identify if there is a soil health resource concern and then design a conservation plan if needed. In her estimation, conducting an initial assessment helps farmers establish a baseline to monitor when assessing soil health in their vineyard after making management changes.

  Before starting a soil health evaluation, Moore suggests that growers compile a list of past, present and future management goals. They must also consider the prior land use and different soil types. Moore notes that doing the necessary homework and taking notes can help growers track their progress over time. To help with this evaluation, NRCS has a suite of questions they typically ask the landowner to assess soil health: 

•    What is the current crop rotation/cover crop used in inter-rows? 

•    What is the tillage system?

•    How frequently is the implement used, and at what depth?

•    How long have you been in this management system?

•    Are you considering any changes?

•    How many months per year is the surface covered with at least 75 percent of living vegetation, decaying residues or mulches?

•    If the land is grazed, what type of animals and cover crops are on this land? Are these animals and crops a consistent part of the system, and if so, how many years have they been in place?

•    What’s the method and timing of planting and termination periods for the cover crops? 

•    Are there any issues on the field, such as too little water? 

  Ideally, this evaluation should be conducted on moist soils close to field capacity, such as a few days after a saturating rain once the soil has drained appropriately.

  To get an accurate long-term assessment of the soil, farmers should sample soils multiple times during the first year or two to get an idea of how sampling time, moisture, and temperature impact results. Also, the growth stages of the grapevines and the cover crops being evaluated offer insights into how management practices impact soil results. When conducting soil assessments, farmers must be mindful of extenuating circumstances, such as fires, heat waves and floods that can harm the soil. 

Vineyard Soil Health Trials

  Dr. Miguel Garcia, a sustainable agriculture program manager at Napa Natural Resource Conservation District (NRCD), and Cappellazi summarized data from vineyard trials in the California North Coast and the Willamette Valley, respectively. 

  Garcia conducts research with the North Coast Soil Hub, which is a collective of NRCDs from Napa, Sonoma, Mendocino and Humboldt counties. His project involves 500 soil samples from 70 different vineyards throughout these counties. The goal was to develop a tool to showcase how different management practices affect soil health. As these trials were only collected once, they only provide a snapshot of what’s happening with the soil. In order to truly understand how different soil management practices affect soil health over time, longer-term monitoring is necessary.

  The first trial described by Garcia was the long-term project at Huichica Creek Sustainable Demonstration Vineyard, which is owned and operated by the Napa Resource Conservation District and used for educational purposes. Since 1992, half the rows have been tilled, and the remaining rows left undisturbed. For this experiment, they applied a combination of compost and biochar as a soil amendment to select plots that consisted of tilled and no-till rows. A control plot had nothing applied. Over a three-year span, they observed less water pounding and better cover crop growth in those plots that received compost and biochar. Also, they measured an increase in organic carbon and total nitrogen in the no-till and compost plots.

  The second trial was at Gamble Ranch, a no-till vineyard located in the Yountville AVA that is owned by Treasury Wine Estates. Here, they applied compost only. In one section, they experimented by having some of the rows tilled lightly. While this trial continued for several years, preliminary results indicate that the aggregates from the no-till plots remained intact and were better at holding water than tilled plots. Also, they found that no-till with compost produced higher levels of total nitrogen and phosphorus.

  Shannon Cappellazzi reported on trials conducted in the Willamette Valley. The research points to a practical difference between tillage and no-till, although there is often a lot of overlap in the data.   When testing different cover crop treatments, winter annual cereal grains had the highest average water-stable aggregates. However, as Cappellazzi remarked, “It’s hard to get a really significant difference since there’s so much variability because of these differences in apparent features. I would really encourage people to think about comparing soils within a similar climate, texture and slope.”

  When performing this research, Cappellazzi points to the need to assess the difference by slope, as vineyards are typically on hillsides in the Willamette Valley. “When you are comparing two soils to each other, pair them with a similar type of slope and way that that hill faces. That’s going to allow you to actually determine whether or not the management practices that you’re making are making a difference.”

  Cappellazzi likes to use carbon mineralization or CO2 burst tests to assess soil health. “That carbon is the total amount of food that’s available. You need to know whether or not these microbes are using that food. Then the aggregate stability tells you what they are doing with that food. Are they building those structures and allowing that space for the plants and the roots and the water and the microbes to move around there?”

How Growers Can Perform Their Own Soil Tests

  According to Moore, an in-field assessment can be done with tools that many growers have on hand, though one may want to add a penetrometer, a device that measures soil compaction. Also, Moore recommends carrying around a notebook to jot down notes and taking pictures to illustrate soil status.

  In order to evaluate the soil accurately, one must dig holes in multiple places across the land that’s being evaluated. Soil compaction can be evaluated anywhere with a penetrometer or using a wire test flag to depth. If a wire flag can go into the soil to about 10 inches with relative ease, it is assumed that compaction is not a concern. This evaluation should be conducted on moist soils close to field capacity, such as a few days after a saturating rain once the soil has drained appropriately.

  Using dry soil, one can evaluate soil aggregate stability by doing a slake or a slump test. This can provide a quick snapshot of the soil’s overall health. This can be done with simple tools, including a cup and strainer. Get the soil clod wet, and then flip it over. The more the soil retains its shape, the better the aggregate stability and, typically, the more organic matter the soil has.

  One of Cappellazzi’s favorite infield tests is the infiltration rate test which uses a six-inch piece of PVC placed one inch into the soil. Simply filling the column with water allows growers to see how long it takes to get into the soil. If it pools or takes a long time, there may be less soil structure and more compactness, indicating a less healthy soil condition.

The Till Versus No-Till Debate

  Garcia avoids making broad generalizations, citing that an analysis has to be site-specific. “The management practices that you implement, the type of tillage, the kinds of organic matter, the soil and slope of the vineyard, the climate and other factors are driving the change,” he said. “That’s why I like to see soil samples from an individual site.”

  When asked about till versus no-till, Garcia suggested staying away from extremes. “It’s a little bit problematic because tillage could be a valuable tool, assuming that there’s a good reason for doing it and you’re aware consequences to what you’re doing,” he said.  For example, he supported a light pass less than three inches depth in those instances where a grower needs to grow a cover crop on difficult soil. He cites recent evidence that growers can accomplish their objectives with strategic tilling. “When you have high compaction and tried everything you could do to counteract that, then sometimes a light till might be a lifesaver. I think the problem is when we abuse these kinds of techniques and overdo it.”

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