Vineyard views

Wines & Vines, July, 1999 by Cliff Ohmart

Now that the grape-growing season is in full swing I feel compelled to write something about disking, one of the most common vineyard practices in many winegrape growing regions. The urge becomes particularly strong when I drive by vineyards where every square inch of soil has been cultivated, including right up to the edge of the highway, so that the only green things in sight are grape leaves. I also feel inspired to put pen to paper when I drive through vineyard country on a windy day and the air is filled with dust, or when I park my car for a few hours and get back to find it covered with a nice coat of dirt.

You might ask what disking has to do with sustainable viticulture. The answer? An awful lot, since disking has a tremendous impact on the soil, which is literally and figuratively the foundation of sustainable viticulture.

Tilling the soil, as some soil text books state, is as old as written history. Disking, one form of tilling, has long been considered a fundamental crop production practice, and it can benefit vineyards in several ways:

(1) Weed management. Disking is the primary way that grape growers control weeds between the vinerows. Actively growing weeds can compete with the vines for nutrients and water. Furthermore, growers often talk about weeds encouraging vineyard pests, such as omnivorous leafroller or nematodes. Therefore disking helps manage these problems.

(2) Incorporation of organic matter. Disking is used to incorporate a cover crop, vine prunings, or compost, which increases the speed of their decomposition.

(3) Breaking up surface compaction. Disking keeps the soil surface friable for easy ditch establishment in vineyards where furrow irrigation is used. Furthermore, many growers feel that well disked soil is much smoother than untilled soil for operating machinery, such as sulfur dusters.

(4) Preparing a seedbed for cover crops. It is important to prepare a good seedbed for cover crop establishment and disking is an essential part of this preparation.

(5) Discouraging rodents. Frequent disking tends to keep voles out of the vineyard and discourages gophers.

The saying goes that there is no such thing as a free lunch, and disking is no exception. Frequent disking of the soil can have negative effects on soil and the vineyard:

(1) Decreases soil organic matter. Disking increases the rate of organic matter decomposition thereby decreasing the level of organic matter in the soil. Organic matter provides carbon and an energy source for soil microbes, it aides the growth of crops by improving the soil's ability to store and transmit air and water, and it stores and supplies nutrients to crops by providing cation-exchange capacities.

(2) Decreases aggregate stability. Frequent disking reduces soil organic matter and microbial populations which reduces aggregate stability. Soil aggregates are groups of soil particles that bind to each other more strongly than they do to surrounding particles.

The spaces between the aggregates provide pore space for water and air. Aggregate stability refers to the ability of these aggregates to resist breakdown. Organic matter and soil microbes enhance soil aggregation and encourage aggregate stability.

(3) Decreases water infiltration. Natural channels are formed by cracking of the soil, decaying roots and earthworms, and they allow soil to absorb water quickly. Disking - particularly if it is done when the soil is wet - breaks up these natural channels, and frequent disking causes the formation of a plow pan or impenetrable layer.

(4) Increases soil erosion. Disked soil is much more prone to wind and/or water erosion than soil under a reduced tillage program, non-tilled soil, or soil occupied by plants or plant residue. Erosion declines by 50% when 30% of the soil surface is covered by plants or crop residue. Studies in corn systems have shown a 400% decrease in erosion when growers use minimum tillage.

So what is the right thing to do when it comes to deciding whether to disk your vineyard or not? As with most other viticultural practices, you need to determine the priorities for your vineyard and then weigh the positives and negatives of your choice for vineyard floor management. There is no question that the conventional method for vineyard floor management has been disking. I am sure that many winegrape growers grew up on a tractor pulling a disk. There is something very satisfying about moving through a vineyard row by row, turning up the clean soil, leaving a fluffy bed behind the tractor. However, as we learn more about how to grow high-quality winegrapes, and drip irrigation systems become the conventional way to water a vineyard, disking may not be the necessity that it used to be.

For example, establishing a drip irrigation system in the vineyard changes things quite a bit. The soil does not need to be disked to keep it in a condition for easy ditch establishment. With this issue out of the way, the next most common concern about vineyard floor management is that weeds or cover crops will compete with the vines for water and nutrients if disking is not done. It is natural for growers to want to make sure that all the water and nutrients make it to the vine and not to the weeds. In most crops, there is a direct correlation between production and increased water and nutrients. Winegrapes, however, are different. Premium winegrapes generally come from vines whose vigor has been reduced and vine balance has been achieved, either by natural aging or through canopy management. Furthermore, recent research has shown that winegrape quality is enhanced when the vines get about 30% less than full water use. Therefore it is not necessary to get every drop of water and every bit of nutrient from the soil into the vine. In fact, in many cases this is undesirable. Some growers want vegetation to dry out the soil in the spring, either so it is easier to get equipment on the vineyard without compacting the soil, or so that they have complete control over how much water the vines receive by using drip irrigation. Disking becomes less crucial and possibly a problem under these circumstances.