Cover crops provide an array of important benefits to vineyards. These include direct benefits that improve soil tilth, build soil structure, enhance the root environment, limit soil erosion, and modify the vineyard environment. Through these, we can gain additional, indirect benefits in the management of soil moisture, mineral nutrients, grapevine growth vigor and balance, wine grape yield and quality, and grapevine pests. While any type of cover crop will provide a number of benefits, certain cover crops accentuate certain benefits. In addition, certain management activities will enhance particular benefits of a cover crop. In this article, we will consider both cover crop selection and management to meet specific goals.
Soil conservation is a concern for all vineyards because topsoils normally contain much greater quantities of mineral nutrients readily accessible to grapevines than the subsoils they overly. For vineyards on sloping ground, soil conservation is the cover crop goal that surpasses all others and under these circumstances, the cover crop ought to have a dense fibrous root system that binds soil particles and holds them in place (fig 1). This requirement limits cover crop possibilities primarily to forage grasses.
Some sloping vineyards occupy former rangeland where, with a little starter nitrogen fertilizer and light cultivation, an appropriate cover crop will grow from seed banks built over many years. Others will require planting or, at least, a low rate of supplemental seed to achieve a stand of grass that is sufficiently thick to protect the soil surface.
Soil Borne Pest Suppression
Aside from soil conservation, limiting damage by high populations of soil-borne pests of grapevines is perhaps the only other cover crop management objective that will exceed all others. In this instance, the most effective cover crop option is a blend of diverse plant species that will promote a diverse population of soil inhabitants. The more diverse the population of soil inhabitants, the less likely any single group inhabitants will be present in large numbers, including groups that damage vine roots. It also helps if the cover crop plants are non-hosts for the soil pest of concern.
This article is a reproduction of the Mid Valley Agricultural Services Viticulture Newsletter, February 2011.
Common blended cover crops include mixtures of forage grasses or cereal grains, legumes, and members of the mustard family (fig. 2). Plants of the mustard family typically have large taproots that make large openings in the soil and substantially increase its porosity, which is an additional benefit that can be particularly valuable for soils high in clay.
Plants in the legume family are critical for vineyards managed with organic inputs and for any grower wishing to capture fertilizer value from a cover crop investment. Leguminous plants include clover, vetch, pea, and bean. They form symbiotic associations with a specific group of bacteria (Rhyzobia), which reside in root nodules. These bacteria capture nitrogen from the atmosphere and provide it to the legume plant in exchange for some of the carbohydrates it produced through photosynthesis. As dead, nitrogen-rich leguminous tissues decompose in the soil, nitrogen becomes available to vines.
The quantities of nitrogen added with legume cover crop residues can be substantial and far exceed the seasonal requirement of grapevines. Blending legumes with grasses or cereal grains both regulates the amount of nitrogen in cover crop residues and substantially reduces the cost of cover crop seed (Fig 3). Blending also slows the rate of decomposition and extends the availability nitrogen by increasing the ratio of carbon to nitrogen (C:N) in the cover crop residue.
Timing of Incorporation
The greatest number of simultaneous benefits from a cover crop comes not from its composition, but from its management. Early cover crop incorporation, while tissues are still succulent and tender, will have the greatest stimulatory effect on soil microorganism populations, including the production of the persistent organic matter called humus (fig. 4). Microbial stimulation and humus production will, in turn, produce the greatest increase in soil tilth and friability, moisture holding capacity, and mineral nutrient availability. At the same time, there will be increases in soil porosity, water infiltration rate, and permeability.
Early incorporation is an essential ingredient of dry farming and maximum water use efficiency in any vineyard because it conserves moisture stored from winter rains. For soils with a high shrink-swell potential, early incorporation creates a dust mulch that minimizes soil cracking, channeling of water, disruption of mineral nutrient movement, and damage to grapevine roots. For all vineyards, early incorporation increases vineyard temperatures and lowers the potential for frost damage. After early incorporation, avoid any subsequent cultivation to minimize organic matter losses and diminished benefits associated with it.
The direct benefits of managing cover crops with non-tillage are limited, but there are some indirect benefits that may be important. Non-tillage allows competition between the cover crop and grapevines for soil resources, including water and mineral nutrients. Such competition is only beneficial for vineyards or sections of vineyards where grapevine growth is overly vigorous and excessive canopy development is the result, and it is proportionate to the growth vigor of the cover crop. Non-tillage also cools the vineyard during the growing season, which may enhance wine grape quality in warm wine grape growing areas. In cool areas, however, it may slow fruit maturation. Finally, non-tillage limits dust arising from the vineyard floor, limiting air pollution and dust deposition on leaves that encourage pest mites.
Actually, true no-till is practiced in very few vineyards because most long-term cover crops require periodic rejuvenation through aeration or fertilization followed by light tillage (fig. 5). Moreover, the floors beneath no-till cover crops become rutted over time and require periodic smoothing to ensure uninhibited tractor access.
Unquestionably, the composition and management of a cover crop will determine the benefits it provides. Often, the short-term value of cover crop benefits is difficult to determine, but over the long term, their benefits are clearly evident as uniform, consistently productive vineyards.
This article was originally published in the Mid Valley Agricultural Services February 2011 newsletter.
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