Management of Vineyard Soils
Organic Fertility Management
The foundation of organic viticulture lies in the health of the soil and organic growers are required to maintain or improve the soil that they manage. Organic farmers have different approaches to supplying grapevine needs compared to conventional growers who provide fertility by numerous synthetic fertilizers. However, organic growers are limited only to those inputs that build up and maintain organic matter. Thus increasing soil organic matter is a key aspect of an organic production system. Adding organic materials such as cover crops, crop residues, and composts to cultivated soils over time builds soil organic matter and improves the ability of the soil to supply nutrients. The ultimate goal is a healthy, fertile, biologically active soil with improved structure and enhanced nutrient availability.
Role of Organic Matter in Soil Fertility
Increasing soil organic matter is a key aspect of an organic production system. The formation and decomposition of soil organic matter are fundamental life-promoting processes that store and release energy derived from photosynthesis.
Humus is the most resistant and mature fraction of soil organic matter. It is very slow to decompose and may last for hundreds of years. Humus is responsible for giving the soil that rich, dark, spongy feeling and for properties such as water retention and cation exchange capacity. Plant residues that are high in carbon (C) and low in nitrogen, such as straw or cornstalks, decompose slowly but are efficient producers of humus. Residues that contain high levels of nitrogen, such as young cereals and legumes, decompose quickly, producing less humus.
The active fraction is responsible for the release of most nitrogen where it is converted into plantavailable mineral forms such as ammonium (NH4+) and nitrate (NO3¯) through the process of mineralization.
Factors Affecting the Accumulation of Soil Organic Matter
Generally it takes 3 to 5 years of soil-building program (compost applications and cover cropping) before the soil organic matter levels (SOM) begin to make noticeable differences in a vineyard. Factors affecting the accumulation of soil organic matter include (McGourty et al., 2011):
Cover Crops and Green Manures
The terms cover crop and green manure are frequently used synonymously. They perform many similar functions and many of the same plant species are used as both cover crops and green manure crops. The main difference between the two is that cover crops fix and trap nutrients, add organic matter to soils, and reduce nitrate leaching, nutrient runoff, and soil erosion. The primary purpose of a green manure is as a soil-building crop to produce organic material for incorporation into the soil. Examples of green manure crops include grass mixtures (e.g., annual ryegrass, winter wheat, etc.) and legume plants (e.g., vetch, clover peas, alfalfa, etc.).
Manure is a valuable source of organic matter and plant nutrients. Most common manures are beef, dairy, hog, chicken, and turkey. Properly managed manure can add plant nutrients and improve soil quality. Raw manure is high in nutrients, especially readily available nitrogen. Fresh, non-composted manure will generally have a higher N content than composted manure. However, the use of composted manure will contribute more to the organic matter content of the soil.
Growers are finding compost to be indispensable aspects of vineyard floor management programs as source of N-P-K and micronutrients. However, composts contain relatively low concentrations of nitrogen, phosphorous, and potassium compared to livestock manure. The nitrogen content of composts will vary according to the source material and how it is composted. In general, nitrogen in the form of ammonium (NH4+) or nitrate (NO3¯) is low in composts.
Commercial Organic Fertilizers
A number of approved organic fertilizers are available; common examples are listed in Chapter 17, Fertilizer Management for Grapevines. Most of these materials are by-products of fish, livestock, and food processing industries.
For many growers the transition from conventional to organic production methods, nitrogen management is the greatest difference in nutrient management and perhaps the entire vineyard operation. It is the plant nutrient that is often most limiting to efficient and profitable crop production. Rather than rely on synthetic soluble nitrogen sources obtained from the combustion of natural gas, nitrogen is recycled primarily from three sources: cover crops, livestock manure usually applied as compost, and nitrogen-fixing cover crops. Nitrogen applied in this way is stable and slowly released. While organic nitrogen is less likely to leach or volatilize, as synthetic fertilizers, it is also not as readily available to the vines.
Nitrogen availability is typically the most limiting factor in the decomposition of organic materials since its availability depends on the material’s carbon-to-nitrogen ratio. The carbon-to-nitrogen ratio (C:N) is important since decomposition of organic matter will either release or immobilize nitrogen. Soil microbes readily release the excess nitrogen in a plant-available form when organic matter has a low C:N ratio or when nitrogen is in excess. When the organic material has a high C:N ratio, microbes will immobilize nitrogen through assimilation resulting in a decline in soil nitrogen until the decomposition process lowers the C:N ratio. A material having 30 times as much carbon as nitrogen, for example, is said to have a C:N ratio of 30:1.
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