Management of Vineyard Soils
Organic Matter Management
Organic matter (OM) normally constitutes less than 5 percent of most mineral soils. Soil organic matter is that portion of the soil that consists of carbon-based compounds from the remains of plants, animals, and microorganisms. Through decomposition, organic materials added to a soil change to soil organic matter (SOM) while releasing nutrients. The organic component of the soil includes living organisms, fresh organic residue, and active and stabilized OM fractions. The living component includes microbes although it constitutes a small fraction of the soil. The active OM is generally unstable and more than 85 percent of it disappears quickly as decomposition progresses. Humus is the most abundant and stable form of SOM and it is resistant to further decomposition.
Benefits of Soil Organic Matter
The content of SOM in an organic crop production system is the most important factor because it is the primary nutrient supplier and soil conditioner. Organic matter in the soil balances various chemical and biological processes and helps to maintain soil quality parameters at an ideal level.
Classification of Soil Organic Matter
Several types, or “pools,” of soil organic matter exist, each important for different soil functions. Think of these pools of soil organic matter on a continuum from fresh, active organic matter that is gradually transformed to welldecomposed humus (i.e., stable fraction).
Active Fraction
The active fraction of organic matter is composed of a range of material, including recent plant litter and highly decomposed unrecognizable plant and other organic residues that break down in a very short time, from a few weeks to a few years. This kind of organic matter is associated with biological activity in the soil. The active organic matter accounts for only a small fraction of the total organic matter in the soils but is much more sensitive to soil management practices and is closely related to nutrient cycling and soil tilth-promoting functions of soil organic matter.
Stable Fraction
Many soil organisms assist in the process of decomposing plant and animal tissues. During the process of decomposition, chemical transformations take place, creating new organic compounds in the soil. These organic compounds are relatively resistant to decomposition because of either their chemical structure, their adsorption to clay particles, or their protection within microaggregates. This chemical complex is referred to as the stable fraction, also known as humus, and is not very biologically active.
Mineralization
Organic matter mineralization is a process where microbes metabolize organic carbon and convert organic nitrogen compounds into simple inorganic ions (nitrate and ammonium),which are available for plant uptake. Sources of mineralizable organic nitrogen include native soil organic matter, crop residues, and organic amendments like manure and compost.
Conditions Favoring Build-up of Soil Organic Matter
Carbon-to-Nitrogen Ratio
The carbon-to-nitrogen ratio (C:N) is a major factor that determines the speed of organic material decomposition as well as nutrient release patterns. Low C:N ratio (< 20:1) favors fast decomposition resulting in quick release of nutrients. Many beneficial organisms responsible for decomposition can multiply fast and get their food by decomposing materials with a low C:N ratio.
Temperature
The activity of many microbes acting on organic materials is a function of temperature. Low temperatures slow down decomposition, while warm temperatures speed up decomposition.
pH
While OM decomposition is halted with pH extremes, the process progresses well in the pH range of 5.5 to 6.8. However, individual groups of microbes have specific pH requirements for optimal decomposition.
Moisture and Aeration
Proper soil moisture enhances the growth of microorganisms that break down OM into humus. On the other hand, excess water can lead to anaerobic conditions which slow down the degradation process.
Soil Organisms
Different groups of organisms have different capacities in the process of decomposition. The product of one group can be the source of food for others to further break down organic material. Added OM will be colonized and decomposed by a community of organisms that will change over time as the composition of the residue changes.
Nitrogen Supply
Nitrogen is one of the main substrates needed by microbes to decompose organic materials. When decomposing high C:N ratio material, microbes need an external supply of nitrogen. For organic growers, this nitrogen must be supplied from organic sources, as organic crop production does not allow the use of synthetic fertilizers such as urea, ammonium nitrate, etc.
Organic Matter Additions
There are wide ranges of options that a grower can use to add OM to the soil. Organic materials are highly variable in mineralization pattern, nutrient content, and availability. That is why it is important to set a goal and develop a best management plan for the vineyard. Cover crops, green manure, livestock manure, and compost are typically some of the major materials for building SOM.
Building Soil Organic Matter with 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.
Building Soil Organic Matter with Manure
Livestock manure contains most elements required for plant growth including nitrogen, phosphorous, potassium, and micronutrients. However, it is manure’s organic carbon that provides its potential environmental value to contributing to soil organic matter.
Building Soil Organic Matter with Compost
Compost is an organic matter resource that has the unique ability to improve the chemical, physical, and biological characteristics of soils. Compared to livestock manure, it may have low nutrient levels. Nutrients from compost are often less available to the crop; thus compost may be more useful for building SOM.
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