Organic matter decline

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Soil organic matter – general information
Soil organic matter refers to organic constituents in the soil, including any plant or animal material that returns to the soil and goes through the decomposition process. Soil organic matter is one of the most important soil substances, as it affects the soil structure and porosity, the water infiltration and moisture holding capacity, the diversity and biological activity of soil organisms and plant nutrient availability.

In addition to providing nutrients, high organic matter content favors high level of soil aggregation which in turn is essential for good soil structure, aeration, water infiltration and resistance to erosion and crusting. In most soils the organic matter varies in the range 2 10 percent. However even in small amounts, organic matter is a very important constituent of the soil.

Depending on the organic matter content, soils are characterized as mineral or organic. Mineral soils form most of the cultivated land worldwide and may contain from trace to 30 percent organic matter, usually below 10 percent. Organic or peat soils have been developed under special natural conditions, they need special management in order to sustain the high organic matter level and because of their rarity they are not vital cropping soils. LIFE SoS concentrates on the organic matter of mineral cropping soils in the semi-arid region of the South-Eastern Mediterranean. The aims of this part of the project are firstly to explore the actual organic matter content in the cropland of Anthemountas region and secondly to discover and propose these farming practices that will help improve the quality of the soil, regarding to the organic matter content.

Farming systems tend to mine the soil for nutrients and to reduce soil organic matter levels through repetitive cultivation of soils, harvesting of crops and inadequate efforts to replenish nutrients and restore soil quality. The decline continues until management practices are implemented or improved or until fallow period allows a gradual recovery through natural ecological processes. However, rebuilding soil quality and restoring adequate organic matter levels through appropriate farming practices may take several years, especially in areas that farming takes place during hot and dry periods.

Soil organic materials consist of a variety of substances in varying proportions and intermediate chemical and physical stages, such as live organisms and their un-decomposed, partly decomposed and completely decomposed remains as well as products of their transformations:

  • Living (soil) organisms and microorganisms
  • Soil organic matter: non-living components, as a heterogeneous mixture resulting from physical (fragmentation), microbiological and chemical transformations of organic debris
    • raw plant (and animal) residues, or older non-transformed organic debris (unaltered identifiable materials)
    • active or transformed organic fraction, sometimes referred as the humification process products, and
    • resistant or stable organic matter also referred as humus, that consists of relatively high molecular weight, brown to black colored substances, formed by secondary synthesis or reactions.



Soil organic matter role
Soil degradation related to the organic matter declination has become a major concern in cultivated agricultural land, especially in arid and semiarid regions. On most soils suitable physical properties may occur even at relatively low levels of organic matter (2-4%). A level of organic matter higher than required to maintain suitable physical properties is beneficial in that the soil has a greater buffering and nutrient holding capacity, but it does not contribute directly to soil productivity.

The active and some of the resistant soil organic components, together with microorganisms are involved in binding small soil particles into larger aggregates favoring good soil structure, aeration, water infiltration and resistance to erosion and crusting.

The resistant or stable fraction of soil organic matter – humus - contributes mainly to nutrient and water holding capacity and soil color. This fraction of organic matter decomposes very slowly and therefore has less (direct) influence on soil fertility than the active organic fraction. Soil organic matter serves in two main ways in relation to nutrient supply to the plants:

Soil organic matter is mainly derived from plant residues and therefore it contains all of the essential plant nutrients. Upon decomposition the nutrients are released in a plant-available form.
•    The stable organic fraction usually referred as humus, has a great cation exchange capacity (CEC), in other word may absorb, hold and later release a big amount of nutrients in plant-available form.

The main physical, chemical and biological functions that organic matter of the soil serves are:
•    Binds soil particles together into stable aggregates
•    Positively influences water holding capacity and aeration
•    Positively affects soil temperature
•    High cation exchange capacity (Humus CEC: 100-300cmol+/kg, Kaolinite CEC: 2-15cmol+/kg)
•    Source of pH buffer
•    Binding site for heavy metals and pesticides
•    Food source for the micro-organisms of the soil and small animals
•    Major reservoir of plant nutrients

Levels of organic matter in soils
The amount of organic matter in a soil depends on a range of factors and reflects the balance between accumulation and breakdown of organic substances. The main factors that control this balance are:

  • Climate. For similar soils under similar management, carbon is greater in areas of higher rainfall and lower in areas of higher temperatures. The rate of decomposition doubles for about every 8 or 9°C in mean annual temperature.
  • Soil type. Clay helps protect organic matter from breakdown in various ways, such as by strongly binding organic matter, by keeping higher moisture levels, preventing aeration etc.
  • Vegetative growth. The more vegetative production the greater the carbon inputs to the soil. The kind of vegetation, i.e. the carbon to nitrogen ratio (C/N) plays significant role to organic matter rise.
  • Topography. Soil at the bottom of slopes have generally higher carbon, because these areas are generally wetter and have higher clay contents. Poor drainage slows down organic matter decomposition.
  • Tillage. Tillage will increase organic matter breakdown. At some extent this negative effect may be balance by the amount of plant residues (carbon input to soil) that are incorporated in the soil.

Organic matter in virgin soils is usually higher than in cultivated soils and furthermore when cultivation of such soils begins organic matter content usually declines. On cultivated soils there are several agricultural practices that contribute to increased soil organic matter content. Such practices in general are:

  • Incorporation of plant residues (after harvest)
  • Addition of organic materials such as manure, compost or other organic substances
  • Mulching, thus the coverage of the soil with raw plant residues
  • Crop rotation that includes pasture and fodder species
  • Less or minimal soil cultivation, especially during dry and hot periods (no till farming may also be adopted)
  • Maintaining high soil humidity levels

Perennial forages are the most beneficial in terms of increasing soil organic matter, as a result of:

  • The extensive and deep route system they leave in the soil
  • The fibrous nature of the root system of perennial grasses
  • The increased permeability of dense subsoils because of the deep penetrating roots
  • The reduced rate of organic matter decomposition in the absence of tillage

Practices like burning or removing of the plant residues, intensive soil cultivation, especially under dry and hot conditions and in-depth drying and heating of soil, negatively affect soil organic matter content. Another factor contributing to soil organic matter depletion is the summer-fallowing, since aeration, increase in soil temperature and loss of moisture, results in increased organic matter decomposition.

Soil erosion is another factor that contributes to organic matter declination, whereas during the erosion process the top and usually the richest in organic matter soil horizon is taken away.

The decomposition of the organic materials only takes place if conditions are suitable and the rate of the transformations is also critically affected by these conditions. Decomposition occurs when temperature, humidity and alkalinity lay in specific limits. The type of microorganisms involved in the process depends on these conditions.

Soil organic matter cannot be increased quickly when management practices that conserve soil organic matter are adopted. At the end only a small fraction of crop residues added to soil remains as soil organic matter. Only after an extended period of time the return of crop residues combined with suitable agricultural practices and if possible crop rotation of forages with cereals and oilseeds may increase the soil organic matter.
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