Soil organic matter


An important biological property is the soil organic matter (SOM) content.

SOM is the foodstuff for a large variety of soil organisms ranging upwards in size from bacteria, archaea, and fungi (the microorganisms) to earthworms, ants, termites, and wood lice. This hierarchy of organisms constitutes a soil food web. As SOM is consumed by these organisms, organic molecules with negative electrical charges are formed and combine with mineral particles such as clay and silt to form aggregates. Other complex molecules such as polysaccharide gums and mucilages are also produced that aid in stabilizing the aggregates. This is all part of soil structure formation.

Organic matter is usually expressed as percent carbon, its main building block. As microorganisms decompose organic matter, they derive energy for growth and absorb mineral nutrients such as Nitrogen (N), Phosphorus (P), and Sulphur (S) which is important for nutrient cycling.

Conducting soil analysis will provide your baseline %SOM, and testing every three years will enable you to chart your improvement over time. It can be measured in different ways but common methods include ‘loss on ignition (LOI)’ or ‘dry combustion (Dumas)’. It is important to use the same laboratory and same method each time, to give comparable results. A well-mixed representative sample from a number of samples for each block is also important to avoid skewed results from the presence of roots, etc.

In detail

Building up soil organic matter (SOM) is the foodstuff of soil organisms, from earthworms down to bacteria. Clean-cultivation of interrow and undervine generally leads to a decrease in SOM, whereas SOM is increased under cover crops, especially permanent swards, growing interrow and undervine. Cover crops can be grazed by sheep in winter, which helps to control weeds and enhances nutrient cycling. Compost, mulches and manures also increase SOM. Compost can be made from grape marc, straw, bark chips, grass cuttings and manure (chicken, cow or pig). It is most effective when applied undervine where the soil is kept moist and encourages the growth of soil organisms. Mulches of straw or bark chips are slow to decompose and therefore more effective in building up SOM than herbaceous mulches that decompose more quickly.

Soil health is promoted when there is an active chain of organisms from the smallest (bacteria) to the largest (earthworms), described as the soil food web. This makes nutrient cycling more efficient and if the organic substrate is of low C:N ratio, mineral N is made available to the vine. The diverse population of microorganisms (bacteria, archaea, fungi, actinomycetes, protozoa and nematodes) is called the soil microbiome. A healthy microbiome stimulates mineralisation of SOM and may also produce low concentrations of growth-promoting compounds. In some cases, natural soil microorganisms can suppress soil-borne bacterial and fungal pathogens and parasitic nematodes.

There are two main aspects to increasing SOM:

  1. Minimising losses: using absolutely the minimum tillage necessary to control weeds.
  2. Actively adding new SOM:
    • Compost: adding compost to the undervine strip increases the SOM whilst adding nutrients and improving soil structure
    • Mulching: mulching adds SOM to the surface, to be worked in by soil fauna. It also suppresses weeds and lowers the surface temperature of the soil
    • Cover crops: allowing vegetation to grow on the vineyard floor increases photosynthetic activity, improves soil structure, and reduces soil temperature
    • Biochar: biochar stores carbon in the soil long-term and improves water holding capacity

Another important biological property is the microbial biomass carbon (MBC). Organic C that is rapidly processed through the microbial biomass can form an association with soil mineral particles. The result is a dark brown topsoil consisting of small friable aggregates. Such a topsoil is excellent for water infiltration, aeration and root growth.

Much of the above information on SOM is derived from the following AWRI factsheets:

Further information

To find lab for analysis contact:

In the UK labs include:

Please note that this list of laboratories is for information purposes. The RVF does not endorse, recommend or guarantee the suitability, accuracy or quality of the services provided. You should exercise due diligence before engaging any advisors.