what the SOM?

soil organic matter as an indicator of soil health
photo credit https://www.sciencenewsforstudents.org/article/dirt-soil

So what is SOM? The USDA defines SOM as: the organic component of soil consisting of three primary parts:

  • (fresh) plant residues and small living organisms
  • (active) decomposing organic matter
  • (humus) stable organic matter

Most of us understand the importance of sufficient nutrition in the soil – nitrogen, phosphorus, and potassium – to the health of our gardens and landscape, but the value of soil carbon is often underestimated.

Through photosynthesis, plants get carbon from the air, but soil plays an important role in the health of plants by supporting a living system.

Carbon, the primary ingredient in soil organic matter, is the unifying factor in a soil’s capacity to function in plant growth and decomposition, water filtration and erosion resistance, plant, animal and human health. The organic matter is what gives your soil its dark-brown color and rich earthy smell. SOM sources include manures, compost, dead plants, and even living plant roots. All of these contain different forms of carbon-rich materials, including lignin, cellulose, sugars, lipids, and humic acids.

down into the dirt

Here at the Skagit project you will finds mounds of arbor chips and organic mulch placed beside planting beds and near riparian areas seasonally and eventually you will
see shovels at work as our Grounds Crew amends the landscape and turf with SOM. Our garden maintenance program includes building our soils organic material in combination with a twice application of Organic Slow Release Fertilizer, to build a healthy sustainable growing foundation for all our planting and restoration projects.

photo – l. fowler

Soil is a complex living environment where decaying organic matter, microorganisms and insects, water, air, and a mixture of minerals support life, so on the basis of organic matter content, soils are characterized as either mineral or organic.

Mineral soils derived from minerals or rocks and containing trace to small percentage of organic matter or humus, make up majority of the world’s cultivated land.

Organic or “living” soils contain more than 30 % organic matter creating a nutrient rich mini-ecosystem with microorganisms that feed and breath life back into the soil.

We garden in soil – We wash dirt out of our clothes

Just to be clear…soil is not dirt! Dirt which is basically dead, dirt, contains sand, silt, and clay, sometimes in the form of rocks – but lacks the minerals, nutrients or living organisms found in soil.

The foundation for all life on our planet, soil supports a network of ecosystems from rainforest to deserts that service biodiversity above and below ground in plants and animals. Soil is an integral part of the water cycle – removing from the atmosphere and through absorption, it removes and filters out pollutants, and in the soil, water is available for plant uptake. It has an affect on the planets waterways and lakes, in how they flow and retain water.

Soil contains a “history” of our planet and evidence of human development through which we may find the answers to understanding changes in our climate through soils historic testimony.

scoop on the big ‘O’

Organic is not a new concept, although the term “organic farming or gardening”, in modern western culture, has morphed into a definition that promotes the production of crop, animal, or other products without the use of synthetic chemical fertilizers and pesticides, transgenic species, or antibiotics and growth-enhancing steroids, or other chemicals, the concept of organic was pioneered by Sir Albert Howard (1873-1947) and many of his generation, including F.H. King, Walter Northbourne, and Lady Balfour among others who played an important role in the development of organic agriculture practices. Historically, organic, was promoted as methods of building soils through composts, green manures, mulches, and cover crops to increase the levels of SOM as a primary technique. Howard’s concept of soil fertility centered on building soil hummus and emphasized how soil life was connected to the health of crops, livestock, and mankind – he believed it was a birthright. Northbourne coined the term ‘organic’ from Howard’s system of agriculture and ‘having a complex but necessary interrelationship of parts, similar to that in living things’. Lady Eve Balfour compared organic and non-organic farming and helped to popularize organic farming with the publication of The Living Soil (1942)

J.I. Rodale’s global magazine Organic Farming and Gardening first published in 1940, has long promoted the historic definition of organic farming – so for many who have practiced these methods (often subjected to ridicule), enjoyed particular satisfaction in the 68th UN Assembly (2015) International Year of Soils with a theme of “Healthy Soils for a Healthy Life”.

HEALTHY SOILS FOR A HEALTHY LIFE Food and Agriculture Organization of the UN has been nominated to implement the IYS 2015, within the framework of the Global Soil Partnership and in collaboration with Governments and the secretariat of the United Nations Convention to Combat Desertification. The IYS 2015 aims to increase awareness and understanding of the importance of soil for food security and essential ecosystem functions.

The objectives of the IYS were to raise awareness about the importance of soil to human life and our continued existence. The education of soils crucial role in food security, climate change, adaptation and mitigation, essential ecosystem services, poverty alleviation and sustainable development – and the investment in sustainable soil management activities were among the IYS goals.

There is a large body of science and numerous studies that show SOM provides benefits for building soil health. It improves the number and diversity of beneficial organisms that provide nutrients for plants, fixing nitrogen, and controlling soil-borne plant diseases.

so what does this mean to the gardener?

If you are interested in limiting the need for synthetic chemical fertilizers, especially nitrogen fertilizers that are linked to numerous environmental problems, than building your soil is key. Decomposing plant and animal residues into the SOM can provide all the nutrients needed by your plants and turf.

SOM improves your soil structure so it is more resistant to erosion, crusting, and compaction. With good SOM levels your soil can absorb and retain moisture – storing it for your plants to use in dry periods, and increase water infiltration into soil.

taking soils ‘temperature’

Sampling, testing and the interpretation of SOM tests can indicate the overall health of your garden and landscape soil but there is no laboratory test to directly quantify SOM, but estimates of SOM can be made through a measurement of soil organic carbon (SOC), or total soil carbon. The most common method used to estimate the amount of SOM in a soil sample is by measuring the weight lost when it is heated to 400°C – ‘loss on ignition’, and essentially the organic matter is burnt off.

Testing methods differ in cost, accuracy and reproducibility – and improper soil sampling methods can produce inconsistent and misleading SOM test results. If you believe the SOM test benefits would outweigh the cost factor than choose a soil testing laboratory and SOM testing method that will provide consistent data quality. (Tabatabai, M.A. 1996 – Soil Organic Matter Testing)


SOM in your garden

Increasing the SOM in your landscape or garden can be accomplished with the below list of do’s and don’t:


  • Mulch gardens and landscapes
  • Add compost or manure to the garden
  • Use sheet mulching to build no-till
  • garden beds
  • Leave undiseased stalks and leaves
  • in the garden as mulch to protect soil
  • and feed the worms and microbes
  • Use annual weeds that have not yet
  • gone to seed as mulch
  • Compost or bury vegetable kitchen
  • scraps in the garden
  • Save your coffee grounds and add
  • them to soil


  • Avoid excess tillage – use only when and where it’s necessary
  • Dig garden beds by hand
  • Build “no-till” garden beds
  • Don’t rototill the garden in the fall
  • Never leave soil bare – mulch with leaves, straw, grass clippings, etc.


Humus – Highly decomposed portion of soil organic matter. Humus is made of
humic and fulvic acids. These are very large, complex molecules that hold nutrients and
water in the soil, making them more available to plants.

Organic – (of food or farming methods) produced or involving production without the use of chemical fertilizers, pesticides, or other artificial agents.

Photosynthesis – the process by which green plants and some other organisms use sunlight to synthesize foods from carbon dioxide and water. Photosynthesis in plants generally involves the green pigment chlorophyll and generates oxygen as a byproduct

Soil tilth – term used to describe the favorability of soil conditions for growing plants. A soil with good tilth has good structure, nutrients, and water
holding capacity.

Tillage – is the mechanical disturbance and mixing of soil. The degree and results
of disturbance depend on the implement used. For example, a shovel causes much less
disturbance and damage than a rototiller.

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