© 2019 Australian Soil Management

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What is Soil Organic Matter (SOM)?

Plants use CO2, water and sunlight energy to make organic matter. Plants (and animals as part of the food chain) eventually die and return to the soil where they are decomposed by small organisms to form soil organic matter. There is a continuous turnover of SOM in soil. It is a complex mixture of organic matter at different stages of decomposition.

What are Soil Organic Matter pools?

It is convenient to divide Soil Organic Matter into different pools dependent on their ease of decomposition. These are the labile pool, slow pool and inert pool. The labile pool is all the freshly added plant and animal residues as well as small soil organisms. These are easily decomposed into nutrients for plant roots. The slow pool is already well decomposed and called humus. The inert pool is old and resistant to further breakdown. The important point to note is that soils differ not only in total Soil Organic Matter but in the size of each pool. A large labile pool is preferred because it means the soil can provide nutrients for plant growth.

Why is Soil Organic Matter important?

Soil Organic Matter is essential for soil quality and is the basis of sustainable agriculture. A high quality soil has a good structure (soil crumbs or aggregates), supports more plant growth, stores moisture, has plenty of air pockets, and does not erode easily. Soil Organic Matter is a major natural store of carbon for the Earth. There is twice the carbon stored in Soil Organic Matter than in air or plants and animals.

What is the difference between Soil Organic Matter and carbon?

Soil Organic Matter is, on average, made up of 58 per cent carbon. Soil organic carbon in soil test results can be converted to Soil Organic Matter by multiplying your Soil Organic Carbon result by 1.72.

How can I convert percentage Soil Organic Carbon in my soil test results to tonnes of carbon per hectare?

Tonnes carbon per hectare = Soil Organic Carbon % x Soil bulk density (Mg/m  ) x Sampling depth (cm). One Mg = 1000 kg = 1 tonne. For example, 1% Soil Organic Carbon in a soil with a bulk density of 1.4 Mg/m  sampled to a depth of 30 cm contains 42 tonnes of carbon per hectare.

How much carbon can be stored in soils?

Some soils can store more carbon than others. Soil Organic Carbon ranges from about 10 per cent in alpine soils to less than 0.5 per cent in desert soils. Some soils in Australia have fallen down below 1 per cent after decades of agriculture. Soil Organic Carbon in farmland, parks and sports fields is usually lower than adjacent areas still with natural vegetation. The challenge is to change soil management practices to increase Soil Organic Carbon and Soil Organic Matter. Some Land Managers are already doing this. Australian soils offer a big opportunity to capture and store extra carbon.

What management practices increase Soil Organic Carbon?

  • Keep soil covered with live healthy plants. Healthy plants capture more carbon

  • Avoid soil cultivation, plowing or using a rotary hoe

  • Do not burn off dead plant material. The carbon is removed as CO2  and the soil is left bare

  • Do not cut grass too short or overgraze. Plants under stress capture less CO2

  • Protect soil from erosion by wind or water. Lost soil is lost carbon

  • Direct addition of carbon in the form of composts, manures and other recycled organics

  • Any practice that improves plant growth because more growth means more CO2 captured. Avoid chemical fertilizers and pesticides if possible because large quantities of CO2 are generated in their manufacture