The soil-climate connection

While there are many components to maintaining productive and sustainable soils, soil organic matter deserves special attention because it affects the chemical and physical properties of the soil and its overall health. Soil Organic Matter (SOM) consists of a whole series of materials ranging from undecayed plant and animal tissues to the fairly stable brown/black material normally defined as humus, and also includes the organisms that live in the soil. SOM enhances water and nutrient holding capacity-soil organic matter holds 10 to 1,000 times more water and nutrients than the same amount of soil minerals. SOM also improves soil structure, stores carbon out of the atmosphere, can enhance productivity and environmental quality, and can reduce the severity and costs of natural phenomena, such as drought, flood, and disease.

While the consumption of fossil fuels has taken the brunt of the blame for anthropogenic greenhouse gas emissions, land-use changes have been responsible for about one-third of the increase in atmospheric carbon dioxide over the last 150 years, mainly through the loss of soil organic carbon. In fact, more than twice as much carbon is stored in the Earth’s soils as is stored in living vegetation or the atmosphere. The European Conservation Agriculture Federation suggests that over about 20 years, most agricultural soils will have lost about 50% of their organic carbon because of the reliance of industrial agriculture on inorganic fertilizers, rather than organic composts and manures, as a source of crop nutrients, and the extensive use of tillage.

Solution: Compost sequesters carbon in the soil.

Soils provide natural storage for carbon and other nutrients. More than twice as much carbon is stored in the Earth’s soils as is stored in living vegetation or in the atmosphere. However, the amount of carbon stored in soils has been decreasing due to urbanization, land use changes, conventional agricultural practices, open pit mining and other activities that degrade soils. In fact, more carbon entered the atmosphere from soils than from fossil fuel combustion from the 1860s until the 1970s.

The application of compost to the soil increases its carbon storage capacity in three ways. First, harvesting crops removes carbon from the soil that would otherwise return to the soil when the plant dies and decomposes. Compost in the proper amount returns all of this organic matter to the soil. Second, the nitrogen in compost can increase soil productivity, which can lead to increased crop residues and an increased return of carbon to the soil. Third, composting increases the formation of stable carbon compounds, or humus, that remain bound in the soil for long periods of time and comprise 60-80 percent of soil organic matter. This soil carbon then increases soil fertility, water-holding capacity, drought resistance and nutrient cycling, and reduces soil erosion. This lead the European Commission’s Working Group on Organic Matter to conclude, “Applying composted EOM [exogeneous organic matter] to soils should be recommended because it is one of the effective ways to divert carbon dioxide from the atmosphere and convert it to organic carbon in soils, contributing to combating greenhouse gas effect.”

In the world’s first full lifecycle study on the environmental impacts of windrow composting (using long rows to produce large volumes of compost), the Australian Department of Environment and Conservation found commercial composting of organic waste and the application of compost to agricultural soils resulted in net greenhouse gas reductions, even if the recycled materials have to be transported up to 600 km (370+ miles) for agricultural applications. The study also determined commercial composting and agricultural application resulted in other benefits including the “reduction in use of fertilizers, herbicides, water, and electricity resulting from compost applications, and therefore reducing release of GHGs, nutrients and toxic chemicals to environment (air, water, and soil) during production and use of these avoided inputs.”