By Rob Ellison
Some of the increase in atmospheric CO2 can be ascribed to a decline in soil carbon following land conversion to intensive farming methods (grazing or cropping). The decline in soil carbon content following farming is seen in Figure 1.
Figure 1: Shows the relative declines in soil carbon following conversion to farming (Source: Lal and Follett, 2008)
‘Organic matter is the lifeblood of fertile, productive soil. Without it, agricultural production is not sustainable.
Organic matter is any living or dead animal and plant material. It includes living plant roots and animals, plant and animal remains at various stages of decomposition, and microorganisms and their excretions.
On farms the main sources of organic matter are plant litter (plant roots, stubble, leaves, mulch) and animal manures. Earthworms and microorganisms decompose these materials. The process of decomposition releases nutrients which can be taken up by plant roots. The end product of decomposition is humus, a black crumbly material resistant to further decomposition. A complex chemical substance, humus stores plant nutrients, holds moisture and improves soil structure
Benefits of organic matter
- Improve soil structure As organic matter decays to humus, the humus molecules ‘cement’ particles of sand, silt, clay and organic matter into aggregates which will not break down in water. This cementing effect, together with the weaving and binding effect of roots and fungal strands in the decomposing organic matter, makes the soil aggregates stable in water.
- Improves drainage These larger, stable aggregates have larger spaces between them, allowing air and water to pass through the soil more easily.
- Holds moisture The aggregates are also very effective in holding moisture for use by plants. Humus molecules can absorb and hold large quantities of water for use by plant roots.
- Provides nutrients Organic matter is an important source of nitrogen, phosphorus and sulfur. These nutrients become available as the organic matter is decomposed by microorganisms. Because it takes time for this breakdown to occur, organic matter provides a slow release form of nutrients. If crops are continually removed from the soil, there is no organic matter for microbes to feed on and break down into nutrients, so fewer nutrients are available to plants.
- Improves cation exchange capacity Humus molecules are colloids, which are negatively charged structures with an enormous surface area. This means they can attract and hold huge quantities of positively charged nutrients such as calcium, magnesium and potassium until the plant needs them. Clays also have this capacity, but humus colloids have a much greater CEC than clays.
How to increase soil organic matter levels
- Grow perennial pasture A period under perennial, grass-dominant pasture is an effective way of increasing organic matter in farm soils. Short-lived annual grasses are a source of dead roots; perennial grasses are a source of leaf matter. Even short periods (1–2 years) under pasture can improve soil structure, even though the actual increase in organic matter may be small.
- Grow cereal crops Cereal crops leave significant amounts of organic matter in their dead roots and stubbles after harvest.
- Grow green manure crops Green manure crops provide protective cover until they are ploughed into the soil. Initially they provide a large increase in organic matter levels, but they break down rapidly to give only a small increase in long-term organic matter levels; also, the ploughing operation can do more harm than the good done by the organic matter.
- Spread manure Bulky organic manures will increase organic matter, but frequent and heavy applications are needed to produce significant changes.
- Use organic fertilisers Organic fertilisers applied in large amounts can boost organic matter levels but are generally less cost-effective as supplies of nutrients than inorganic fertilisers. Applied in small quantities, they are unlikely to have a significant effect on organic matter levels.
- Keep cultivation to a minimum Cultivation breaks down the stable aggregates, exposing humus in the aggregates to air and faster decomposition. Direct drill techniques allow you to sow seed while leaving stubble residues on top of the soil, and leaving aggregates intact.
- Concentrate organic matter An alternative to increasing inputs is to make more effective use of what is already there. Retain all organic additions, whether roots, stubble or manure, close to the surface. The stability of soil structure is related to the concentration of organic matter at the surface, not the total quantity present in the soil.
‘Agricultural lands occupy 37% of the earth’s land surface. Agriculture accounts for 52 and 84% of global anthropogenic methane and nitrous oxide emissions. Agricultural soils may also act as a sink or source for CO2, but the net flux is small. Many agricultural practices can potentially mitigate greenhouse gas (GHG) emissions, the most prominent of which are improved cropland and grazing land management and restoration of degraded lands and cultivated organic soils. Lower, but still significant mitigation potential is provided by water and rice management, set-aside, land use change and agroforestry, livestock management and manure management. The global technical mitigation potential from agriculture (excluding fossil fuel offsets from biomass) by 2030, considering all gases, is estimated to be approximately 5500–6000Mt CO2/yr.’ (Smith et al, 2008)
Figure 2: Global Greenhouse Gas Emissions (http://www.epa.gov/climatechange/science/indicators/ghg/global-ghg-emissions.html)
- Carbon dioxide (CO2) – Fossil fuel use is the primary source of CO2. The way in which people use land is also an important source of CO2, especially when it involves deforestation. Land can also remove CO2 from the atmosphere through reforestation, improvement of soils, and other activities.
- Methane (CH4) – Agricultural activities, waste management, and energy use all contribute to CH4 emissions.
- Nitrous oxide (N2O) – Agricultural activities, such as fertilizer use, are the primary source of N2O emissions.
- Fluorinated gases (F-gases) – Industrial processes, refrigeration, and the use of a variety of consumer products contribute to emissions of F-gases, which include hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6).
‘Agricultural land… denotes the land suitable for agricultural production, both crops and livestock. It is one of the main resources in agriculture. The standard classification (used, e.g., by FAO — Food and Agriculture Organization of the United Nations) divides agricultural land into the following components, with their respective global land area in 2009:
- Arable land (13,812,040 km²) – land under annual crops, such as cereals, cotton, other technical crops, potatoes, vegetables, and melons; also includes land left temporarily fallow.
- Permanent Crops (1,484,087 km²) – Orchards and vineyards (e.g., fruit plantations).
- Permanent Pastures (33,556,943 km²) – areas for natural grasses and grazing of livestock, such as Meadows and pastures.’
The total is 4.9 billion hectares on which future global food security depends – an absolute necessity to increase the productivity of agricultural lands this century to provide for increased population and changed consumption patterns. There is an opportunity as well to address nitrous oxide and methane emissions and make substantial inroads on carbon dioxide emissions.
Figure 3: Masai with cattle (Source: Wikipedia)
Here’s a guy who is all cattle and no hat. Although pastures can be restored globally to mitigate CO2 – it works only as much as the economic benefits of newer pasture techniques works for individual farmers.
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