Research has shown that declining crops yields are, in most cases, exponentially linked to loss of soil quality. Soils are threatened by water and wind erosion, nutrient depletion and salinisation, among other things.
Although, on a global scale, soil loss is unlikely to be a major threat to food security, the impacts are probably going to be felt acutely at the local and regional levels. This is especially so where farmers are too poor to curb or overcome the damage.
Mechanized agriculture has caused billions of tonnes of carbon previously trapped in soil to be released to the atmosphere as carbon dioxide and methane. Lack of carbon in the soil makes crop production inefficient and constrains yields.
Global warming is likely to make things worse. As the ground heats up, organic matter decomposes more easily, reducing soil fertility, releasing carbon dioxide and further exacerbating the warming effect. In fact, soil erosion already contributes to global warming, as some of the carbon in soil-laden water running off fields inevitably escapes into the atmosphere. Deserts are expected to expand as the interiors of continents become drier, and erosion would worsen if soils dried out and violent storms became more frequent.
Inevitably, a hotter world is likely to have less organic matter in its soils. This will have tremendous consequences for farmers the world over, particularly for those farming in the marginal areas of developing countries. Critical regions of soil degradation and hence a high priority for soil restoration and carbon sequestration (storing carbon in soil securely so it is not immediately reemitted) include sub-Saharan Africa, central and south Asia, China, the Andean region, the Caribbean, and the acid savannas of South America.
Fortunately, something can be done, by managing soils to stem erosion and retain more carbon. Carbon sequestration can potentially offset global fossil fuel emissions by 5-15% each year, while raising yields. Once sequestered, carbon can remain in the soil, for the next 20-50 years at least, as long as appropriate management techniques are used.
One proven method of resisting erosion, restoring soil fertility and boosting crop productivity is the use of organic compost. Compost is a natural fertilizer, supplying soils with vital plant nutrients, and aiding them to retain water and air. It restores soil carbon, crucial in maintaining consistent yields through improvements in water and nutrient holding capacity, soil structure and biotic activity. Compost improves soil texture and structure, and helps control weeds, pests and diseases.
The implications of practices such as composting could be enormous for food security. According to Rattan Lal of the Carbon Management and Sequestration Centre at Ohio State University, a one tonne increase of the soil carbon pool in degraded soils may increase crop yield by 20 to 40 kilograms per hectare (kg/ha) for wheat, 10 to 20 kg/ha for maize, and 0.5 to 1 kg/ha for cowpeas. In Ethiopia, farmers using compost increased yields for various food crops, sometimes doubling or even tripling yields. The yields from using compost are comparable, and often higher, than those from chemical fertilizer.
Already, agricultural experts are urging Malawian subsistence farmers to use organic compost, instead of manufactured fertilisers. Samuel Saka, agricultural development officer for Nkhunga extension planning area in Nkhota-Kota district in central Malawi said, "Based on the successful demonstration plots we had, many farmers have started implementing this method and the results speak for themselves - high maize yields."
A nationwide campaign is being promoted, as prices of manufactured fertilisers are skyrocketing, putting them beyond the reach of most Malawian subsistence farmers. As compost is affordable and can be prepared from a combination of readily available material such as dry leaves, grass, maize stalks and other biodegradable substances, it can be made and used by poor farmers. There is no need to borrow money, so farmers do not have to take credit and get into debt as they often do when using chemical fertilizers.
The materials to make compost are mixed with soil and water, then left to decompose for some time. The heat generated during the process destroys most weed seeds, fungal diseases, parasites and pests. Although farmers do have to invest time and labour into making good compost, the rewards are worth the effort. The positive effects of compost on yields last for two or more growing seasons, so soil given compost in one year need not be treated again the next year.
Other strategies exist for combating soil degradation, and should be used together with compost. For example, in developing countries, sowing cover crops, using green manures and crop residues, and planting trees have restored soil fertility and stemmed erosion. In general, recommended strategies to rehabilitate degraded soils and increase soil carbon are also sustainable agricultural practices. The combined benefits of restoring soils, increasing yields and helping limit climate change are too important to ignore.