Soil Fertility and Erosion
One handful of healthy soil contains more micro-organisms than there are people living on earth. The thin layer of topsoil, that we walk on and through which plants send their roots, is the result of enduring, age-old processes of decomposition, transformation and accretion through countless organisms. Most of these organisms are microscopic and at present we only know a fraction of them."Soil is the source of nutrients required for plant growth and itself the result of organic processes of living organisms. It is therefore the primary environmental stock that supports agriculture. Soil condition varies widely but global estimates suggest that 23% of all used land is degraded to some degree, which is a cause of serious concern. The key soil degradation processes include: erosion, salinization and water logging, compaction and hard setting, acidification, loss of soil organic matter, soil nutrient depletion, biological degradation, and soil pollution. Agricultural activities influence all these processes“ (Global, p. 39)
Since the transition from hunter-gathering lifestyles to settled agricultural societies, human civilisations have time and again learnt from painful experience that healthy soils are more fragile than they might appear. The long-term fertility of a soil, its resilience and capacity to regenerate form the foundation of all forms of agriculture which is built on a very sensitive base.
Errors in the management and care of soils often become visible when it is too late to avoid the consequences. Some proven methods are no longer adequate to meet the demand of a growing world population for fertile soil. These include slash and burn agriculture, which was traditionally used to convert forest areas into arable land for are restricted period of time, thereby adding nutrients to the soil, as well as the practice of leaving land fallow for several years to enable soils to regenerate."Soil degradation in one form or another occurs in virtually all countries of the world. About 2,000 million hectares are affected by soil degradation. Water and wind erosion accounted for 84% of these damages, most of which were the result of inappropriate land management in various agricultural systems, both subsistence and mechanized." (Global, p. 24)
Some of the world's most vulnerable farmlands are tropical areas, where most of the organic matter is found on and above the surface, overlying a very thin topsoil layer, as well as the oldest soils in the world in the subtropical, dry plains of Africa. European soils, on the contrary, are deeper and richer in organic matter and for this reason more resilient."Scientists estimate the global cost of soil erosion at more than US$400 billion per year. This includes the cost to farmers as well as indirect damage to waterways, infrastructure, and health.“ (Global, p. 518)
Building up, maintaining and conserving the fertility of different soil types around the world in the face of diverse climatic conditions is the biggest challenge to agriculture today. The key to soil fertility lies mostly in the humus content of soils. The wide variety of essential nutrients can only be available if soils contain a sufficient proportion of decomposed organic matter. And only then can these nutrients dissolve in the water and be absorbed by the plant."Innovative soil and crop management strategies can increase soil organic matter content, hence maintaining or enhancing crop performance.The organic matter content of the world's agricultural soils is typically 50-65% of pre-cultivation levels (Lal, 2004). Strategies to increase soil organic matter (carbon) include the integration of crop and livestock production in smallscale mixed systems (Tarawali et al., 2001, 2004); no-till farming; cover crops, manuring and sludge application; improved grazing; water conservation and harvesting; efficient irrigation; and agroforestry.“(Global, p. 175)Generations of farmers and soil scientists from different cultural backgrounds have been researching and experimenting with tilth, the optimum soil condition for the cultivation of plants. Animal and human manure, nitrogen-fixing leguminous plants, mulching techniques, composting and adequate crop rotation can all play an important role. Equally important factors for soil fertility are the preparation and protection of the soil structure, root penetration, aeration, shadow and water absorption and storage, wind protection, run-off prevention, and terracing. To be truly healthy, soil also needs to be teeming with a wide variety of soil-dwelling organisms such as worms, spring tails, woodlice, as well as the right combination of soil microorganisms and fungi. >>more
Facts & Figures
Land degradation and desertification threaten fertile land and the benefits human society derives from it throughout the world. On a global scale, around 10 - 20% of drylands and 24% of the world’s productive lands are degraded.
The European Union is losing 970 million tonnes of soil per year due to water erosion, an amount equivalent to a one metre-depth loss of soil from an area the size of the city of Berlin or enough to cover an area twice the size of Belgium with one centimetre of soil.
Currently the cost of land degradation reaches about US$490 billion per year, much higher than the budget for action to prevent it. Roughly 40% of the world’s degraded land occurs in areas with the highest incidence of poverty. Land degradation directly impacts the health and livelihoods of an estimated 1.5 billion people.
Our most significant non-renewable geo-resource is productive land and fertile soil. Each year, an estimated 24 billion tonnes of fertile soil are lost due to erosion. That’s 3.4 tonnes lost every year for every person on the planet.
Soils store more than 4000 billion tonnes of carbon. By way of comparison, the forests store 360 billion tonnes of carbon as woody biomass, and the atmosphere more than 800 billion tonnes in the form of carbon dioxide.
Agroforestry of maize associated with fast growing and nitrogen-fixing shrubs (e.g. Calliandra and Tephrosia) has spread among tens of thousands of farmers in Cameroon, Malawi, Tanzania, Mozambique, Zambia and Niger. These soil-improving plants resulted in an improvement in total maize production, with yields of 8 tonnes per hectare compared with 5 tonnes over a five-year period.
The current high erosion rates throughout the world are of great concern because of the slow rate of topsoil renewal; it takes approximately 500 years for 2.5 cm layer of fertile topsoil to form under agricultural conditions.
With increasing population growth, the amount of arable land available for each person is continually dropping. Currently, each human being has only 2000m² at his or her disposal; in 1961, that figure was 4000m². The amount of arable land available per person will decrease to 1500m² by 2050.
Yield reduction in Africa due to past soil erosion may range from 2–40%, with a mean loss of 8.2% for the continent.
