Soil consists of a mineral component, which is generally a mixture of sand, silt and clay. Since soils vary considerably in their composition, they may be compared universally using a "soil texture triangle" http://upload.wikimedia.org/wikipedia/commons/8/80/SoilTexture_USDA.png. A good fertile soil is a loam, which has an approximate composition of 20% clay, and of the order of 40% each of silt and sand. These terms do not reflect the chemical composition of the three components, but their particle size. Thus, sand grains are of the order of millimeters or less in diameter, while clay particles are of nanometer dimension; particles of silt are of a size somewhere in between these orders. Soil also contains organic material, referred to as soil organic matter (SOM), or soil carbon (SC). This ranges from identifiable plant components, to their more indistinct decomposition products, created by passage through the bacterially replete bodies of earthworms, and by the actions of the other creatures, and microbes that live within soil http://ergobalance.blogspot.co.uk/2014/04/deep-down-and-dirty-science-of-soil.html. The mass of the living creatures themselves is also accounted in the reckoning of SOM.
Soil is indeed both fragile and living. It is fragile because it is prone to degradation, principally by erosion. It is living because it contains an amazing number of different species within it, collectively termed the soil food web http://www.sciencereviews2000.co.uk/blog_v2/view/science-progress-news/62/soil-erosion-climate-change-and-global-food-security-challenges-and-strategies/806#.VFDC3xbyVco. It is thought that a single teaspoonful of soil may contain a billion microbes. A major source of soil erosion is deforestation, which may occur naturally, e.g. through forest fires, or by human actions, to clear land to grow commercial crops on, e.g. soya beans in South America. Slash and burn methods are employed on a smaller scale by groups of farmers who are poor and need to feed their families. More exquisite methods are sometimes employed to clear ground of trees, shrubs and all else, e.g. the process of "chaining" where a ship's anchor chain, maybe 400-600 feet in length, is strung between two 100 tonne tractors. If 2 inches in diameter, such a chain will have a weight of 7-10 tonnes http://www.sotra.net/products/tables/weight-for-studlink-anchor-chain, and so by driving the tractors, the chain is dragged along the ground, where it simply rips out anything in its path by the roots.
Soil that has been so denuded is vulnerable to erosion, all the more so when it is left uncovered as a result of monoculture cropping, and during the winter, when the erosive elements of wind and water are at their most forceful. While erosion is the major (85%) cause of soil degradation globally, other impacts are important, e.g. compaction, crusting (hardsetting), loss of nutrients, contamination, salinization and loss of biological life in the soil. Once the productivity of the land has been lost, the strategy is to move on and clear yet more forest. According the the United Nations, one third of the world's cropland has been abandoned over the past 40 years due to degradation and desertification http://www.giz.de/expertise/downloads/giz2013-en-german-national-report-unccd.pdf, while 52% of the land used for agriculture is moderately to severely affected by soil degradation http://www.unccd.int/Lists/SiteDocumentLibrary/WDCD/DLDD%20Facts.pdf.44% of the world's food production systems and 50% of its livestock are considered to be vulnerable to climate change, while 12 million hectares of crop land are lost per year (23 hectares per minute), where 20 million tonnes of grain might have been grown. All of this against an estimated requirement to produce 60% more food by 2050, not only to match a population that is predicted to increase from around 7 billion now, to 9.5 billion, but that there is a rising "middle class" who are likely to have greater expectations from their diet, i.e. want to eat more meat, which is far more land intensive to produce than growing crops to be eaten directly http://ergobalance.blogspot.co.uk/2014/04/the-soil-land-water-climate-honey-bees.html. Thus, we are destroying the productivity of that same land from which we demand a relentless increase in production: two powerful forces pulling in opposite directions, with an inevitable "give" at some point.
There is a close connection between soil and water. In the natural order of things, water evaporates from plants and wetlands (evapotranspiration), rivers and oceans, forming clouds in the atmosphere from which rain falls. The rain is absorbed through the soil surface (infiltration), and is taken up by movement through the soil (transmission): when the rates are high for both processes, the soil acts as an effective "sponge" and can absorb large amounts of water http://agroecologygroup.org.uk/wp-content/uploads/Briefing-on-flooding-soil-and-farming-pr.pdf. When the soil becomes compressed, e.g. by heavy farm machinery or over-intensive grazing, a process called "compaction", the water cannot enter it so easily and tends to move elsewhere, as "runoff". Water runoff contributes to soil erosion: when the soil is unable to retain water, it runs off the surface taking large amounts of soil with it. This soil ends up downstream in rivers (silting-up) where it creates bottlenecks and causes the water to back up. Since it is in the surface layers of the soil that nutrients, e.g. phosphate, are concentrated, essential fertility is lost from the soil in the process. Soil may also be eroded to below the optimum rooting depth of plants, which stunts their growth. Production of maize has expanded in the U.K. since the 1970s http://anewnatureblog.wordpress.com/2014/02/06/lost-in-the-drainage-maize/, mostly grown to feed cattle. The stubble is left over autumn and winter, and is not ploughed into the ground, meaning that fine soil is exposed to be washed downstream, when the impacts of wind and rain are at their strongest. Regions in the South West of England, including the catchment that drains into the Somerset Levels http://ergobalance.blogspot.co.uk/2014/02/flooding-on-somerset-levels-and.html, have some of the greatest concentrations of maize crops in the country, and it has been estimated that some 50% of the sediment in the Tone and Culm rivers are from soil eroded from fields of maize grown to feed concentrated and highly producing dairy herds.
Uplands, if well manged, can absorb large quantities of water, and planting trees and shrubs has a hugely positive effect in reducing the volume of water that ends up downstream. Rather than intensive grazing, which can urge soil compaction, a loss of vegetation and increased soil erosion, less intensive and all year round grazing can help to improve the soil structure and its all over quality. The presence of soil organic matter (SOM) is a critical feature of good soil structure and its ability to retain nutrients. This improved soil structure also enhances the ability of soil to absorb and hold water, so that a high SOM content can make a dramatic effect in the capacity of soil to hold and transmit water, while the loss of SOM has a large and detrimental effect on the holding capacity of soil for water and its ease of becoming compacted. In simple terms, a good soil is like a springy sponge, and in its ability to hold water, not only is runoff retarded but the lower lying aquifers are able to recharge, so preserving supplies of freshwater, as well as mitigating surface flooding. As a point of note, however, in some locales that were inundated during the 2013/2014 winter, in the U.K., even after the surface flood-waters had been pumped away, flooding still remained a problem due to "groundwater pressure", where the raised water table pushes up through the ground, and up through the floors of houses that have only floorboards as their ground-level barriers, rather than solid concrete.
In a 1937 letter to U.S. State Governors, urging soil conservation laws, President Franklin D. Roosevelt wrote, "The nation that destroys its soil destroys itself." This was around the time of the dust bowls that arose in the northern United States and Canada, where former prairie lands had been turned over to farming. The impact of ploughing, loss of prairie grasses, and drought literally caused the soil to blow away. The resulting effects on the populations were dramatised by John Steinbeck in his "Grapes of Wrath", where some 2.5 million people were displaced as they made their way toward California, looking for work and for land on which they could grow food. Good soil has been described as "the real World Bank", meaning that it is the one thing, of all things, that we must invest in. Without soil we have nothing.
The main critical factor in mitigating soil erosion, is to keep it covered throughout the year. In Utah and Montana, it has been demonstrated that ground cover has a profound protective influence, and for uncovered ground some 17 tonnes of soil/hectare/year can be lost http://www.stopclearcuttingcalifornia.org/bca/research/Ecology%20of%20Soil%20Erosion.pdf (Figure 1). This amounts to an annual loss of soil depth of about 1 mm, and at this rate, about one inch of soil would be lost in 25 years. However, this decreases markedly with increasing ground cover, so that it is nearer 1 tonne/ha/year for 100% coverage. At around 0.1 mm/ha/yr (about the thickness of a human hair), this implies that it would take 250 years to lose that same inch of soil. Under natural conditions, soil is created at a similar rate to that at which it is eroded, and it takes 200-1,000 years to form an inch of soil, depending on climate. Thus, by covering the ground, the rate of erosion is brought back to the natural balance point, rather than it being lost at 10-40 times the natural rate, as it is on agricultural lands presently.
The following are some notes, taken from a talk that I give on the broad subject of soil and other resource depletion and what we might do to cope with it. The emphasis is on Permaculture and the Transition Towns movement, so to move from the global village to a village of globes, i.e. to more resilient, self-sufficient localised communities.
Some ways to protect and regenerate soil:
•No bare ground. Reforestation (Plant Trees!). Planting cover crops (peas, beans, buckwheat, clover, etc.).
•Building Soil Organic Matter (SOM). No-till farming.
•Shielding the soil through the use of sand fences, shelter belts, woodlots and windbreaks. Plant Trees!•Farmer-Managed Natural Regeneration: “reforested” five million hectares barren land (Niger) → 40 trees/hectare.
•Protecting forests: soil/carbon plus “lungs of the Earth”.•Mulch from pruned trees, and straw to cover fields: increasing soil water retention and reducing evaporation.
•Plant Trees! Help soil to absorb water, reduce flooding.•Building the “Soil Food Web” (one billion microbes/tsp.).
•Permaculture = Permanent (Agri)Culture.
•Permaculture = a good design!
•“You cannot solve a problem from the same consciousness that created it. You must learn to see the world anew.” Albert Einstein.
•Seeing the whole picture, and placing design elements together to support one another.
•“The problem is the solution.”
•Companion planting; no-till, building soil structure, efficient use of water, smaller PNK inputs; capturing carbon; best use of light; exploit “3rd dimension”.
•“Three sisters”: bean + corn + squash.
Forest Garden (Principle of “layers”). http://en.wikipedia.org/wiki/Forest_gardening