Soil is not merely dirt - if only it were so simple! There are more organisms in a kilogram of soil, for instance, than there are human beings on Earth. Soil is a complex mixture of mineral and organic components (some solid, some gaseous) and the health of the soil directly relates to human survival, largely in matters agricultural. The health of soil is threatened by a number of factors. Erosion, salinisation, acidification and structural decline are foremost amongst them.
Weathering is the process by which objects are broken into smaller pieces, whereas erosion refers to the transportation of these pieces. Erosion, in terms of an open field, occurs in three forms: sheet (occurs when water moves evenly across a broad front), rill (where water flows in channels or rivulets) and gully (essentially a more dramatic manifestation of rill) erosion. More broadly, the primary causes of erosion are water (flow or wave action), moving ice, wind and gravity - loose, sandy soils (ill-suited to food production at any rate) are the most frequently lost, and the deposition of these soils as sediments in water bodies can lead to other issues, such as water salinity and turbidity.
Erosive forces are most dangerous against bare soils; many land managers engage in the practice of burning crop stubble in order to accommodate for later yields, but this leaves the soil surface unprotected and soil particles are more easily loosened by wind or water (especially heavy rain). The retention of ground cover and maintenance of wind breaks (usually in the form of vegetation, which fulfils both functions) significantly reduces both wind and water erosion. Direct drilling, a method of crop plantation which does not require soil to be disturbed, is a worthwhile investment to this end (although the necessary machinery is expensive). Additionally, soil not ploughed along the contour lines of sloped land increases water speed and erosive force.
Here in Australia, it is (scientifically, just so we’re clear) said that three bags of topsoil are lost with every one bag of exported wheat. In a country which relies heavily upon agriculture as the basis of its economy, this is a very substantial problem. Moreover, soils here form very slowly, are generally nutrient-poor and are very shallow - a very substantial problem indeed.
Much of the world’s arable land now suffers (or will imminently suffer) reduced productivity based on dry land salinity. Salinity issues naturally occur in arid climates, where there is low and sporadic rainfall. The problem is exacerbated by inward (or sluggish) drainage where water bodies move inland, instead of quickly cleaning catchments and depositing matter in the ocean.
Dry land salinity is caused by overgrazing (and the removal of flora - and, in dry climates, native flora in particular), clearing for agricultural production or any measure which reduces ground cover and water absorption. Plants removed from areas with potential salinity problems are usually more salt-tolerant than food crops. Even so, it is the lack of deep root systems which is the main issue. Grasses and most crops have shallow roots and fairly slow absorption rates, meaning that intensive irrigation or heavy rainfall causes water to sink into the watertable, which will in turn rise and bring mineral salts with it. This rise can occur through capillary action, which means that (in general, depending upon soil type) a watertable a metre below the surface can visibly manifest as white salt crust on the ground surface. High evaporation rates cause the salt to remain there, degrading the soil in perpetuity.
Usually, saline water will prove fatally toxic to food crops and will result in reduced, stunted or even non-existent yields. On occasion, this saline water will filter into rivers and cause similar damage to aquatic ecosystems. The diversion of water systems for damming or irrigation purposes means even more sluggish drainage and so a lingering problem in the catchment.
Soil pH is a pivotal factor in crop growth through a number of biological and chemical factors. The tolerable range of soil acidity varies between plants. In acid soils, there is an overabundance of aluminium, manganese, copper, zinc and boron as other elements are leached from the soil. The latter four elements (as well as iron) are necessary for plant growth, but aluminium is not and can actually prove to be a potent toxin. Other effects are arguably indirect, such as the effect of acidity upon vital bacteria in the roots of legumes. Many such organisms thrive in neutral to slightly alkaline soils.
The nutrients present in a soil are determined by the parent rock from which it was formed. Alkaline soils are usually formed from rocks high in calcium, magnesium and (to a significantly lesser extent) sodium and potassium. Acid soils form from rocks which lack these elements. Leaching, initiated by natural processes and accelerated by human activity (crops get their nutrients somewhere, and fertilisers are not an insignificant factor), removes them and turns a soil acid. To correct this, limestone can be ground up and worked into a soil, although it is important not to turn the soil overly alkaline.
Structural decline happens when soil is physically disturbed. If churned to a significant depth (during construction work, ploughing or similar activity) or compacted (by agricultural machinery), structure is lost. In the former case, it may mean greater exposure to erosion, whereas in the case of the latter it means both added erosion and lowered productive capacity; crops naturally rely on the spaces between soil particles to establish root systems and water will not percolate through heavily compacted soul as well as otherwise it would, instead moving more quickly across the soil surface and taken soil particles with it. Broader tracks on machinery and restriction on movement over wet, bare land can alleviate this problem.