Definitions

residual soil

Soil conservation

Soil conservation is set of management strategies for prevention of soil being eroded from the earth’s surface or becoming chemically altered by overuse, salinization, acidification, or other chemical soil contamination. The principal approaches these strategies take are:

other ways are

  • no till farming
  • contour plowing
  • wind rows
  • crop rotation
  • the use of natural and man-made fertilizer
  • resting the land

Many scientific disciplines are involved in these pursuits, including agronomy, hydrology, soil science, meteorology, microbiology, and environmental chemistry.

Decisions regarding appropriate crop rotation, cover crops, and planted windbreaks are central to the ability of surface soils to retain their integrity, both with respect to erosive forces and chemical change from nutrient depletion. Crop rotation is simply the conventional alternation of crops on a given field, so that nutrient depletion is avoided from repetitive chemical uptake/deposition of single crop growth.

Cover crops serve the function of protecting the soil from erosion, weed establishment or excess evapotranspiration; however, they may also serve vital soil chemistry functions. For example, legumes can be ploughed under to augment soil nitrates, and other plants have the ability to metabolize soil contaminants or alter adverse pH. The cover crop Mucuna pruriens (velvet bean) has been used in Nigeria to increase phosphorus availability after application of rock phosphate. Some of these same precepts are applicable to urban landscaping, especially with respect to ground-cover selection for erosion control and weed suppression.

Windbreaks are created by planting sufficiently dense rows or stands of trees at the windward exposure of an agricultural field subject to wind erosion. Evergreen species are preferred to achieve year-round protection; however, as long as foliage is present in the seasons of bare soil surfaces, the effect of deciduous trees may also be adequate. Trees, shrubs and groundcovers are also effective perimeter treatment for soil erosion prevention, by insuring any surface flows are impeded. A special form of this perimeter or inter-row treatment is the use of a “grassway” that both channels and dissipates runoff through surface friction, impeding surface runoff, and encouraging infiltration of the slowed surface water.

Erosion prevention

When surface planting is not feasible, there are a variety of mechanical management tactics to protect surface soils from water and wind erosion. Need for these tools arises on construction sites and other situations of transition, where bare soils are exposed. The primary tactics applied are mulching of soil surfaces and use of surface runoff barriers. From 1990 to 2005 considerable innovation has occurred in manufacture of plastic confined hay-bale products, so that a variety of shapes and sizes of runoff barriers can be delivered to the construction site.

There are also conventional practices that farmers have invoked for centuries. These fall into two main categories: contour farming and terracing, standard methods recommended by the U.S. Natural Resources Conservation Service , whose Code 330 is the common standard. Contour farming was practiced by the ancient Phoenicians, and is known to be effective for slopes between two and ten percent. Contour plowing can increase crop yields from 10 to 50 percent, partially as a result from greater soil retention.

Keyline design is an enhancement of contour farming, where the total watershed properties are taken into account in forming the contour lines. Terracing is the practice of creating benches or nearly level layers on a hillside setting. Terraced farming is more common on small farms and in underdeveloped countries, since mechanized equipment is difficult to deploy in this setting.

Human overpopulation is leading to destruction of tropical forests due to widening practices of slash-and-burn and other methods of subsistence farming necessitated by famines in lesser developed countries. A sequel to the deforestation is typically large scale erosion, loss of soil nutrients and sometimes total desertification.

Salinity management

The ions responsible for salination are: Na+, K+, Ca2+, Mg2+ and Cl-. Salinity is estimated to affect about one third of all the earth’s arable land. Soil salinity adversely affects the metabolism of most crops, and erosion effects usually follow vegetation failure. Salinity occurs on drylands from overirrigation and in areas with shallow saline water tables. In the case of over-irrigation, salts are deposited in upper soil layers as a byproduct of most soil infiltration; excessive irrigation merely increases the rate of salt deposition. The best-known case of shallow saline water table capillary action occurred in Egypt after the 1970 construction of the Aswan Dam. The change in the groundwater level due to dam construction led to high concentration of salts in the water table. After the construction, the continuous high level of the water table led to soil salination of previously arable land.

Use of humic acids may prevent excess salination, especially in locales where excessive irrigation was practiced. The mechanism involved is that humic acids can fix both anions and cations and eliminate them from root zones. In some cases it may be valuable to find plants that can tolerate saline conditions to use as surface cover until salinity can be reduced; there are a number of such saline-tolerant plants, such as saltbush, a plant found in much of North America and in the Mediterranean regions of Europe.

Soil pH

Soil pH levels adverse to crop growth can occur naturally in some regions; it can also be induced by acid rain or soil contamination from acids or bases. The role of soil pH is to control nutrient availability to vegetation. The principal macronutrients (calcium, phosphorus, nitrogen, potassium, magnesium, sulfur) prefer neutral to slightly alkaline soils. Calcium, magnesium and potassium are usually made available to plants via cation exchange surfaces of organic material and clay soil surface particles. While acidification increases the initial availability of these cations, the residual soil moisture concentrations of nutrient cations can fall to alarmingly low levels after initial nutrient uptake. Moreover, there is no simple relationship of pH to nutrient availability because of the complex combination of soil types, soil moisture regimes and meteorological factors.

The important observation is that pH is the regulatory mechanism to plant nutrient uptake, and that the theoretical concentration of soil nutrients is meaningless until pH levels are in the optimum range for uptake. Soil pH can be raised by amendment by agricultural lime; The pH of an alkaline soil is lowered by adding sulfur, iron sulfate or aluminium sulfate, although these tend to provide costly short term benefits. Urea, urea phosphate, ammonium nitrate, ammonium phosphates, ammonium sulfate and monopotassium phosphate also reduce soil pH.

Soil organisms

Promoting the viability of beneficial soil organisms is an element of soil conservation; moreover this includes macroscopic species, notably the earthworm, as well as microorganisms. Positive effects of the earthworm are known well, as to aeration and promotion of macronutrient availability. When worms excrete egesta in the form of casts, a balanced selection of minerals and plant nutrients is made into a form accessible for root uptake. US research shows that earthworm casts are five times richer in available nitrogen, seven times richer in available phosphates and eleven times richer in available potash than the surrounding upper150 mm of soil. The weight of casts produced may be greater than 4.5 kg per worm per year. By burrowing, the earthworm is of value in creating soil porosity, creating channels enhancing the processes of aeration and drainage.

Soil microorganisms play a vital role in macronutrient wildlife. For example, nitrogen fixation is carried out by free-living or symbiotic bacteria. These bacteria have the nitrogenase enzyme that combines gaseous nitrogen with hydrogen to produce ammonia, which is then further converted by the bacteria to make other organic compounds. Some nitrogen fixing bacteria such as rhizobia live in the root nodules of legumes. Here they form a mutualistic relationship with the plant, producing ammonia in exchange for carbohydrates. In the case of the carbon cycle, carbon is transferred within the biosphere as heterotrophs feed on other organisms. This process includes the uptake of dead organic material (detritus) by fungi and bacteria in the form of fermentation or decay phenomena.

Mycorrhizae are symbiotic associations between soil-dwelling fungi and the roots of vascular plants. The mycorrhizal fungi increase the availability of minerals, water, and organic nutrients to the plant, while extracting sugars and amino acids from the plant. There are two main types, endomycorrhizae (which penetrate the roots) and ectomycorrhizae (which resemble 'socks', forming a sheath around the roots). They were discovered when scientists observed that certain seedlings failed to grow or prosper without soil from their native environment.

Some soil microorganisms known as extremophiles have remarkable properties of adaptation to extreme environmental conditions including temperature, pH and water deprivation.

The viability of soil organisms can be compromised when insecticides and herbicides are applied to planting regimes. Often there are unforeseen and unintended consequences of such chemical use in the form of death of impaired functioning of soil organisms. Thus any use of pesticides should only be undertaken after thorough understanding of residual toxicities upon soil organisms as well as terrestrial ecological components.

Killing soil microorganisms is a deleterious impact of slash and burn agricultural methods. With the surface temperatures generated, virtual annilation of soil and vegetative cover organisms are destroyed, and in many environments these effects can be virtually irreversible (at least for generations of mankind). Shifting cultivation is also a farming system that often employs slash and burn as one of its elements.

Soil contamination

There are thousands of anthropogenic chemicals that enter soil systems, most of which have an adverse effect upon soil quality and plant metabolism. While the role of pH has been discussed above, heavy metals, solvents, petroleum hydrocarbons, herbicides and pesticides also contribute soil residues that are of potential concern. Some of these chemicals are totally extraneous to the agricultural landscape, but others (notably herbicides and pesticides) are intentionally introduced to serve a short term function. Many of these added chemicals have long half-lives in soil, and others degrade to produce derivative chemicals that may be either persistent or pernicious.

Typically the expense of soil contamination remediation cannot be justified in an agricultural economic analysis, since cleanup costs are generally quite high; often remediation is mandated by state and county environmental health agencies based upon human health risk issues.

Mineralization

To allow plants full realization of their phytonutrient potential, active mineralization of the soil is sometimes undertaken. This can be in the natural form of adding crushed rock or can take the form of chemical soil supplement. In either case the purpose is to combat mineral depletion of the soil. There are a broad range of minerals that can be added including common substances such as phosphorus and more exotic substances such as zinc and selenium. There is extensive research on the phase transitions of minerals in soil with aqueous contact.

The process of flooding can bring significant bedload sediment to an alluvial plain. While this effect may not be desirable if floods endanger life or if the eroded sediment originates from productive land, this process of addition to a floodplain is a natural process that can rejuvenate soil chemistry through mineralization and macronutrient addition.

See also

References

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