Modified: 11/07/98
Water falling on the surface of the land will either soak into the soil (infiltrate 1) and percolate downward through the soil profile or runoff 1 the surface. Water that infiltrates into the soil profile will raise the water content. If the rate water falls on the surface exceeds the rate water enters the soil and percolates downward, then the soil at the surface becomes saturated and excess water must runoff, even if the lower part of the soil profile is dry.
All water movement responds to some driving force that causes water to move in some particular direction. Gravity pulls water downward through the soil profile. The adhesion 1 of water to soil particles surfaces and cohesion 1 of water (the forces holding a liquid together) draws water from the filled pores of moist soil into the empty pores of dry soil. This can cause water to move against the pull of gravity. Water vapor diffuses from outward from soil pores or the interior of leaf tissue, where humidity is always 100%, into the atmosphere above the soil surface if the humidity in the air is lower than 100%.
The answer can be found in the text Miller & Gardiner (p. 117, Figure 4-5). At saturation, the water potential is zero. Lowering the water content to field capacity, to the wilting point 1 and, finally, to oven-dry creates a negative water potential 1. At water contents above field capacity capillary forces 1 (which draw water into small pores) and gravity forces both affect the water potential, but below field capacity gravity has little effect and capillary forces (the matric potential 1) is dominant. See Miller & Gardiner (pp. 112-113).
The soil pores in dry soil are mostly empty while most pores in wet soil are filled with water. Adhesive forces 2 cause water to enter empty soil pores where it coats the surfaces of soil particles. Cohesive forces 2 in liquid water draw additional water into these pores causing them to fill up. The combined effect of these adhesive and cohesive forces on water movement into small soil pores is called capillary forces 2.
Saturated flow 1 is water movement driven by the pull of gravity through soil pores that are mostly saturated by water, meaning at water contents above field capacity 1. Because the pores are mostly filled by water, the rate of saturated flow depends on the size of the pores. The larger the pores, as in a sandy soil, the more rapidly water flows under saturated conditions.
Unsaturated flow 1 is water movement driven by capillary forces (the so-called matric potential) through soil pores that are mostly empty, meaning at water contents below field capacity. Because the pores are mostly empty, the rate of unsaturated flow is very slow. You can easily imagine that water in a thin film coating an empty pore will move more slowly than water in a filled pore.Percolation 1 is the actual movement of water, typically saturated flow at water contents above field capacity under the pull of gravity. As the water percolates downward through the soil profile substances in the soil can dissolve into the water and travel downward with the percolating water. This latter process is leaching 1. If the substances that dissolve into the water are contaminants, the percolating water will leach them from the soil and carry them downward to the water table and into the groundwater 1.
Forces between water molecules and atoms at the surface of soil clays and humus cause the water molecules to adhere 3. This is explained in the text Miller & Gardiner (pp. 109-119, Figure 4-1).
The amount of water adhering to the surface of soil particles increases with the surface area of the particles. Smaller particles, such as clay, have a much larger surface area per weight than larger particles, such as sand, and have a proportionately larger amount of water clinging to them. There amount of space between particles does not depend on the size of the particles, but the pores size 1 does depend on particles size. Capillary forces 3 (adhesion + cohesion) hold water more strongly against the pull of gravity, meaning the water-holding capacity 1 is larger in fine-textured soils.
Soils with a lower water-holding capacity 2, meaning sandy soils, store less water for plant use and allow water to drain more rapidly through the soil. These soils require more careful water management to conserve water and prevent groundwater 2 contamination.
Plants withdraw water from soil pores, working against capillary forces that hold water in the soil. Plant-available water 1 is the amount of water in a soil between field capacity 2 and the wilting point 2. Plants have increasing difficulty withdrawing water as the water content decreases. This level of difficulty is simply a measure of the work needed to overcome capillary forces 4, expressed as the matric potential 2. Eventually, the forces pulling water into plant roots cannot overcome the capillary forces holding water in soil pores and the plant wilts for lack of sufficient water.
Plants can efficiently remove water from soil within its root system 1. Plants growing in climates with abundant water typically have shallower root systems than plants growing in arid climates for this reason. There is a limit to how deep plants will grow roots and even in arid climates the water table may lie beyond the reach of the most deeply rooting plants.
Plants use energy from sunlight to transform carbon dioxide from the atmosphere and water from the soil into biomass. This process, called photosynthesis 1, occurs in leaf tissue. The stomata 1 are specially adapted cells on the lower surface of leaves that provide an opening through which carbon dioxide can enter and oxygen can escape. The special feature of stomata is their capacity to close up when the plant is loosing too much water. This prevents water loss (transpiration 1) but also prevents carbon dioxide from entering the leaf. As a result, plants that are under water stress stop growing because they cannot absorb carbon dioxide when their stomata are closed to prevent water loss.
Relative humidity 1 is the ratio between the actual amount of water vapor 1 in the air (P) and the amount of water vapor the air can potentially hold (P0): relative humidity = P/P0. If the air is saturated with water vapor the humidity is 100% and liquid water begins to condense as fog or rain. The amount of water the air can potentially hold depends on the temperature: warm air can hold more water vapor than cool air.
If the amount of water vapor in the air is constant, then raising the temperature will increase the amount of water vapor the air can potentially hold causing the relative humidity to decrease. Lowering the temperature will decrease the amount of water vapor the air can potentially hold causing the relative humidity to increase.Raising the temperature lowers the relative humidity 2 of the air, creating a driving force that draws water vapor 2 from the interior of plant leaves, through the stomata 2, into the air causing water loss. This process is called transpiration 2.
High humidity in the air reduces the loss of water by plants (transpiration 3) and evaporation 1 from the soil surface, thereby reducing water loss from the soil and preserving the water stored in the soil.
Water regulation by plants, anything that limits water loss through the leaves or stores water in plant tissue, permits the plant to continue to photosynthesize 2 and grow under conditions where there is little water in the soil and relative humidity 3 is low.
Undesirable water loss occurs when water is lost by runoff 2 without ever entering the soil or percolating 2 deep into the soil profile beyond the rooting depth of plants growing in the soil. This is water that never enters or is used by plants. Adding irrigation 1 water too rapidly or compacting the soil increases runoff. Adding irrigation water in excess of the actual requirements of the plants growing in the soil results in the excess simply percolating downward because the plants cannot use it.
The amount of water in the groundwater 4 is the balance between what is added and what is withdrawn. If the rate water is withdrawn by wells exceeds the rate water can replenish the groundwater, then water table will gradually move downward as the volume of water in the ground diminishes.
Growing crops require both nutrients and water. Fertilizing the soil replaces nutrients extracted from the soil by crops and removed by harvest. Dams store water for the crop growing season, providing a predictable reserve of water for irrigation 3 that supplements rainfall and provide water needed by the growing crop.
Plant cells contain large amounts of dissolved compounds that lower the osmotic pressure of water within them. Water enters plant roots 2 because the osmotic pressure within root cells is lower than the matric potential 3 of water in soil pores. The plant wilts when the matric potential decreases to the point where capillary forces 5 holding water in soil pores equals the osmotic forces 1 that would draw water into plant roots. Usually the amount of salt dissolved in soil water is so low it can be ignored, but there enough dissolved salts in sea water lower its osmotic potential 1 making it difficult for plants to absorb water. Terrestrial plants are not adapted to overcome the low osmotic potential in sea water and, therefore, will wilt for lack of water if grown in sea water. That is why sea water is not used to irrigate 4 crops.
Understanding the global carbon cycle 1 is important because it is closely tied to the global energy cycle 1. Visible light (short-wavelength radiation 1) from the sun passes through the Earth atmosphere to the Earth surface where it is absorbed. The absorption of visible light warms the Earth surface. Any object, including the earth, emits radiation depending upon its temperature. The Earth emits invisible infrared (long-wavelength 1) radiation back into space. If the Earth atmosphere contained no carbon dioxide 1, it would be as transparent to infrared radiation emitted by the Earth as it is transparent to the visible radiation from the sun, meaning the Earth atmosphere would be much cooler than it is now.
The global carbon cycle controls the quantity of carbon dioxide in the atmosphere, which in turn determines the transparency of the atmosphere to infrared radiation emitted by the Earth surface. Carbon dioxide permits the efficient heating of the Earth atmosphere, effectively trapping energy that would otherwise escape into space. In short, the amount of carbon dioxide in the atmosphere has a huge effect on both the energy cycle and atmospheric temperatures.Each substance has a characteristic capacity to absorb radiation. When a substance absorbs radiation, the energy in the light is transformed into heat, causing the substance to heat up. For example, water is transparent to visible light. Intense visible light can pass through water without raising it's temperature. Water efficiently absorbs microwave radiation, a long wavelength form of radiation, causing the water to heat up. That is the principle behind microwave ovens. Anything containing water will absorb microwave radiation, resulting in the efficient heating of the substance.
Think of the Earth as a radiation source, similar to the transmitter in a microwave oven, except that the radiation being emitted is infrared rather than microwave. Carbon dioxide 2 in the atmosphere absorbs infrared radiation that passes through it just as water absorbs microwave radiation passing through it. Carbon dioxide in the atmosphere provides an efficient means of heating the atmosphere 1 and maintaining a much higher temperature at the Earth surface than if the Earth atmosphere contained no carbon dioxide.The global carbon cycle transforms carbon from one form into another, using energy supplied by the sun. The primordial atmosphere, before plants and photosynthesis, contain much more carbon dioxide than today and the mean Earth temperature was correspondingly much higher. Plants use the energy in solar radiation to transform carbon dioxide 3 and water into biomass, releasing oxygen into the atmosphere. The carbon in biomass, residue, humus 1 and fossil fuels is "reduced" carbon 1. If all of the "reduced" carbon (biomass, residue, humus and fossil fuels) was combusted the oxygen in the atmosphere would be consumed and carbon dioxide produced, returning both the oxygen and carbon dioxide contents in the atmosphere to their original primordial levels.
Humus is important because there is twice as much "reduced" carbon stored as humus than is stored in biomass. Anything that would increase the rate humus is oxidized will increase atmospheric carbon dioxide levels, causing further global warming 1.The amount of humus 2 stored in a soil depends on the amount of biomass in the ecosystem and the rate of biological activity 1. Arid ecosystems create less biomass than humic ecosystems, consequently arid soils contain less humus. The level of biological activity in increases with soil temperature 1, leading to increased production of biomass and more rapid decomposition of residue and humus. The amount of humus in humid ecosystems reflects the rate of decomposition rather than biomass production. Biomass production in tundra ecosystems is slower than in humid tropical ecosystems, but the rate of decomposition is much slower in cool climates leading to a buildup of humus.
Scientists are concerned, but currently cannot prove, that a major consequence of global warming will be increased humus decomposition rates (i.e., increased soil respiration 1) in the upper latitudes of the northern hemisphere. Soil respiration already accounts for ten times the carbon dioxide 4 release into the atmosphere and accelerated soil respiration in the northern hemisphere will have unpredictable effects on global warming, potentially accelerating the rate of heating far beyond that produced by burning fossil fuels.Respiration 2 by vegetation and soil organisms in natural ecosystems releases ten times as much carbon dioxide annually than the burning of fossil fuels or forests. The net effect of both on atmospheric carbon dioxide levels is the same, but respiration by natural ecosystems is a far more complex phenomena than industrial emissions. Natural and agricultural ecosystems contain plants that can absorb carbon dioxide from the atmosphere, slowing the rate of carbon dioxide buildup in the atmosphere. Increased carbon dioxide levels and warmer temperatures may actually increase the efficiency of carbon dioxide removal (photosynthesis 3), but scientists cannot accurately predict the pattern of warming or the effect on rainfall which means they cannot predict the sequestration of carbon by ecosystems. Scientists know even less about the storage and release of carbon in humus and the effects of climate changes on soil respiration.
Global change encompasses a broad range of events that include growing populations, industrial emissions of carbon dioxide and other substances into the atmosphere, degradation of natural ecosystems, and the expansion of agriculture. Growing populations drive the other processes through increased demand for energy, water, food and land. To the extent that the loss of forests (deforestation 1), the expansion of deserts (desertification 1), and other forms of environmental degradation effect respiration by these ecosystems they effect the global carbon cycle 2 and, ultimately, the global energy cycle 2.
Concern about global warming 2 arises from the inherent unpredictability of the phenomena and the perception, probably accurate, that these changes cannot be quickly reversed. Two decades ago, before global warming became a priority issue, scientists and government leaders were concerned about unchecked population growth and the potential for widespread famine. That threat has not abated, with the consequence that global warming increases both the stakes and unpredictability.The unpredictability of increasing atmospheric carbon dioxide 5, both on global climate and on global ecosystems, makes it impossible to answer these questions. Global warming 3 is a grand experiment whose outcome no one can predict. While it is true that increased carbon dioxide levels in the air will increase plant growth (photosynthesis 4), it is impossible to extend such research findings to the response of large-scale ecosystems, be they natural or agricultural.
Biomass production at the ecosystem scale depends heavily upon climatic factors. In principle, plants grow faster and produce more biomass in atmospheres containing higher carbon dioxide levels, but what effect will global warming have on rainfall? Even if global warming were not a concern, growing populations will unquestionably place growing demand on limited water supplies. What effect will unpredictable changes in climate and rainfall have on the response to growing demand for food and water?Three forms of energy play important roles in the Earth's energy cycle 3: radiant energy 1, sensible heat 1 and latent heat 1. Heat transport requires matter (gas, liquid, solid) because heat is atomic or molecular motion. Radiation can pass through a vacuum, such as the near vacuum of space. The sun produces both radiant and heat energy, but only the radiant energy reaches the Earth as visible light. Visible light (short-wavelength radiation 2) that passed through the Earth atmosphere and absorbed by the Earth surface causes sufficient heating for the Earth to emit invisible (long-wavelength 2) infrared radiation that is so efficiently absorbed by carbon dioxide, heating the atmosphere 2.
All surfaces (clouds, oceans, land, and snow) reflect some visible radiation. Light reflected by clouds and other surfaces does not cause heating.
Soil surfaces heat the lower atmosphere in three ways. First, any material--including the soil surface--radiates energy at an energy depending on the material's temperature. Terrestrial surfaces on Earth radiate invisible infrared radiation that is absorbed by carbon dioxide 6 in the air, heating the air. Second, there is sensible heating 2 of the air directly in contact with warm soil surfaces. Sensible heat is heat you can feel. Finally, water evaporating from the soil surface absorbs energy in a form called latent heat 2. When the water vapor 3 condenses in the upper atmosphere it releases this latent heat, heating the air. Latent heat is absorbed and released whenever a substance changes phase, as when water evaporates from liquid to vapor and condenses from vapor to liquid.
Direct absorption of solar radiation heats the soil surface, but the heating of the soil depths occurs because of thermal (heat) diffusion 1 downward into the soil profile. Heat flows from warm to cool at a rate determined by the thermal conductivity 1 of the material. The thermal conductivity of soil changes with its water content because soil pores can be either filled with air or water and water is a better conductor of heat than air.
At midday, when solar heating of the soil surface is greatest, heat diffuses into the air above the soil and downward into the soil profile. At night, when the soil surface cools, heat diffuses upward to the soil surface. Temperature 3 fluctuations at the soil surface are much greater than deep in the soil because heat diffuses slowly through soil. For the same reason, the maximum and minimum temperatures at depth are delayed relative to the soil surface. Heat diffusion has biological significance because temperature affects biological activity and, hence, respiration. All things being equal, cool soils respire less than warm soils.Seasonal soil temperatures 4 mirror diurnal (daily) temperature cycles. Both the magnitude of temperature variations and the timing of maximum and minimum temperatures change with depth. The magnitude of seasonal temperature changes decreases, eventually reaching the mean annual temperature for the location, with depth. The delay of maximum and minimum temperatures relative to the soil surface increases with depth. This is because heat diffuses into the soil depths during summer (daytime) and out of the depths during winter (night).
Half of the volume of most soils consists of pores that may contain either air or water, depending on the moisture content. Air has a negligible capacity to absorb heat compared to soil minerals and water. Water has twice the capacity to absorb heat compared to soil minerals. That means moisture content strongly influences the actual heat capacity 1 of a soil.
Each substance has a characteristic heat capacity. Suppose a fixed quantity of heat is absorbed by a gram of soil minerals, causing the temperature of the soil minerals increases by 10 degrees. The temperature of one gram water absorbing the same amount of heat would increase only 5 degrees because the heat capacity of water is about twice that of soil minerals. Replacing air with water in soil pores means the temperature of moist soil will increase less than a dry soil when both absorb the same amount of heat.The significance of soil heat capacity 2, soil moisture content and soil temperature 5 comes from its effect on biological activity 2 in the soil ecosystem. We already understand that much of what happens in soil arises from biological activity. Biological activity is influenced not by the heat stored in the soil, but by its actual temperature. The temperature of moist soils, including those in wetlands, will increase less than dry soils because the water has a very high heat capacity. Heat absorbed in soil is stored during the warming stage (morning or Spring) and is released during the cooling stage (evening or Fall).
Colloids 1 consist of very small particles, giving them their distinctive property: a very high surface area to volume ratio. Surface properties are unusually important for colloids such as soil clays and humus 3. Clay and humus surfaces represent most of the surface area in soils, surfaces where water and nutrients adhere.
The meristem 1 is the root tip where cell division (growth) occurs. The extension zone is where the new cells enlarge and begin to differentiate into root structures. Nutrients are not absorbed in either zone because the cells are not mature.
Plants absorb nutrients as charged atoms called ions, but they cannot absorb more negatively-charged ions (anions 1) than positively-charged ions (cations 1). Plants must remain charge neutral. They do this by excreting the acid cation H+ for every nutrient cation absorbed and excreting the bicarbonate anion HCO3- for every nutrient anion absorbed [Miller & Gardiner, p. 317, Fig. 10-1].
Root hairs 1 are specialized cells at the root surface that increase the contact between the plant root and the soil. Mycorrhiza fungi 1 [Miller & Gardiner, pp. 184-186] infect the roots of certain plant species to form a mutually beneficial relationship. Biologists call mutually beneficial relationships between different species: symbiosis 1. The fungi grow hyphae (filaments) that are functionally equivalent to root hairs. The mycorrhizae fungi increase the capacity of the plant to extract nutrients from the soil while the plant supplies the fungi with sugars and essential amino acids.
The rhizosphere 1 is volume of soil in direct contact with plant roots. Plants modify the acidity and biological activity in the rhizosphere to increase the solubility 1 (biological availability) and mineralization 1 of essential nutrients.
The cycling of nutrients 1 continually converts them from one chemical form into another. Some chemical forms are readily available to growing plants while other forms are not. A soil is fertile if biological activity can easily release nutrients stored in plant residue or humus 4 (mineralization 2), if there is an abundance of rock minerals that can release nutrients as they dissolve (mineral weathering 1), or if soil contains colloids 2 that loosely bind nutrients to their surfaces.
Mineralization 3 is the microbial decay of plant residue that releases nutrients in a form that plants can absorb through their roots.
Synthetic fertilizers are highly soluble 2 inorganic salts that quickly and completely dissolve in the water for uptake by plant roots. Once they are absorbed by plants they enter the nutrient cycle 2 and are transformed like any other form of the nutrient. All nutrients absorbed by plant roots must become soluble salts identical to the chemical form in synthetic fertilizers, the difference is that synthetic fertilizers are more concentrated.
The three most abundant elements in plant tissue are carbon, oxygen and hydrogen that plants obtain from the carbon dioxide in the air and water from the soil. Carbon, oxygen and hydrogen are structural elements, not nutrients. The next most abundant element in plant tissue is nitrogen. Nitrogen is the nutrient most likely to be deficient 1 because plants need relatively large amounts to build biomass and because it is not readily available. Before plants can use the nitrogen in the atmosphere, it must be converted into ammonium or nitrate by biological fixation 1 (symbiosis 2 between rhizobia bacteria 1 and legume plants [Miller & Gardiner, pp. 189-190] or certain non-symbiotic soil bacteria [Miller & Gardiner, pp. 191-192]) or a non-biological process used in the fertilizer industry.
Soil nitrogen can exist as a structural part of plant residue or humus 5 (organic nitrogen), as ammonium (an inorganic cation 2 soluble in water) or as nitrate (an inorganic anion 2 soluble in water). Soil phosphorus can also exist as a structural part of plant residue or humus (organic phosphorus), as phosphate (an inorganic anion soluble in water) or as insoluble 3 phosphate minerals. Ammonium nitrogen is not very mobile because it is strongly attracted to negatively-charged clay surfaces. Phosphate reacts with the soil, forming insoluble phosphate minerals that are inaccessible to plants and immune to leaching. Nitrate, however, is not attracted to clay surfaces and does not form insoluble minerals, leaving it dissolved in water and easily leached by water percolating through soils.
The solubility 4 of soil minerals changes with pH. Some nutrients are most soluble in acid soils, iron [Miller & Gardiner, p. 359, Fig. 11-2] or manganese for instance, and deficiencies occur only in alkaline soils. Some nutrients are more soluble at neutral pH values, phosphorus [Miller & Gardiner, p. 337, Fig. 10-10] for instance, and deficiencies occur in acid or alkaline soils. The solubility, i.e., the biological availability, of each nutrient has its maximum within a specific pH range.
Aquatic plant growth is limited by the availability of phosphorus. When suspended soil particles containing phosphorus adhering to their surfaces enter a lake, the phosphorus stimulates algae and other aquatic plants to grow. When the inorganic phosphorus supply is depleted by its conversion into biomass the bloom ends. When the aquatic biomass dies it stimulates the rapdi growth of microbes that degrade the residue. These microbes require oxygen and rob water of dissolve oxygen (hypoxia 1 = lack of oxygen) needed by aquatic fauna such as fish, causing them to die. The build up of nutrients in aquatic systems is called eutrophication 1.
Macronutrients 1 (nitrogen, phosphorus, sulfur, potassium, magnesium, and calcium) constitute anywhere from a few tenths of a percent to a few percent by (dry) weight. Micronutrients 1 (boron, iron, zinc, copper, manganese, molybdenum, etc.) are trace constituents. Plant requirements for micronutrients range from a thousand- to ten thousand-times less than macronutrients. Simply put, the quantity required determines whether the nutrient is a macronutrient or a micronutrient.
Regardless of the quantity required by the plant, nutrient deficiencies 2 display several physiological effects. Mild deficiencies will reduce plant growth and crop yield. Moderate deficiencies may cause leaf discoloration (chlorosis = yellowing) or abnormal growth. Severe deficiencies cause tissue necrosis (death) and, ultimately, death of the whole plant. Macronutrient deficiencies will stunt plants or kill them just as surely as micronutrient deficiencies.Erosion is a part of the rock cycle and a perfectly natural process. Natural erosion is slower than the soil forming process, otherwise there would be not soils. Human activity that removes protective vegation and disturbs soil structure may accelerate soil erosion, leading to land degradation 1 and pollution by eroded sediments 1.
Clay and humus, soil colloids 3, act as adhesives that bind together silt and sand grains in to aggregates. Soil aggregates 1 are difficult to transport and less easily eroded than mineral grains by themselves. Soils naturally low in clay (loess deposits) or humus are more erodable because they lack of adhesives makes it easier to detach and transport mineral grains. The impact of rain drops 1, or bouncing mineral grains (saltation 1), or tillage can destroy aggregates and detach mineral grains for subsequent transport.
Runoff begins as sheets of water flowing over than soil surface that soon builds velocity and becomes riverlets that erode small channels (rills 1) that grow in width and depth to become large channels (gullies 1). Water velocity increases with the size of the channel. Water flowing in the largest channels (gullies) has the most erosive power and can carry the heaviest sediment load, while runoff flowing in sheets have the least erosive power and carry the lighest sediment load.
Clay and humus bind larger mineral grains together until impact (rain drop 2 or saltation 2) destroys the aggregate and detaches the mineral grains. Once detached, the smallest particles, humus and clay, are most easily suspended 1 in wind and runoff.
Water falling on the land surface either soaks into the soil (infiltration 2) or runs off. Runoff 3 causes erosion. If the soil contains a clay-rich B-horizon, removal of the surface layer, exposing the clay layer, will decrease infiltration and increase runoff because water cannot rapidly enter clayey soil.
Runoff flowing over the land surface transports detached mineral grains. The greater the runoff 4 velocity, the greater the sediment load it can carry. Increasing the slope angle increases runoff velocity.
Contour or strip farming (alternating strips of perennial crops with annual crops) and terraces reduce runoff velocity. The slope length within which runoff 5 can build velocity is the distance between perennial crop strips or the terraces 1.
Vegetation reduces erosion by shielding soil from rain drop impact 3, blocking saltating 3 particles, and holding the soil in place with its roots. Crop residue also shields the soil from rain drop impact, reducing the risk of erosion. Tillage practices that leave some plant and residue cover ("no-tillage" and "minimum tillage") increase reliance on pesticides while reducing the risk of erosion because insects, weeds and plant diseases flourish when plant and residue cover remains.
Removal of vegetation and the exposure of the land surface causes extreme soil loss in highly erodable 1 land. The wisest use of such land is to revegetate and minimize disturbance. Some erodable land can be used for pasture or as wood lots, but some are so susceptible to erosion that grazing or tree harvest is too great a disturbance.
The USLE 1 and related equations allow conservationists to identify erodable 2 land and to determine the effect of erosion control practices on soil loss. Some land is moderately erodable and can still be used for a variety of purposes without serious soil loss provided appropriate conservation practices are used.
Dams constructed on the upper reaches of the Nile River had slowed river velocity and reduced the sediment load. Because the Nile deposits less sediment 2 in the delta, ocean waves erode the delta. Of course, the dams are quickly filling with sediment and will eventually become useless.
Soil loss by wind or water remove the surface layer containing the greatest amount of nutrients and pesticides. Wind and water effectively transports the fine-grained particles containing the most nutrients vast distances where they can disrupt nutrient cycling 3 upon deposition. This is called nutrient pollution.