Land or soil is one of the natural bases for human life and social development. Soils are defined and characterized on the basis of their morphological profiles because the assemblage of obvious physical features represented by these units are often related to the less obvious features of their chemical composition, chemical properties, and fertility.
Men have tilled the soil and irrigated and drained it for at least six millennia. This is basic to civilization. Systematic scientific study of agriculture began in the first half of the nineteenth century, along with physical studies of the soil. In its natural state, the soil is normally a three-component porous medium consisting of solid soil particles, water, and air. Much of the water involved in the hydrologic cycle is located in soil between the time of its arrival as rain at the soil surface and that of its return to the atmosphere. The processes of water movement in soil play a central part in the scientific study of the terrestrial sector of the hydrologic cycle and in the problems of dry-land and irrigated agriculture, of plant ecology, and of soil biology. These determine the transport of materials in solution such as natural salts, fertilizers, and urban and industrial wastes through the soil. Properties such as infiltration, drainage, and retention of water in the soil layers; extraction of water by plant roots; and the evaporation of water from the soil are also important.
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The solid phase of the soil has mineral and organic matter, which is usually highly colloidal, seldom exceeds 5-10% by weight of soil. In an agricultural context, the main interest in soil structure is in terms of soil tilth, which is related to the ability of aggregates to maintain their integrity when the soil is irrigated, tilled, or otherwise worked so that water retention and drainage and aeration are kept at favorable levels.
As soil is a complex mixture of many components there is usually little value in determining the amount of a chemical element present without any indication of the fraction of the soil in which it occurs and its form of combination. Indeed, fractions that comprise only a small proportion of the total soil mass are often very important in determining its behavior. The following is a convenient classification of soil fractions:
The Organic Fraction,
The Mineral Fraction,
Soluble in water: Simple inorganic ions,
Soluble in dilute acids: Carbonates,
Insoluble in dilute acids.
Primary minerals mainly occur in sand and silt fractions; secondary minerals usually occur in the clay fraction (< 2 µm diameter).
Organic materials are added to soils as dead plant and animal remains. They are decomposed by the microflora and microfauna to form humus, an amorphous material distinct from undecomposed litter. Well-humified organic matter contains about 58 % carbon, so the amount of the soil organic fraction is usually specified by determining the organic carbon content and multiplying it by 1.73. Organic contents range from zero in some mineral subsoils, through 1 to 10 % in arable topsoils, to nearly 100% (of the dry weight) in some peat and muck soils. The amounts in surface soils depend on the balance between accumulation and decomposition, and these processes in turn are influenced by temperature and moisture content.
Apart from carbon, hydrogen, and oxygen, the organic fraction contains nitrogen, sulfur, and phosphorus. The proportions of these elements are often expressed as ratios compared to nitrogen taken as 10, and typical values are C:N = 80-150:10, S:N = 1.2-1.5:10, and P:N = 0.2-3.0:10. Metals such as aluminum, iron, manganese, and copper are also found in small amounts in humic complexes.
The organic compounds in humus are very different. The main portion appears to consist of polymers, some of which are formed by random condensation of phenols, amino acids, and other related microbial degradation products. A large number of compounds have been isolated from humus extracts, but many of these must be artifacts. Of particular interest, apart from the polyphenols, are amino acids (implying that humus contains protein), sugars (indicating carbohydrate fractions), and amino sugars. The sulfur seems to be part of the main humus fraction, probably as sulfur-containing amino acids and organic sulphates. In some soils, much of the organic phosphorus is present as inositol polyphosphates, which appear not to be an integral part of the humus.
The soluble-salt content of most soils is low so that the soil solution typically contains between 5 and 25 mmol/L of calcium and magnesium salts, mainly as nitrate. In saline soils, however, the salt content is of the order of 100 mmol/L, and although still less than 1% of the soil mass, the soluble salts dominate the behavior of the soil and include also sodium (Na+), chloride (Cl-), bicarbonate (HCO3-), and sulphate (SO4–) ions.
The salt content is normally determined in a saturation extract prepared by wetting the soil until it is just saturated with water and filtering off the extract under reduced pressure. The filtrate may be analyzed chemically, but a rapid indication of the degree of salinity is given by measuring its electrical conductivity. Conductivity values above 4 milliSiemens (mS) indicate that crop production may be reduced by salt damage, while above 20 mS only salt-tolerant species can survive. The approximate conductivity at 25°C of a 100 mmol/L solution referred to above is 8-10 mS.
The “reaction” of soil is one of its most important diagnostic parameters. It is given by a pH measurement on the saturation extract or on a suspension of soil in water or in a dilute electrolyte solution. Strongly acid soils may have pH values down to 3.5, and strongly alkaline soils as high as 9.5, but more typical pH values of soils range from 5 to 8.
In soils formed from limestone rocks or other carbonate-containing sediments, carbonates occur mainly as calcite (CaCO3) but sometimes also as dolomite [(Ca, Mg)CO3]. They are important in the buffer system that controls the pH and cation balance of soil, and also for their reactions with anions, particularly phosphate. In their reactions with anions, the particle size and surface area of the soil carbonates are more important than the amount.
Amounts of soil carbonate are estimated from the carbon dioxide evolved when the soil is treated with dilute acid, the results being expressed as a percentage by weight of the soil. In a leaching environment, soil carbonate is gradually removed by solution in carbonated water [CaCO3 + H2O + CO2 = Ca(HCO3)2] so that topsoils contain less carbonate than subsoils or the parent material. The leached carbonate may be concentrated by chemical precipitation at depth in the soil profile.
Soil analysis includes the separation and determination of sand, silt, and clay fractions by sieving and sedimentation. The mineral matter of soils is directly inherited from the parent material, although its composition is usually different depending on the age of the soil and the resistance of minerals to weathering. The minerals in the sand and silt fractions are mainly quartz and feldspars, plus a host of accessory minerals. Only the most resistant primary minerals remain in advanced stages of soil development, i.e., quartz (SiO2) as the major component, with smaller amounts of heavy metal oxides such as hematite (Fe2O3), magnetite (Fe3O4), and rutile (TiO2).
The clay-sized (< 2 µm effective diameter) fractions of many soils contain microcrystalline aluminosilicate layer minerals, and smaller amounts of hydrous oxides such as goethite, (FeO.OH), and gibbsite, (Al(OH)3). The gibbsite is more abundant in soils formed from basic rocks than from acidic rocks and are often amorphous. The clay fractions of some soils such as those derived from volcanic ash consist almost wholly of amorphous aluminosilicates of variable composition (allophane), and similar material is associated with the surfaces of many crystalline particles.
Land degradation making the land unsuitable for habitat construction and agriculture has become a major problem in recent times. This has threatened the world food production as soil quality degradation results in severe reduction in crop yield. It is estimated that 15 percent of the world’s total land area has not maintained its quality due to a number of problems that include erosion, nutrient decline, salinization and physical compaction. The countries which are mainly dependent on agriculture as a national resource suffer more from the effects of land degradation.
Some of the major soil degradation processes and the causes for them are given below.
Loss of topsoil by erosion/surface wash. This results in a decrease in depth of the topsoil layer due to more or less uniform removal of soil material by run-off water. The possible causes are inappropriate land management especially in agriculture (insufficient soil cover, unobstructed flow of run-off water, deteriorating soil structure) leading to excessive surface run-off and sediment transport.
“Terrain deformation” is an irregular displacement of soil material (by linear erosion or mass movement) causing clearly visible scars in the terrain. The possible causes are inappropriate land management in agriculture forestry or construction activities, allowing excessive amounts of run-off water to concentrate and flow unobstructed.
Fertility decline and reduced organic matter content resulting in a net decrease of available nutrients and organic matter in the soil. This is likely to be due to imbalance between output (through harvesting, burning, leaching, etc.) and input (through manure/fertilizers, returned crop residues, flooding) of nutrients and organic matter.
Soil contamination indicates the presence of an alien substance in the soil without significant negative effects and soil pollution signifies soil degradation as a consequence of location, concentration and adverse biological or toxic effects of a substance. The source of pollution may be waste dumps, spills, factory wasted, etc. The source can also be diffuse or airborne (atmospheric deposition of acidifying compounds and/or heavy metals.
Eutrophication with the presence of an excess of certain soil nutrients, impairing plant growth. The possible causes are imbalanced application of organic and chemical fertilizer resulting in excess nitrogen, phosphorus; liming.
Compaction resulting in deterioration of soil structure by trampling by cattle or the weight and/or frequent use of machinery. The possible causes are repeated use of heavy machinery, having a cumulative effect. Heavy grazing and overstocking may lead to compaction as well. Factors that influence compaction are ground pressure (by axle/wheel loads of the machinery used); frequency of the passage of heavy machinery; soil texture; soil moisture; climate.
Sealing and crusting which is clogging of pores with fine soil material and development of a thin impervious layer at the soil surface obstructing the infiltration of rainwater. The possible causes are poor soil cover, allowing a maximum “splash” effect of raindrops; destruction of soil structure and low organic matter.
Waterlogging that results from effects of human induced hydromorphism (i.e. excluding paddy fields). The possible causes are rising water table (e.g. due to construction of reservoirs/irrigation) and/or increased flooding caused by higher peak-flows.
Lowering of the soil surface resulting from subsidence of organic soils, settling of soil. The possible causes are oxidation of peat and settling of soils in general due to lowering of the water table; solution of gypsum in the sub-soil (human-induced) or lowering of soil surface due to extraction of gas or water
Loss of productive function which results from soil (land) being taken out of production for non-bio-productive activities, but not the eventual “secondary” degrading effects of these activities. The possible causes are urbanization and industrial activities; infrastructure; mining; quarrying, etc.
Aridification, which is the decrease of average soil moisture content. The possible causes are lowering of groundwater tables for agricultural purposes or drinking water extraction; decreased soil cover and reduced organic matter content.
Salinisation / alkalinization which is a net increase of the salt content of the (top)soil leading to a productivity decline. The possible causes are a distinction can be made between salinity problems due to intrusion of seawater (which may occur under all climate conditions) and inland salinisation, caused by improper irrigation methods and/or evaporation of saline groundwater.
Dystrification, which is the lowering of soil pH through the process of mobilizing or increasing acidic compounds in the soil.
Worldwide, almost 2,000 million hectares of land show at least minor signs of degradation, corresponding to approximately 1% of the ice-free surface. Around 300 million hectares of land surface are already seriously degraded. Soil degradation situation in India is shown in Fig. 2.10.
Population growth and soil
Population growth exerts enormous pressure on soils, and the soil degradation is due to additional migration and urbanization processes. The higher the rate of global population growth, the higher is the demand on the soil functions. There is already a growing disparity between growth-related demand and the availability of land. Many states are no longer capable of feeding their own populations with domestic agricultural products because they do not have enough land. Given the speed of population growth and the level of soil degradation already apparent, an increasing scarcity of soils available for meeting competing demands is expected.
Two case studies of soil degradation
1. The Sahel Region
The problems of soil degradation and desertification in the Sahel can be attributed to changes in nature as well as to socioeconomic causes.
The nomadic groups in the Sahel are increasingly restricted in the mobility and flexibility that once provided them with a secure basis for ecological adaptation. Growing competition from other forms of land use, political measures and unclear or disadvantageous land-use rights led to their sedentarisation; they were pushed into more marginalized locations much less suitable for grazing livestock. The sensitive soils and ecosystems in the region are degraded as a result, mainly due to overgrazing.
Subsistence farmers are similarly affected by displacement to marginal land that is unsuitable for farming. Greater mechanization without parallel soil protection measures (erosion protection, and suitable irrigation) has negative effects on the soils.
Finally, “cash crops” (cotton, groundnuts) on fertile soils is not pursued in a sustainable fashion. These monocultures are farmed with the help of machines and pesticides, both of which can cause great problems.
The Sahel also undergone tremendous social changes caused by internal and external conditions. Of importance is the general neglect of rural concerns and the orientation to agrarian export production through large-scale capital-intensive projects in the agricultural sector. External factors can be identified both in the global economic conditions (agricultural subsidies and/or export policies of the industrial nations, international debt) and in the practice of international development organizations, which in the past were not geared to the principle of sustainability, and which through their orientation to production technology gave too little consideration to the existing development potential. If the complex problems faced by the Sahel are to be solved, greater attention must be given to the socioeconomic causes and to organizational and financial decentralization.
2. The “Leipzig-Halle-Bitterfeld” region
The soils in the “Leipzig-Halle-Bitterfeld” region are contaminated, in some cases alarmingly, by depositions of airborne pollutants through deliberate depositing of inorganic and organic substances. A prime cause of this contamination was the concentration of chemical industries, mining and energy production, all of which used outdated production methods. Since the turn of the century, there have been five brown coal mining fields, and large-scale chemical plants developed in Bitterfeld (paints and dyes), Leuna (methanol, nitrogen) and Buna (synthetic rubber). For economically and environmentally sound development of the region, soil remediation and the removal of contaminated soil are a matter of urgency, which requires considerable support from the state or from outside the region.
Fig. 2.10. Soil degradation in India
In a landslide, masses of rock, earth, or debris move down a slope. Landslides may be small or large, slow or rapid. They are activated by:
alternate freezing or thawing, and
steepening of slopes by erosion or human modification.
Debris and mudflows are rivers of rock, earth, and other debris saturated with water. They develop when water rapidly accumulates in the ground, during heavy rainfall or rapid snowmelt, changing the earth into a flowing river of mud or “slurry.” They can flow rapidly, striking with little or no warning at avalanche speeds. They can travel several miles from their source, growing in size as they pick up trees, boulders, and other materials.
Landslide problems can be caused by land mismanagement, particularly in mountain, canyon, and coastal regions. In areas burned by forest and brush fires, a lower threshold of precipitation may initiate landslides. Land-use zoning, professional inspections, and proper design can minimize many landslide, mudflow, and debris flow problems.
Protection from a landslide or debris flow
(a) Guidelines for the period following a landslide:
Stay away from the slide area. There may be danger of additional slides.
Listen to local radio or television stations for the latest emergency information.
Watch for flooding, which may occur after a landslide or debris flow. Floods sometimes follow landslides and debris flows because they may both be started by the same event.
Check for injured and trapped persons near the slide, without entering the direct slide area. Ask for rescuers and give them correct locations.
Help a neighbor who may require special assistance – infants, elderly people, and people with disabilities. Elderly people and people with disabilities may require additional assistance. People who care for them or who have large families may need additional assistance in emergency situations.
Inform appropriate authorities about damaged roadways, railways, electricity lines and other utilities. Reporting potential hazards will get the utilities turned off as quickly as possible, preventing further damage.
Check building foundation, chimney, and surrounding land for damage. Damage to foundations, chimneys, or surrounding land may help assess the safety of the area.
Replant damaged ground as soon as possible since erosion caused by loss of ground cover can lead to flash flooding and additional landslides in the near future.
Seek advice from a geotechnical expert for evaluating landslide hazards or designing corrective techniques to reduce landslide risk. A professional will be able to advise you of the best ways to prevent or reduce landslide risk, without creating further hazard.
(b) During a Landslide or Debris Flow
What one should do if a landslide or debris flow occurs:
Stay alert and awake. Many debris-flow fatalities occur when people are sleeping. Listen to radio or television for warnings of intense rainfall. Be aware that intense, short bursts of rain may be particularly dangerous, especially after longer periods of heavy rainfall and damp weather.
If you are in areas susceptible to landslides and debris flows, consider leaving if it is safe to do so. Remember that driving during an intense storm can be hazardous. If you remain at home, move to a second story if possible. Staying out of the path of a landslide or debris flow saves lives.
Listen for any unusual sounds that might indicate moving debris, such as trees cracking or boulders knocking together. A trickle of flowing or falling mud or debris may precede larger landslides. Moving debris can flow quickly and sometimes without warning.
If one is near a stream or channel, he should be alert for any sudden increase or decrease in water flow and for a change from clear to muddy water. Such changes may indicate landslide activity upstream, so be prepared to move quickly. Don’t delay! Save yourself, not your belongings.
Be especially alert when driving. Embankments along roadsides are particularly susceptible to landslides. Watch the road for collapsed pavement, mud, fallen rocks, and other indications of possible debris flows.
(c) What to do in case of Imminent Landslide Danger
Contact your local fire, police, or public works department. Local officials are the best persons able to assess potential danger.
Inform affected neighbors. Your neighbors may not be aware of potential hazards. Advising them of a potential threat may help save lives. Help neighbors who may need assistance to evacuate.
Evacuate. Getting out of the path of a landslide or debris flow is your best protection.
Curl into a tight ball and protect your head if escape is not possible.
(d) Before a Landslide or Debris Flow
Protect yourself from the effects of a landslide or debris flow:
Do not build near steep slopes, close to mountain edges, near drainage ways, or natural erosion valleys.
Get a ground assessment of your property.
Contact local officials, geological surveys or departments of natural resources, and university departments of geology. Landslides occur where they have before, and in identifiable hazard locations. Ask for information on landslides in your area, specific information on areas vulnerable to landslides, and request a professional referral for a very detailed site analysis of your property, and corrective measures you can take, if necessary.
If you are at risk from a landslide talk to your insurance agent. Debris flow may be covered by flood insurance policies.
Minimize home hazards
Have flexible pipe fittings installed to avoid gas or water leaks, as flexible fittings are more resistant to breakage (only the Gas Company or professionals should install gas fittings).
Plant ground cover on slopes and build retaining walls.
In mudflow areas, build channels or deflection walls to direct the flow around buildings.
Remember: If you build walls to divert debris flow and the flow lands on a neighbor’s property, you may be liable for damages.
Recognize Landslide Warning Signs
Changes occur in your landscape such as patterns of storm-water drainage on slopes (especially the places where runoff water converges) land movement, small slides, flows, or progressively leaning trees.
Doors or windows stick or jam for the first time.
New cracks appear in plaster, tile, brick, or foundations.
Outside walls, walks, or stairs begin pulling away from the building.
Slowly developing, widening cracks appear on the ground or on paved areas such as streets or driveways.
Underground utility lines break.
Bulging ground appears at the base of a slope.
Water breaks through the ground surface in new locations.
Fences, retaining walls, utility poles, or trees tilt or move.
Faint rumbling sound that increases in volume is noticeable as the landslide nears.
The ground slopes downward in one direction and may begin shifting in that direction under your feet.
Unusual sounds, such as trees cracking or boulders knocking together, might indicate moving debris.
Collapsed pavement, mud, fallen rocks, and other indications of possible debris flow can be seen when driving (embankments along roadsides are particularly susceptible to landslides).
The most critical and increasing threat to sustainable land use is desertification. It is estimated that desertification affects one-quarter of the total land area of the world, or about 70 percent of all dry lands, and threatens the livelihoods of over 1 billion people in more than 100 countries. Desertification is closely linked with rural poverty and hunger. It exacerbates conditions leading to famine, migration, internal displacement, political instability and conflict.
Desertification is the degradation of land in arid, semi arid and dry sub-humid areas resulting from various climatic variations, but primarily from human activities. Current desertification is taking place much faster worldwide and usually arises from the demands of increasing population that settle on the land in order to grow crops and graze animals.
A major impact of desertification is loss of biodiversity and productive capacity, for example, by transition from grassland to perennial shrubs. The change in vegetation induces desertification. In the Madagascar, 10% of the entire country has been lost to desertification due to zoom agriculture by indigenous people. In Africa, with current trends of soil degradation, the continent will be able to feed just 25% of its population by 2025 according to one estimate.
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Deserts may be separated from the surroundings by less arid areas, mountains and other landforms. In other areas, there is a gradual transition from a dry to a more humid environment, making it more difficult to determine the desert border. These transition zones have very fragile, delicately balanced ecosystems. Desert fringes are a mosaic of microclimates. Small hollows support vegetation that picks up heat from the hot winds and protects the land from the prevailing winds. After rainfall the vegetated areas are distinctly cooler than the surroundings. In these marginal areas human activity may stress the ecosystem beyond its tolerance limit, resulting in degradation of the land. By pounding the soil with their hooves, livestock compact the substrate, increase the proportion of fine material, and reduce the percolation rate of the soil, thus encouraging erosion by wind and water. Grazing and collection of firewood reduce or eliminate plants that help to bind the soil.
In large desert areas, sand dunes can encroach on human habitats. Sand dunes move through wind. In a major dust storm, dunes may move tens of meters. And like snow, sand avalanches, falling down the steep slopes of the dunes that face away from the winds, move the dunes forward.
Droughts by themselves cannot cause desertification. Drought is just a contributing factor. The causes are social and economic, having to do with access to resources, power and economics. Droughts are common in arid and semiarid lands, and well-managed lands can recover from drought when the rains return. Continued land abuse during droughts, however, increases land degradation. Increased population and livestock pressure on marginal lands has accelerated desertification. In some areas, nomads moving to less arid areas disrupt the local ecosystem and increase the rate of erosion of the land. Nomads are trying to escape the desert, but because of their land-use practices, they bring the desert with them.
Some arid and semi-arid lands can support crops, but additional pressure from greater population or decreases in rainfall can lead to the disappearance of the few plants present. The soil becomes exposed to wind, causing soil particles to be deposited elsewhere. The top layer becomes eroded. With the removal of shade, rates of evaporation increase and salts become drawn up to the surface. This is salinisation, which inhibits plant growth. The loss of plants causes less moisture to be retained in the area, which may change the climate pattern leading to lower rainfall.
The degradation of formerly productive land is a complex process. It involves multiple causes, and it proceeds at varying rates in different climates. Desertification may intensify a general climatic trend toward greater aridity, or it may initiate a change in local climate. Desertification does not occur in linear, easily mappable patterns. Deserts advance erratically, forming patches on their borders. Areas far from natural deserts can degrade quickly to barren soil, rock, or sand through poor land management. The presence of a nearby desert has no direct relationship to desertification. Unfortunately, an area undergoing desertification is brought to public attention only after the process is well under way. Often little data are available to indicate the previous state of the ecosystem or the rate of degradation.
Combating desertification is complex and difficult. Over-exploitation of the land and climate variations can have identical impacts, which makes it very difficult to choose the right mitigation strategy. Measures like reforestation cannot achieve their goals if global warming continues. Forests may die when it gets drier, and more frequent extreme events could become a threat for agriculture, water supply, and infrastructure.
Overgrazing and to a lesser extent drought in the 1930s transformed parts of the Great Plains in the United States into the “Dust Bowl”. During that time, a considerable fraction of the population abandoned their homes to escape the unproductive lands. Improved agricultural and water management have prevented a disaster of the earlier magnitude from recurring, but desertification presently affects millions of people with primary occurrence in the less developed countries.
Desertification is widespread in many areas of the People’s Republic of China. The populations of rural areas have increased along with an increase in the livestock; the land available for grazing has decreased. Importing of European cattle, which have higher food intakes, has made things worse.
Human overpopulation is leading to destruction of tropical wet and dry forests, due to widening practices of zoom cultivation. Deforestation has led to large scale erosion, loss of soil nutrients and sometimes total desertification.
Overgrazing has made the Rio Puerco Basin of central New Mexico one of the most eroded river basins of the western United States and has increased the high sediment content of the river. Overgrazing is also an issue with some regions of South Africa such as the Waterberg Massif, although restoration of native habitat and game has been pursued vigorously since 1980.
The Desert of Maine is a 40-acre dune of glacial silt near Freeport, Maine. Overgrazing and soil erosion exposed the cap of the dune, revealing the desert as a small patch that continued to grow, overtaking the land. Ghana and Nigeria currently experience desertification; in the latter, desertification overtakes about 1,355 square miles of land per year. The Central Asian countries, Afghanistan, Kazakhstan, Kyrgyzstan, Tajikistan, Turkmenistan, and Uzbekistan, are also affected. More than 80% of Afghanistan’s land is subject to soil erosion and desertification. In Kazakhstan, nearly half of the cropland has been abandoned since 1980. In Iran, sand storms were said to have bur
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