eChapters

4.1 INTRODUCTION

All soil components contribute in some measure to cation exchange. The source, extent, and quality of the contribution of each to the cation exchange capacity is varied and complex. Ocasionally, reactions between components alter the extent and quality of cation exchange. Also, in some this is strongly influenced by the pH of the soil-water complex, and the electrolyte content of the soil solution. Indeed, when ‘simple and rapid’ methods, empirical or allegedly fundamental in origin, appear in the literature, they must be viewed circumspectly. Mostly, they provide an incomplete, or a one-sided, picture of the phenomenon. To justify his invention, the experimenter usually provides correlations with other intrinsic soil properties (e.g. the concentration by weight and the mineralogy of the <2 jxm particles, the organic matter content, the surface area) or with the reaction of soil with its environment (e.g. plant growth and composition). However convincing such statistical parameters might be for the population of soils investigated, they do not help elucidate, and may indeed mask, the true extent and quality of cation exchange.

This chapter deals with current views on the sources, extent and quality of cation exchange, and with comprehensive methods for evaluating these. The relationship between the properties thus observed and values obtained from simple and rapid procedures is considered.

3.1 INTRODUCTION

The chemistry of the soil is much more complex than can be represented simply by precipitation, adsorption, or exchange processes. The complexity arises from the time scale of soil chemical reactions in which the structure and dimensions, the extent and distribution of pores, and the content and mobility of water in them, interact immensely with the reaction chemistry. The fourth process adding to this complexity is the dissolution of native and reprecipitated soil constituents by percolating water containing solutes accumulated by natural processes or added by man. The fifth complicating factor compounding the time scale is that very often the resultants of soil chemical reactions are large complex molecules, occasionally polymeric, made up of several constituent groups or atoms. It is by no means clear which of the four processes participate in a chemical reaction and in which order. That a process reoccurs during a prolonged reaction is very likely.

Such complexities explain why no single model related rigorously to the retention and release of cations and anions, can provide a basis for a laboratory procedure to test and compare the chemical properties of soils. In this chapter, some of the soil reactions that are important to plant nutrition and pedological processes are discussed, principally those leading to relatively simple compounds accumulating in soil.

2.1 INTRODUCTION

Mass flow and diffusion are processes which cause the redistribution of many components within the soil profile. They are usually investigated because they cause a redistribution of materials such as fertilizers and pesticides which are added to the soil, or because of the need to reduce salt concentrations to levels safe for crop growth. This has given an economic incentive to the investigations, but the processes are also basic to our understanding of the development of soil properties. For example, diffusion is the main process for the movement of gases into and out of the soil, which in turn affects many soil properties (Chapter 7), and mass flow of water is an important cause of the formation of horizons in soil profiles (Chapter 8). Hence there are both academic and economic reasons for attempting to understand the way the processes operate in soils.

Over the last few years our understanding has increased considerably. It is the purpose here to give the principles which have been evolved in this recent work, and to discuss their application. It will also be shown that many problems remain to be solved.

Diffusion means a spreading out, and is caused by random thermal motion, as is the Brownian movement of colloidal particles observable under the microscope. In unstirred liquids, which are our main concern in this chapter, all the molecules and ions of the solvent and solute have this random movement (molecules and ions in gases and some solids behave similarly). As a result of this motion, which occurs randomly in all directions, any irregularities in the concentration of a solution or mixture of gases eventually disappear.

1.1 INTRODUCTION

This volume is concerned with the chemistry of soil processes. These are rather arbitrarily divided and discussed in the first part of this book in terms of mass flow and diffusion, precipitation, cation, anion and ligand exchange, adsorption, translocation of metals, and oxidation and reduction. The way in which these processes operate to determine the fate of different substances added to soils is described in the remaining part of the book. In this initial chapter an attempt is made to show how the individual chemical processes described later combine with physical and biological processes to give soils their ability to support plant life, and to accept the detritus of the world and to convert it to harmless constituents of the soil mass.

14.1 Changes in policy and strategy

14.1.1 Policy changes

In the discussion on land use planning we suggested that the ideal pattern of development is to establish a long-term national or regional policy, which leads to medium-term strategies, and short-term tactical operations. In the real world a curious situation is arising in that the adoption of the new land husbandry approach is growing so fast that the practitioners have moved ahead of the politicians and opinion formers, with the result that the implementation of strategies by Non-Government Organisations, aid agencies, and senior field officers, is helping to shape new policies.

There is always the problem that national policies ought to be long term, but are created by politicians usually working within a short time frame to the next election. Fortunately, the land husbandry approach allows planners at all levels from farmers to governments to appreciate the long-term issues.

The new approach was mentioned in section 9.2.1 in connection with land use planning. It is summed up in the slogan From soil conservation to land husbandry, and two reviews using that title list the main points as the need for:

Damage to arable land tends to attract most attention but degradation of non-arable land can also have serious effects, whether economic, or social, or on the food supply. In dry seasons, loss of grazing capacity is a common and serious problem, forest land may suffer through over-exploitation, roads can cause much damage particularly on steep land. In this chapter we will look at the more important erosion problems on non-arable land.

13.1 Erosion on grazing land

On well-managed high-yielding pasture it is unlikely that there will be any erosion, for the ground will have a good uniform cover. But land which is not arable because it is too poor, too infertile, too stony, or too arid, and is only described as grazing land because it is unfit for more intensive use, is a very different matter. Again, erosion control can be directly and precisely equated to improved management - anything leading to a better water balance and a better ground cover will lead to reduced erosion.

12.1 The nature of gully erosion

Gully erosion is spectacular and widespread, and so it is often used by conservationists as a characteristic symptom of erosion. As a result there is a danger of its importance being over-emphasized. Certainly it is most important as a source of sediment in streams, but in terms of damage to agricultural land or reduction of agricultural production, it is usually not very important for the simple reason that most land subject to severe gully erosion is of little agricultural significance. The really spectacular erosion so popular in non-technical reports shows large areas cut up by deep interconnecting gullies, but this situation is usually found in semi-arid climates unsuitable for any serious agriculture, or on soils which have such adverse chemical or physical properties that their potential production is very low. Added to this is the fact that gully control is always difficult and expensive, so the cost of reclamation usually exceeds the value of the land. Certainly in the sense of reducing the amount of eroded soil choking the dams and rivers it may be highly desirable to do something about gully erosion, but it is very much a case of prevention being better than cure. Limited resources in manpower, money and materials can usually be better employed in preventing future gullies than in curing existing ones.

The worldwide occurrence and recognition is well demonstrated in the variety of names: in English and American - gully, in Egypt - wadi, in South Africa - donga, in French-speaking lands - ravine, in India - nulla, in South America - carcava, or arroyo, and many others. The definition is the same in any case, a steep-sided eroding watercourse which is subject to intermittent flash floods. They are more common in semi-arid climates with infertile soils and sparse vegetation, but some spectacular examples are found in tropical forests in deep soils with dense vegetation.

11.1 The farmer’s point of view

In the last chapter possible conservation practices were reviewed in groups according to their purpose in terms of influencing runoff or soil loss. Now, looking at ways of limiting soil degradation by improved land husbandry, the primary consideration is to look at the alternatives from the farmer’s point of view. In fact we need to be very wary of manual-type recommendations. A better approach is to offer a menu of alternatives, and the role of the professional is to help the farmer choose what is appropriate for his or her circumstances. A recent review of technologies acceptable to resource-poor farmers points out that combining practices into a farming system must take account not only of the physical factors such as soil and climate, but equally the available resource inputs, especially cash and labour, and the farmer’s objectives (STOCKING 1993).

The objectives of a commercial farmer are usually to maximize yield and income, but the subsistence farmer is likely to be more interested in improving food security by reducing the risk of failure, or improving the return on inputs of seed, fertilizer and labour, or improving the quality of life by reducing drudgery, particularly relevant in the case of women farmers.

We need a new approach to the farmer when recommending changes to farming practice. Instead of recommending practices because they will reduce erosion, we should show how they can lead to increased production. For example, practices which lead to better soil structure, more organic matter, or more moisture-holding capacity should be put forward because they improve yield, not because they reduce the soil erodibility. Increasing infiltration from better ground cover or the use of contour farming or ridging can result in better crops as well as reducing soil loss. Improved crop management could include more use of fertilizer and manures or crop rotations. These practices will make more sense to the farmer if they are recommended as leading to better production, which he can instinctively appreciate as desirable, rather than because they will reduce soil erosion which is likely to be a less important objective.

10.1 Different practices for different purposes

There are so many different practices used in erosion control that to describe them needs some form of grouping. For a long time mechanical protection was used to describe all the methods which involved earth moving, such as digging drains, building banks, levelling sloping land, and so on. Anything else was lumped together under biological methods for want of a better title.

This is appropriate for large mechanized farms where the approach is to use machines to do the earth-moving and follow up with improved farming methods. But this division does not suit the concept of erosion control through better land husbandry supported by mechanical protection, and the division is artificial when we start talking about progressive terracing using grass strips or live hedges.

An approach more suited to today’s thinking is to group methods and techniques according to their main objective. All have an element of saving soil loss, but an important difference is the handling of surface runoff. This chapter identifies three kinds of water management objective and two types of soil management on sloping land.

9.1 Planning land use

Modern techniques such as mechanization, better crop varieties, and the scientific use of fertilizers can transform agriculture, but before they can be effective the use of the land has to be right. No techniques will make it possible to grow a good crop if the soil conditions are unsuitable for that crop, and no conservation works can prevent erosion when the basic cause is trying to grow crops on land which is really unsuitable for arable farming (plate 9.1). The object of land use planning is to determine the characteristics of land, its possibilities, and its problems, and try to match these to a form of land use which will be sustainable and economically productive.

9.7.1 Some planning principles

An often quoted definition is that ‘planning is the conscious process of selecting and developing the best course of action to accomplish an objective’. In the case of Land Use Planning the objective is the efficient intensive use of land resources.

It is difficult to imagine any field of human activity where some form of planning is not essential to achieve the objective. Consider going on holiday. Certainly it is possible to come home from work one evening, throw some clothes into a suitcase, go to the railway station or airport and buy a ticket to the first holiday resort which comes to mind. Some fortunate people may indeed enjoy so unplanned a holiday but for most of us it would be disappointing to find oneself in a mountain holiday resort without climbing boots, or on a sunny beach with only winter clothing in the suit case. No, one is more likely to achieve the objective of a happy holiday by a planned approach and this means going through a logical sequence of steps, i.e.

8.1 The purpose of erosion research

The need for research

A soil conservation programme needs to be based on factual information about rates and quantities of erosion. For example some land obviously needs to be protected by mechanical works such as channel terraces, some land equally obviously does not need any protection, and a great deal of land lies somewhere between the two extremes. At some point the terraces become unnecessary or uneconomic, but a logical decision on where to draw the line can only be made if data are available on how much soil erosion is occurring and how much it would be reduced by terraces. Similarly, the effect of channel terraces should be compared with the effect of alternative measures, and the effect should also be measured on the different soils and crops which are likely to be encountered.

Nowadays there are excellent opportunities for research workers to exchange the results of their work. The national and iniernational journals are packed with accounts and results of research reports in their proceedings (as listed in Further reading). Some of the reported research results can be usefully applied in countries other than where the research was done, but care is required in any transfer of technology. If the local conditions of soil, climate, or land use, are different, the results may not apply. However, research in other situations can often serve as a pilot trial to show which factors should be studied locally. Even when local conditions are so different that the results do not apply at all, the techniques and apparatus developed by other workers can still be helpful.

7.1 The purpose and type of models

Sometimes the simplest way to estimate the effect of a physical process is to use an established equation, or formula, or charts and diagrams based on existing knowledge. The current fashion is to call such aids and solutions models, and they tend to become increasingly complicated and consequently dependent on computers to handle the mathematical processes.

It is important to identify the exact objective and purpose of models which are designed to estimate soil erosion. Two main divisions are models for prediction and models to assist with explanation. If the purpose is to predict amounts of erosion under alternative management practices then empirical models such as the USLE are efficient and effective. For an understanding of the processes of erosion it is necessary to turn to physically-based or deterministic models. The two objectives should not be cross-threaded onto the wrong kind of model, i.e. the USLE should not be seen as a vehicle for research studies on the mechanics of erosion. Neither should sophisticated simulation models be used as the basis for evaluating conservation practices which are better designed on the basis of experience, and evaluated by direct testing in the field.

The different kinds of models need a short explanation:

An empirical solution or model is one based on observation or experiment, and not derived from theory. It fits observed facts, and allows us to predict what will happen in certain circumstances, because we know what has happened before in those circumstances. The reliability of such methods depends on the database of experience: we might say This will certainly happen’ (because it always does), or “Will probably happen’ (because it usually does), or ‘it may happen’ (because it sometimes does). An empirical solution may be a simple approximate relationship or a complex multiple regression equation.

6.1 Quantities and rates of runoff

Before a start can be made on the design of the channels and ditches and other works which can deal with surface runoff we need to know the probable quantity of water. If the object is to impound or store the runoff then it may be sufficient to know the total volume of water to be expected, but the usual conservation problem is conveying water from one place to another, and in that case the rate of runoff is more important, and particularly the maximum rate at which runoff is likely to occur. That is the flow which a channel must be designed to accommodate.

In a hypothetical catchment area* with an impervious surface and no losses the maximum rate of runoff would be directly proportional to the rate of rainfall. In natural catchments there are other factors: some of the rain is intercepted by vegetation, some infiltrates into the soil, some starts moving over the surface but is trapped in depressions, and some is lost by evaporation. Estimates of rates of surface runoff therefore depend on two processes: estimating the rate of rainfall, and estimating how much of the rainfall becomes runoff.

There are several empirical methods for doing both and for combining the two factors, but because they are empirical two points must be stressed. Firstly, as with all empirical solutions, data, tables, and formulae developed in one situation should not be applied in different conditions without careful checks on whether the method is valid in the new situation. Secondly, each of the three main methods which will be

5.1 Definitions

The erodibility of soil is its vulnerability or susceptibility to erosion, that is the reciprocal of its resistance to erosion. A soil with a high erodibility will suffer more erosion than a soil with low erodibility if both are exposed to the same rainfall. Whereas the erosivity of rainfall was shown in chapter 4 to be a fairly straightforward measure of the rain’s physical properties, the assessment of erodibility is more complicated because it depends upon many variables. There are alternative uses of the term erodibility. In the widest sense it can be used to include all the variables which effect erosion except the erosivity of rain, as in the first line of figure 2.2. It is also used in the narrower sense as a measure only of the effect of physical characteristics of the soil, as in the erodibility index K of the Universal Soil Loss Equation, where the effects of land management and crop management are assessed separately, as shown in the last line of figure 2.2.

5.2 Factors influencing erodibility

In this chapter we will look at the three broad groups of factors which affect erodibility. First, there are the physical features of the soil, including the chemical and physical composition. Second, there are the topographic features, such as the slope of the land. Third, there is the management of the land, i.e. how it is used.

4.1 Defining erosivity and erodibility

Rainfall erosion is the interaction of two items - the rain and the soil. The amount of erosion which occurs in any given circumstances will be influenced by both, and our study of the processes of soil erosion is simplified by considering the two aspects separately.

We know from observation that one storm can cause more erosion than another on the same land, and we also know that the same storm will cause more erosion on one field than on another. The effect of the rain is called erosivity and the effect of the soil is called erodibility. The way they differ is best explained by examples.

Let us imagine an experiment station which has over its whole area exactly the same soil type and slope. Assume also that the whole of the station is covered by a completely uniform crop. If the amount of erosion taking place during each storm were measured, the amount would be influenced only by the nature of the rain. Comparing the effect of one storm with that of another storm would provide a relative measure of the power of each storm to cause erosion, and that is what is meant by erosivity. This will enable us to say that on this imaginary experiment station, storm A caused a soil loss of X kilograms per hectare and storm B caused a soil loss of Y kilograms per hectare. If Y is half of X, then the erosivity of storm B is half of the erosivity of storm A.

3.1 Physical characteristics

There is obviously an association between the amount of rainfall and the amount of soil erosion, i.e. more rain goes with more erosion, and less rain with less erosion, but in statistical terms the correlation between the two is poor. The same total quantity of rain can on different occasions result in widely differing amounts of erosion, and so other more specific measures are required to describe the ability of rainfall to cause erosion. In this chapter we shall establish the known facts about the main physical properties of rainfall, and then in chapter 4 show how these properties are related to the erosive power of rainfall.

3.1.1 Quantity of rainfall

Any measure of rainfall amount is a sample, and so associated with the inevitable problems of sampling, namely, ⁴is the sample representative of the whole?’ and ‘is the sample measured accurately?’ On both counts the data of rainfall which are usually available are of very doubtful reliability. Considering firstly the size of the sample measured, a very intensive network of rain gauges such as that established in an experimental catchment might be as dense as one gauge per 25 hectares, but this corresponds (with a 125 mm diameter gauge) to a sample of about one in 20 million, which would for any other scientific measurement be considered totally inadequate. The density of gauges in a country which considers itself very well provided might be one gauge per 25 square kilometres - a sample of one in 2 000 million, and in an undeveloped country the density of one gauge per 2 500 square kilometres is a sample of 1 in 200 000 000 000! Secondly, the siting of this remarkably small number of gauges is usually arranged primarily on convenience, eg where they can be easily read and contained, with little or no consideration of whether the site is representative of the whole. Thirdly, the accuracy of the measurement is usually unknown and studies have shown that variations in the shape and size of the instrument, and in the height at which it is mounted, and the way it is shielded from the wind, can seriously affect the recorded amount of rainfall. Clearly, even the apparently straightforward question of how much rain falls is not answered as precisely as might be imagined from a survey of rainfall tables and maps.

2.1 Geological and accelerated erosion

Erosion has always taken place, and always will. The surface of the earth is constantly changing, with mountains rising, valleys being cut deeper and wider, the coast line receding here and advancing there. The physical pattern of the surface of the earth which we see today is not the result of some single cataclysmic sculpturing but the result of changes so infinitely slow that only after centuries is the effect noticeable. Erosion is simply one of the aspects of the constant process of change. It is fundamental to the formation of alluvial soils and sedimentary rocks. Man’s activities seldom slow down or halt the process but frequently speed it up. We usually refer to geological erosion or normal erosion, or natural erosion when we mean that which results only from the forces of nature, and to accelerated erosion when the process is influenced by man.

When trying to predict the probable severity of man-made erosion it helps to consider the rate of geological erosion. If the conditions of climate and topography are such that geological erosion is quicker than usual, then these sameronditions will also lead to particularly severe accelerated erosion. Extreme examples of both kinds of erosion are shown in plates 2.1 and 2.2.

1.1 The relation between man and the earth’s resources

The balance between the demand which a community of plants or animals makes on its environment and the ability to satisfy those demands is not the static equilibrium of a laboratory balance with equal weights on either side. It is more like the unstable balancing of a circus acrobat - a series of swings and overcorrections, resulting in an oscillation on either side of the true balance point. Thus a species of wild life increases in number according to the availability of its food supplies, but then goes beyond the optimum number. The natural correcting factors of starvation or migration reduce the population, but there is an overcorrection, and soon the cycle starts again with increasing numbers.

1.1.1 The Malthusian Thesis

In primitive societies the human population also oscillates about a mean as the limiting factors of starvation, disease, and war, maintain an uneasy balance against the natural tendency to increase. A serious imbalance arises when man learns how to modify the limiting factors, but allows the natural increase to go unchecked. The essence of the Malthusian Thesis is that the resulting instability is the inevitable order for mankind. The thesis is named after the English political economist Thomas Malthus who first gave it formal expression in MALTHUS (1966). In an age when significant improvements in health, hygiene, and the standard of living were just becoming possible, Malthus aroused great argument when he pointed out that these very improvements could lead to mi sery through overpopulation. The key point of Malthus’s argument is that population increases in what he called geometric ratio, what today we would call an exponential curve, because whenever there is an increase in population there are more people to produce the next generation, so the rate of population growth accelerates - in modelling terms a positive feedback loop. On the other hand, when we increase food production there is no feed-back loop, the best we can hope for is for a constant linear increase. The natural sequence is therefore for food demand to increase faster than food supply, leading at some point to food shortage.

18.1 Introduction

The commercially available water quality simulation models with various dimensions (1D, 2D and 3D) and database software designed to quantify movement and concentration of contaminants in lakes, streams, estuaries, and marine environments. The following water quality models were described below (Table 18.1).

18.2 Environmental Fluid Dynamics Code

The Environmental Fluid Dynamics Code (EFDC) is a multifunctional surface water modeling system, which includes hydrodynamic, sediment-contaminant, and eutrophication components. It has been applied to over 100 water bodies including rivers, lakes, reservoirs, wetlands, estuaries, and coastal ocean regions in support of environmental assessment and management and regulatory requirements. IT is a state-of-the-art hydrodynamic model that can be used to simulate aquatic systems in one, two, and three dimensions. It has evolved over the past two decades to become one of the most widely used and technically defensible hydrodynamic models in the world. It uses stretched or sigma vertical coordinates and Cartesian or curvilinear, orthogonal horizontal coordinates to represent the physical characteristics of a waterbody. It solves three-dimensional, vertically hydrostatic, free surface, turbulent averaged equations of motion for a variable-density fluid. Dynamically-coupled transport equations for turbulent kinetic energy, turbulent length scale, salinity and temperature are also solved. The model allows for drying and wetting in shallow areas by a mass conservation scheme.

17.1. Introduction

Throughout human history, the coastal plains and Lowland River valleys have usually been the most populated areas over the world (Wolanski et al., 2004). At present, about 60% of the world’s population live along the estuaries and the coast (Lindeboom, 2002). This is degrading estuarine and coastal waters through pollution, eutrophication, increased turbidity, overfishing and habitat destruction. The pollutant supply does not just include nutrients; it also includes mud from eroded soil, heavy metals, radionuclides, hydrocarbons, and a number of chemicals including new synthetic products.

The impact on estuaries is commonly still ignored when dams and irrigation farming are proposed on rivers. In addition, estuaries are often regarded as sites for future development and expansion, and have been increasingly canalized and dyked for flood protection, and their wetlands infilled for residential areas.

All these factors impact on the biodiversity and productivity and hence, the overall health of estuaries and the ecosystem services they provide to humans. They increasingly lead humans away from the possibility of ecologically sustainable development of the coastal zone. Integrated coastal zone management plans are drawn up worldwide but, in the presence of significant river input, most are bound to fail because they commonly deal only with local, coastal issues, and do not consider the whole river catchment as the fundamental planning unit. It is as if the land, the river, the estuary, and the sea were not part of the same system. When dealing with estuaries and coastal waters, in most countries land-use managers, water-resources managers, and coastal and fisheries managers do not cooperate effectively due to administrative, economic and political constraints, and the absence of a forum where their ideas and approaches are shared and discussed (Wolanski et al., 2004). 

16.1 Introduction

Littoral drift is a geological process by which sediments such as sand or other materials, move along a beach shore.Long Shore Drift is the movement of eroded material, in a zig zag way, along the coast. The process arises when waves approach the shore obliquely. That is to say, at an angle other than 90°. The effect of this is determined by factors such as the direction and fetch of the present wind and, in the long term, of the prevailing wind.

Waves striking the shore at an angle as opposed to straight on will cause the wave swash to move up the beach at an angle. The swash moves the sediment particles up the beach at this angle, while the backwash brings them, solely under the influence of gravity, directly down the beach. Some geologists consider a beach to be merely a shoreline feature of deposited material, but William Bascom (1980) has argued that a beach is the entire system of sand set in motion by waves to a depth of ten meters or more off ocean coasts. Submerged, longshore bars are therefore also part of the beach, and thus beaches can be viewed as either:

15.1 Introduction

A tsunami is a series of waves created when a body of water, such as an ocean, is rapidly displaced. Earthquakes, mass movements above or below water, volcanic eruptions and other underwater explosions, landslides, large meteorite impacts and testing with nuclear weapons at sea all have the potential to generate a tsunami. The effects of a tsunami can range from unnoticeable to devastating. The term tsunami comes from the Japanese words meaning harbor (“tsu”,) and wave (“nami”,). The term is created by fishermen who returned to port to find the area surrounding their harbor devastated, although they had not been aware of any wave in the open water. Tsunamis are common throughout Japanese history; approximately 195 events in Japan have been recorded.

A tsunami has a much smaller amplitude offshore, and a very long wavelength (often hundreds of kilometers long), which is why they generally pass unnoticed at sea, forming only a passing “hump” in the ocean (Fig. 15.1). Tsunami have been historically referred to as tidal waves because as they approach land, they take on the characteristics of a violent onrushing tide rather than the sort of cresting waves that are formed by wind action upon the ocean (Iwan, W.D., 2006).

14.1 Introduction

An oil spill (or slick) is the intentional or unintentional release of oil into the natural environment as a result of human activity. The term often refers to marine oil spills where oil is released into the ocean or coastal waters (Fig. 14.1).

Oil can refer to many different materials, including crude oil, refined petroleum products or by-products, oily refuse, oil mixed in waste, or oily ballast. Oil is also released into the environment from natural geologic seeps on the seafloor, as along the coastline. The fate, behavior, and environmental effects of spilled oil can vary depending upon the type and amount of material spilled. In general, lighter refined petroleum products such as diesel and gasoline are more likely to mix in the water column and are more toxic to marine life, but tend to evaporate relatively quickly and do not persist long in the environment. Heavier crude or fuel oil, while of less immediate toxicity, can remain on the water surface or stranded on the shoreline for much longer. Oil from the Exxon Valdez and Gulf War oil spills, while weathering over time, has persisted along the shoreline for years after the spill.

13.1 Introduction

Eutrophication, means an increase in chemical nutrients — typically compounds containing nitrogen and phosphorus in an ecosystem. It may occur on land or in water. The term is however often used to mean the resultant increase in the ecosystem’s primary productivity (Fig. 13.1), in other words excessive plant growth and decay and even further impacts, including lack of oxygen and severe reductions in water quality and in fish and other animal populations.

12.1 Introduction-Nitrogen

Nitrogen is a chemical element which has the symbol N and atomic number 7. Elemental nitrogen is a colourless, odourless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78.1% by volume of Earth’s atmosphere. Nitrogen is a constituent element of all living tissues and amino acids. Many industrial important compounds, such as ammonia, nitric acid, and cyanides are made up of nitrogen.

12.1.1 History

Nitrogen (Latin nitrogenium, where nitrum (from Greek nitron) means “native soda”and genes means “forming”) is formally considered to have been discovered by Daniel Rutherford in 1772, who called it noxious air or fixed air. That there is a fraction of air that did not support combustion is well known to the late 18th century.

Argon was discovered when it is noticed that nitrogen from air is not identical to nitrogen from chemical reactions. Compounds of nitrogen were known in the Middle Ages. The alchemists knew nitric acid as aqua fortis (strong water). The mixture of nitric and hydrochloric acids is known as aqua regia (royal water), celebrated for its ability to dissolve gold (the king of metals). The earliest industrial and agricultural applications of nitrogen compounds involved uses in the form of saltpeter (sodium- or potassium nitrate), notably in gunpowder, and much later, as fertilizer, and later still, as a chemical feedstock.

11.1 Introduction

Salinity is the saltiness or dissolved salt content of a body of water. The technical term for saltiness in the ocean is halinity, from the fact that halides, chloride specifically are the most abundant anions in the mix of dissolved elements. In oceanography, it has been traditional to express halinity not as percent, but as parts per thousand (ppt ), which is approximately grams of salt per liter of solution. In other disciplines chemical analyses of solutions, and thus salinity is frequently reported in mg/L or ppm (parts per million). Prior to 1978, salinity or halinity was expressed as % usually based on the electrical conductivity ratio of the sample to “Copenhagen water”, an artificial sea water manufactured to serve as a world “standard”(Lewis, E.L 1980). In 1978, oceanographers redefined salinity in Practical Salinity Units (psu): the conductivity ratio of a sea water sample to a standard KCl solution (UNESCO, 1981a) (UNESCO, 1981b). Ratios have no units, so it is not the case that 35 psu exactly equals 35 grams of salt per litre of solution (UNESCO, 1985).

These seemingly esoteric approaches to measuring and reporting salt concentrations may appear to obscure their practical use; but it must be remembered that salinity is the sum weight of many different elements within a given volume of water. It has always been the case that to get a precise salinity as a concentration and convert this to an amount of substance required knowing much more about the sample and the measurement than just the weight of the solids upon evaporation. For example, volume is influenced by water temperature; and the composition of the salts is not a constant. Saline waters from inland seas can have a composition that differs from that of the ocean. For the latter reason, these waters are termed saline as differentiated from ocean waters, where the term haline applies.

10.1 Introduction

Biochemical Oxygen Demand (BOD) is a chemical procedure for determining how fast biological organisms use up oxygen in a body of water. It is usually performed over a 5 day period at 20 C. It is used in water quality management and assessment, ecology and environmental science. BOD is not an accurate quantitative test, although it could be considered as an indication of the quality of a water source.

10.2 Typical BOD values

Most pristine rivers will have a 5-day BOD of less than 1 mg/l. Moderately polluted rivers may have a BOD value in the range of 2 to 8 mg/l. Municipal sewage that is efficiently treated by a three stage process would have a value of about 20 mg/l. Untreated sewage varies, but averages around 600 mg/l in India and as low as 200 mg/l in the U.S., or where there is severe groundwater or surface water infiltration. Slurry from dairy farms is around 8000 mg/l and silage liquor around 60000 mg/l.

9.1. Introduction

Oceans (from Okeanos in Greek) are saline waters that cover almost three quarters (71%) of the surface of the Earth. The area of the oceans is 361 million sq. km., and nearly half of the world’s marine waters are over 3,000 meters deep.

Though somewhat arbitrarily divided into several “separate” oceans, these oceans are in fact one global, interconnected body of salt water, often called the World Ocean. The major divisions are defined in part by the continents and a variety of archipelagos, and are labeled the Pacific Ocean, the Atlantic Ocean, the Indian Ocean, the Southern Ocean, and the Arctic Ocean. Smaller regions of the oceans are called seas, gulfs and straits.

There are a some smaller bodies of salt water that are not interconnected with the World Ocean. These are not considered to be oceans or parts of oceans, though some of them have been given the name, sea.

Geologically, an ocean is an area of oceanic crust covered by water. Oceanic crust is the thin layer of solidified volcanic basalt that covers the Earth’s mantle where there are no continents. From this point of view, there are three “oceans” today: the World Ocean, and the Black and Caspian Seas that were formed by the collision of Cimmeria with Laurasia. The Mediterranean Sea is very nearly its own “ocean”, being connected to the World Ocean through the Strait of Gibraltar, and indeed several times over the last few million years movement of the African Continent has closed the strait off entirely, making the Mediterranean a fourth “ocean”.

8.1 Introduction

A lake is a body of water or other liquid of considerable size surrounded entirely by land (Fig. 8.1). A vast majority of lakes on Earth are fresh water, and most lie in the Northern Hemisphere at higher latitudes. In ecology, the environment of a lake is referred to as lacustrine.

Fig 8.1 A Typical Lakes

Large lakes are occasionally referred to as “inland seas” and small seas are occasionally referred to as lakes. Smaller lakes tend to put the word “lake” after the name, as in Green Lake, while larger lakes often invert the word order, as in Lake Ontario, at least in North America. The term lake is also used to describe a feature such as Lake Dal, which is a wet basin most of the time but may become filled under seasonal conditions of heavy rainfall and snowfall. Many lakes are artificial and are constructed for hydro electric power supply, recreational purposes, industrial use, agricultural use, or domestic water supply.

7.1 Introduction

The coast is one of the most important boundaries in the world. It separates the marine part of the world from the continents. It separates the salt-water communities from fresh water communities; it separates land life from water borne life but amphibians living both environments (Fig. 7.1).

Some of the marine organisms move towards estuary for reproduction with less water salinity. The coastline as such is not a stable. It fluctuates in place over considerable distances due to erosion and sedimentation, but also due to the relative change of sea level with respect to land. The coastal zone is thus a very mobile part of the earth from a geographical point of view. Due to its specific geophysical properties, the coast is also a very important area for the development of flora and fauna. There is hardly any other part of the world that houses such a variety of species, both in flora and fauna.

The abundance of opportunities along the coast has also attracted a lot of human activities. The coastal zone has become an indispensable resource for mankind. It provides excellent opportunities for fisheries, housing, recreation, transport, defense purpose, industrial activities and many others. The influx of man with their social and economic activities, their needs for safe housing, food, drinking water, and with the tremendous quantities of waste they are producing poses at the same time a serious threat to the unique features of the coastal zone. It is therefore necessary to study this valuable region so as to prevent that competing activities are gradually causing depletion of the resources in the coastline. The following figures (Fig. 7.1and 7.2) show the typical coastal area of rocky coast and sandy coast.

6.1. Introduction

A river is a large natural waterway. The source of a river may be a lake, a spring, or a collection of small streams, known as headwaters. From their source, all rivers flow downhill, typically terminating in the ocean. The mouth, or lower end, of a river is known as its base level.

A river water is confined to a channel, made up of a stream bed between banks. Most rainfall on land passes through a river on its way to the ocean. Smaller side streams that join a river are tributaries. The scientific term for any flowing natural waterway is a stream; so in technical language, the term river is just a shorthand way to refer to a large stream. Rivers throughout history have been the source and support for civilization, and many major cities today are near a river of some sort (Fig. 6.1).

6.1.1 Topography

A river conducts water by constantly flowing from a higher to a lower elevation, thereby converting the potential energy of the water into kinetic energy. It starts to form loops and snakes through the plain by eroding the river banks. Sometimes the river will cut off a loop, shortening the channel and forming an oxbow lake from the cut off section. Rivers that carry large amounts of sediment may develop conspicuous deltas at their mouths, if conditions permit. Rivers, whose mouths are in saline tidal waters, may form estuaries. River mouths may also be rias.

5.1 Introduction

The water quality modeling methodologies are started from conceptual models such as defining the problem in a particular region of interest like fresh water ecosystem, coastal ecosystems, ocean ecosystems, the following methodology can be studied carefully for water quality modeling.

Identification of problems in the region of interest

Monitoring of field data using standard methods like APHA, USGS, BIS etc.

Evaluation of primary data for the toxins, higher amount of concentration from the ambient conditions

Processing of primary data to secondary data

Based on the primary data and secondary data, suitable modeling software can be used. If the temperature of the region is higher than the ambient, it should be used Qual2E, MIKE21 etc.

Modeling results can be compared with field data for calibration and validation.

4.1 Introduction

The developed or commercially available models can be applied for any water bodies such as fresh water lakes, ponds, rivers, estuaries and marine ecosystems. The predicted results can be compared with proved with field data and existing approved models and applies to the other such ecosystems. In this chapter, we can discuss about modeling of water bodies, water quality model, spatial discretizafion, modeling pollutants and toxicants, concentration modeling, hydrodynamic mixing zone analysis.

4.2 Modeling of Water bodies

The need for predictive water quality modeling has arisen largely as a result of increased eutrophication of lakes throughout the world (Forsberg, 1987; Canfield and Hoyer, 1988). The most common modelling approach is exemplified by the development and application of steady state, input-output models. Generally, nutrient concentrations are calculated from net inputs and chlorophyll a concentration is predicted by correlation with the limiting nutrient, most often phosphorus (Dillon and Rigler, 1975; Canfield and Bachmann, 1981; OECD, 1982).

Factors that can also influence phytoplankton biomass, such as light, climate, biological interactions and internal loading of nutrients, are not considered. Further more, the assumption that a lake is a continuously mixed system is very restrictive and only applicable at discrete times of the year. As a result, the shortcomings of such approaches include an inability to make predictions in the face of varying physical and biological conditions, and a failure to offer insights into the determinants of changing water quality.

3.1 Introduction

By the end of the twentieth century one of our most important scientific accomplishments has been the explicit realization that the human population is changing the earth system. Some of these changes are occurring at a remarkable rate. We know, with certainty, that human activities are changing land use, habitats, the chemistry of the Earth’s atmosphere and water, rates and balance of biogeochemical processes, and diversity of life on the planet (Vitousek et al. 1997). One prominent mode of human disturbance has resulted from activities that mobilize the nutrient elements nitrogen and phosphorus through land clearing, production and applications of fertilizer, discharge of human waste, animal production, and combustion of fossil fuels (Nixon 1995).

As a result of these activities, surface waters and ground waters throughout the world now have elevated concentrations of N and P compared to concentrations even in the middle of the 20th century. Our mobilization of N and P has accelerated the fluxes of these elements to coastal waters, and fertilization of coastal ecosystems is now a serious environmental problem because it stimulates plant growth and disrupts the balance between the production and metabolism of organic matter in the coastal zone. Coastal eutrophication is a recently recognized phenomenon (Nixon 1995), and scientific investigation of this human disturbance has progressed for only a few decades, so our conceptual model of the problem is evolving rapidly. Here, I describe 2 phases in the evolution of our conceptual model of how nutrient enrichment disrupts coastal ecosystems, and then finish with a view to the future, suggesting critical questions that must be answered as we work to advance and broaden our understanding of this complex environmental problem. This examination of conceptual models is designed as a complement to the excellent general reviews of coastal eutrophication (Vollenweider 1992, Heip 1995, Nixon 1995, Jørgensen & Richardson 1996, National Research Council 2000).

2.1 Historical Development

We have seen in Chapter 1 that modeling is about establishing cause effect relationship. Perhaps one of the earliest attempts in this direction is made as early as 1848 in Germany where it was tried to establish a relation between purity and pollution to the occurrence of certain biological organism. But as far as water quality modeling is concerned the first major breakthrough in this direction is the effort of Streetor and Phelps (1925) in oxygen sag curve. They developed model for estimating DO and BOD in Ohio River. The work of O’Conner (1962) for bacteria models are major milestone in pre-computer era modeling.

With the advent of computer during 1960s complicated kinetics and time variable simulation came into being. Most of the workers presented their efforts in terms of linear programming based water management models concerning waste discharge. Thomann(1963) has formulated linear programming based models for dissolved oxygen in an estuary. So a stage is set for study of eutrophication through modeling, but the problem is determination of values of various parameters in these equations.

During 1970s models expanded in terms of complexities mainly because of enhanced computational facilities. Two dimensional and dynamic framework has started replacing one dimensional steady state configurations. In the same year, Di Toro presented an analysis of the steady state two dimensional concentration distribution that results in the horizontal plane from the continuous discharge of a non-conservative substances (DO and BOD) into an infinite body of water.

A model is something that mimics the behavior of some real system of interest. Our interest here will be in water quality models, which are usually equations, and which predict some result based on factors that influence that result. A water quality model is a representation of a real system. The essence of a good water quality model is that it is simple in design and exhibits the basic properties of the real system that we are attempting to understand. The model should be testable against empirical data. The comparisons of the model to the real system should ideally lead to improved mathematical models. The model may suggest improved experiments to highlight a particular aspect of the problem, which in turn may improve the collection of data. Thus, modeling itself is an evolutionary process, which continues toward learning more about certain processes rather than finding an absolute reality.

Water pollution is a large set of adverse effects upon water bodies such as lakes, rivers, oceans, groundwater caused by human activities. Although natural phenomena such as volcanoes, storms, earthquakes etc. also cause major changes in water quality and the ecological status of water while these are not deemed to be pollution. Water pollution has many causes and characteristics. Increases in nutrient loading may lead to eutrophication. Organic wastes such as domestic sewage and farm waste impose high oxygen demands on the receiving water leading to oxygen depletion with potentially severe impacts on the whole ecosystem. Industrial discharge a variety of pollutants in their wastewater including heavy metals, organic toxins, oils, nutrients, and solids. Discharges can also have thermal effects, especially those from thermal and nuclear power stations, and these too reduce the available oxygen. Siltbearing runoff from many activities including construction sites, forestry and farms can inhibit the penetration of sunlight through the water column restricting photosynthesis and causing blanketing of the lake or river bed which in turns damages the ecosystems the process is called as eutrophication.

Pollutants in water include a wide spectrum of chemicals, pathogens, and physical chemistry or sensory changes. Many of the chemical substances are toxic or even carcinogenic. Pathogens can obviously produce waterborne diseases in either human or animal hosts. Alteration of water’s physical chemistry include acidity, conductivity, temperature, and excessive nutrient loading. Even many of the municipal water supplies in developing countries can present health risks leads to many diseases.

During the last decade organic agriculture has emerged as a dynamic alternate farming system. This has been necessitated as a consequence of resource degradation because of to achieving high yields in various crops to meet the increasing demand of food by the burgeoning population. These have resulted in change in soil structure, imbalance in soil micro-organisms and organic matter, resistance among various weeds to weedicides, increased salinity, sodicity and have affected the water holding capacity of the soil and brought in health and environmental problems. Organic farming is a specified form of diversified agriculture that not only aims to produce nutritious food of high quality but also lead to the generation of more income and employment opportunities for rural population. In true sense, organic farming is a production system which avoids or largely excludes the use of synthetic fertilizers, growth regulators, livestock feed additives and pesticides. Moreover, it relies on agronomic management practices like crop rotation, crop residues, animal manures, legumes, green manures, off-farming organic wastes and aspect of biological pest control to maintain soil productivity.

The inability of Indian agriculture to meet the demand for food in the country during the two and half decades immediately after independence had been a matter of concern at those times. The system of our agriculture based on the traditional knowledge and practices handed down from generation to generation could not produce enough to feed the increasing population. The ignominy of our dependence for food on the western developed nations and the politics of food aid practiced by them added to our determination to be self-sufficient in food production by modernizing agriculture. The green revolution fulfilled our aspirations by changing India from a food importing to a food exporting nation. However, the achievement was at the expense of ecology and environment and to the detriment of the well-being of the people.

Soil conservation in hilly areas

Soil erosion is a major problem on hills because of steep slopes. It has been estimated that about 6 million ha of land requires intensive conservation measures in Himalayan and sub-Himalayan regions of Uttar Pradesh, Uttarakhand, Punjab and Himachal Pradesh. Anthropogenic activities like shifting cultivation, deforestation, over grazing accelerate soil erosion in hilly ecosystem.

Trees are an integral and indispensable constituents of our natural surroundings. The trees keep the environment clean, make the soil healthy and provide food, fibre and shelter to mankind and wild animals. Trees help in stabilizing the soil, a vital natural resource, prevention of soil and water erosion, control run off water in the catchment areas, acts as a shelter belt against wind erosion in arid zone, act as a wind breaker to protect the coastal region during storm and cyclone besides contributing substantially to economic development. The National Commission on Agriculture stress the need for afforestation of about 40 million ha of land presently lying barren because of salinity, sodicity, waterloggings, ravines, coastal sandy areas, stony and gravelly lands, high altitude and slopping lands. Several agencies in various parts of the country are engaged in afforestation of these degraded lands.

Sustainable management of soil and water resources is essential for food, nutritional and environmental security of the country. The total and partial factor productivity of agricultures in India declined over years because of continued degradation of natural resources. The process leading to land degradation are generally triggered by excessive use of land and water resources to meet the growing demand of population for food, fibre and fodder. Various anthropogenic activities leading to accelerated land degradation through salinization, flooding, water logging, erosion of water and nutrients, drought reduced agricultural productivity and nutritional security in the country. These problems can be overcome by optimum utilization of land adopting suitable management practices. For effective land management practices, the soils should be interpreted in forms of their land capability, suitability for irrigation and crop growth. This will not only help the farming community and extension specialists, but also to the administrators and policy makers for using soils for sustainable agricultural production.

Sustainable management of soil resources is essential for food, nutritional and economic security of the country. The continued degradation of soil resources is considered an important factor in lowering the total and partial factor productivity of agriculture in the country. The process like soil erosion, nutrient loss, water logging, desertification, acidification, compaction, crusting, salinization and soil contamination affect food and environmental security in most parts of the country. The future agricultural growth at higher rate cannot be sustained with a deteriorating soil resource base. We need to monitor the quality of soil resources and devise appropriate management strategies for sustainable crop production without deteriorating the environment.

In India, several agencies like National Commission on Agriculture (NCA), Ministry of Agriculture (Soil and Water conservation Division), Society for Promotion of Waste lands Development (SPWD), NBSS&LUP, Nagpur have been constantly inventorizing and characterizing soil resource for drawing up land use and development plans at National and State levels. However, there is a great variance in estimates due to differences in approaches and criteria for assessment. For an example,the total degraded land as estimated by NCA is 148.09 Mha as against, 175.0 Mha by Ministry of Agriculture (Soil and water conservation Division), 129.58 Mha by SPWD and 187.7 Mha by NBSS&LUP, Nagpur.

In soil profile development, we focused on vertical variability in soils - the differences among soil horizons. In this chapter we focused on horizontal variability- how soils differ from place to place across the landscape. Spatial variations in soil properties occurs across distances having geographic meaning for land management-from a few meters to many kilometers. We need to monitor regularly the status of our soil resources employing modern technologies to refine our understanding about them and renew our commitment to their preservation. The applications of Remote Sensing (RS), Geographical Information Systems (GIS) and Global Positioning System (GPS) have greater opportunities to offer in this regard.

Soil health

Soil health is defined as the continued capacity of soil to function as a vital living system, by recognizing that it contains biological elements that are keys to ecosystem function within land use-boundaries. These functions are able to sustain biological activity of soil, maintain the quality of air and water as well as promote plant, animal and human health.

The characteristics of a healthy soil depends on the physical, chemical and biological properties of soils which directly or indirectly influence crop production.

Soil quality and soil health are terms that are discussed in agricultural community. Both these terms encompass a complex, dynamic concept, different to define and hard to measure. The term soil quality and soil health are often used interchangeably in the scientific literature, with scientists in general preferring soil quality and producers preferring soil health. Soil health may be considered as the state of soil at a particular time equivalent to the dynamic properties in short term. In contrast, soil quality may be considered for a particular purpose over a longer time scale, equivalent to static soil quality. In a broader way soil quality may refer to both permanent and static soil properties. (Fig. 11.1).

A static soil property relates to natural characteristic of soil (texture) which are derived from pedogenic processes. These properties cannot be easily amended. The dynamic properties are readily related to management practices. The dynamic component is of most important for the growers because it allows the soil to come to its full potential.

Pollution

Pollution is an undesirable change in physical, chemical or biological characteristics of air, water or land caused by excessive accumulation of pollutants (substances causing pollution).

Kind of pollution

Pollution can be classified in many ways:

In India rice is cultivated under two land situations. In uplands, rice is grown under rainfed conditions and the yield is normally low. In medium or low land conditions rice is grown under wet condition where plenty of rain water or irrigation water is available. Here an intensive tillage is practiced in wet to create a soft puddle condition and then paddy is transplanted. Land submergence is thus an essential condition for successful rice cultivation.

Rice soils are not necessarily soils with water at or near the surface. Many soils have a fairly deep water table, but their surface is kept under submergence maintaining depth of 3-5 cm water. This condition is maintained either by puddling when wet or by having a compacted layer in the sub surface.

A rice soil is characterized by reducing conditions, when it is cropped with rice during wet season and oxidizing conditions during dry season, when the land remained fallow or planted with dry season crops if, irrigation water is available.

Water used for irrigation contain some salts, but quantities and kinds vary greatly depending on the source of water. Since sodic status of soils can be greatly influenced by salts in irrigation waters, successful irrigation practices require the evaluation system to judge irrigation water quality and methods employed. The actual damage done by salt to plants or soils depends on the concentration in soil solution rather than the quality of irrigation water applied. Thus, the use of some water source for irrigation might lead to severe salt problem in soils where drainage is restricted.

Criteria for evaluation of irrigation water

Various criteria considered in evaluating the quality of irrigation water are:

1. Salinity hazard

The most satisfactory method for rating the salt content of irrigation water involves measuring electrical conductivity. Various conductivity limits have been proposed to indicate the degree of salinity problems that might be anticipated with use of irrigation water on crops. There are four classes of salinity hazard such as C₁, C₂, C₃, and C₄.

Saline and alkali soils occur commonly under arid climate due either to the presence of an excess of soluble sodium salts or to the predominance of sodium among the exchangeable bases.

Saline and alkali soils have been classified as follows

1. Saline soils

Saline soils were originally called “White alkali” soils. Russians term them as “Solonchak” soils.

The soils are classified as saline if the electrical conductivity of saturated soil paste is more than 4 dS /m at 25⁰C, exchangeable sodium is less than 15 percent and pH is less than 8.5.

2. Alkali soils

The soils have been classified as alkali if the electrical conductivity of saturated soil paste is less than 4 dS/m, exchangeable sodium is more than 15 percent and pH is 8.5 to 10.0.

Acid soils occupy about 30 percent of the land area of the world out of which 67% acid soils support forests and wood lands and 18 percent are covered by grassland vegetation. Only 4.2 per cent (179 million hectare) of the acid soils across the globe is used for arable crops.

Soil acidity is a major constraint in crop production. The removal of basic cations especially Ca and Mg by leaching and their replacement by acidic cations, H, Al and Fe on exchange complex leads to soil acidity. Soil developed from granite parent material are more acidic than those developed from basalt, shale or lime stone. In acid soils, low pH leading to toxic concentration of H⁺, Al³⁺, Mn²⁺ influences nutrient availability, organic matter decomposition and microbial activity.

Efficient soil amelioration and nutrient management practices are necessary for crops susceptible to soil acidity.

Soil erosion refers to detachment and transport of soil and soil particles from one place to other by the action of water, wind or gravity. Soil erosion consists of (i) detachment of soil particles from surface (ii) their transport from one place to other and (iii) deposition in another place.

Soil erosion are of two types: Natural erosion and accelerated erosion. The natural or geological erosion is steady, nondestructive and slow process of nature. On the contrary the accelerated erosion caused due to human activities like cutting of forest, intensive cropping, construction of roads and building is very rapid and destructive. Surface one inch of soil which is formed in thousand years can be lost within one year due to accelerated erosion.

In India, out of total 175 million hectares of degraded land, soil erosion due to water and wind accounts for 150 million hectare. Soil erosion due to shifting cultivation, salinity, alkalinity, water logging and shifting of water courses account for 25 million hectare.

Average soil loss from various land surfaces in India has been estimated to be 5334 million tonnes per year out of which 29 percent is permanently lost to sea, 10 percent is deposited in reservoirs and 61 percent is transported from one place to other. The plant nutrients lost through 5334 million tonnes of eroded soils was estimated to be 8.4 million tonnes which includes 2.5 million tonnes of N, 3.3 million tonnes of P and 2.6 million tonnes of K.

Siltation of reservoirs through water erosion is a major concern in India. This accelerated silting has considerably reduced the estimated life span of major reservoirs. The rate of siltation depends on the catchment area, soil erodability, topography and rainfall pattern. Larger the catchment area, smaller is the siltation load in the reservoirs because, the coarse fraction of eroded particles may be settle down in the stream valley before the water enters into the reservoir.

Iron Toxic Soils

Lowland rice is being cultivated on approximately 128 million hectares of either irrigated or rainfed land out of which 100 million hectares suffer due to nutrient deficiency or toxicities. Among the toxicities, iron toxicity is well recognized as a nutritional disorder in low land rice production. Iron toxicity is a syndrome of disorder associated with high concentration of Fe²⁺ in soil solution. A wide range of soil types show iron toxicity problem including acid sulphate soils, acid clay soils, peat soil and valley bottom soil. During monsoon rains in the wet season, soluble iron (Fe²⁺) migrate to the surface and its iron content increases to subtoxic or toxic levels.

In rolling topography and with heavy monsoon downpour there is considerable lateral flow of iron containing water from the adjacent uplands to mid – and low lands. This increases the iron content to toxic levels. Poorly drained sandy soils in valleys receiving interflow water from adjacent uplands with laterite horizon (Plinthudults) showed iron toxicity in Srilanka and Kerala and Odisha in India. These soil groups come under the taxonomic orders Alfisols, Ultisols and Oxisols occurring in association with Entisols and Inceptisols. The iron concentration (Fe²⁺) in soil solution that affect low land rice yield ranges from 10 to > 2000 mg L^-1 Iron toxicity induced yield reduction which is associated with poor nutrients . status (deficiency of P, K and Zn) and H₂S toxicities.

Calcareous soils

In arid regions, calcareous soils are developed from limestone or marl parent materials. The content of calcium and magnesium carbonate may be as high as 60 to 70 per cent of the total soil mass. In calcareous soil lime content may vary from a small amount in some places in the profile to appreciable amounts throughout the profile.

Calcium carbonate coatings occur in sand, silt and clay size fractions of calcareous soils. In sub soil, large amounts of lime forms hardpan under high water tables.

In general, pH of the soil increases to alkaline pH with increasing the proportion of water to soil ratio. This may happen in part from a reduction of CO₂ pressure. The dilution of salts in soil solution also permits greater hydrolysis of calcium clay. The dilution of calcium carbonate with production of hydroxyl ions may lead to higher pH as: