Essay on Soil: Introduction and Formation

In this essay we will discuss about soil. After reading this essay you will learn about: 1. Introduction to Soil 2. Components of Soil 3. Functions 4. Microclimate 5. Formation of Soil 6. Types of Pollutants 7. Fertiliser Contamination 8. Greenhouse Effect and Soil Fertility 9. Weathering 10. The Soil Profile 11. Textural Classes 12. Cation Exchange Capacity 13. Soil Reaction 14. Economy of Essential Elements.

Essay on Soil

Essay Contents:

1. Essay on Introduction to Soil
2. Essay on the Components of Soil
3. Essay on the Functions of Soil
4. Essay on the Microclimate of Soil
5. Essay on the Formation of Soil 
6. Essay on the Types of Pollutants of Soil
7. Essay on Fertiliser Contamination
8. Essay on Greenhouse Effect and Soil Fertility
9. Essay on the Weathering of Soil
10. Essay on The Soil Profile
11. Essay on the Textural Classes of Soil
12. Essay on the Cation Exchange Capacity of Soil
13. Essay on Soil Reaction
14. Essay on the Economy of Essential Elements of Soil

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Essay # 1. Introduction to Soil: (500 Words)

Soils form a narrow interface between the atmo­sphere and the lithosphere and possess elements of both: water, a gaseous phase and mineral mat­ter, together with a diverse range of organisms and materials of biological origin.

They continu­ally interact with the atmosphere above and the lithosphere beneath. Soils are that part of the earth’s thin surface ‘rind’ within which organic ma­terials are broken down to form stable humic compounds thereby releasing their contained nu­trient elements for uptake by organisms and dissi­pating their contained energy.

This veneer of ma­terials supports the growth of higher plants and therefore the primary production on which the human population directly depends. Soils provide other important services including the stabilisation of waste materials and part of the excess CO₂ re­leased to the atmosphere by human activities.

The thin veneer of soil is readily damaged or lost by misuse. Following such effects, it does not reform in any major sense within the time frames of human existence and must be considered a non-renewable resource.

Many current systems of agricultural management are not sustainable in the longer term because of the pressures they place on the soil. Production levels may frequently be set on the basis of economic goals rather than the capacity of the soil to withstand particular stresses.

Most cropping systems, for example, require sub­stantial regular inputs of energy and nutrient ele­ments and the sustainability of their use of this purpose is contingent upon continuing inputs. Similar principles apply in other situations. Con­tinued overgrazing in pastoral environments situ­ations soon leads to soil degradation and loss and stresses imposed by chemical contamination may eventually result in impaired functioning.

Faced with such pressures, soils are clearly a threatened resource. From this, one of the emerg­ing challenges facing soil ecologists is the mainte­nance or amelioration of soil fertility in the devel­opment of long-term sustainable agricultural sys­tems. This requires the integration of biological process knowledge into general models of soil functioning and the design of land management systems based on such models.

An understanding of soil functioning and the definition of appropriate management options demand a knowledge of the processes operating in both the above and below ground subsystems and identification and quantification of fluxes of energy and materials between and within them. The maintenance of the physical integrity and fer­tility of soils depends largely on these transfers.

The soil is a unique environment that com­bines solid, liquid and gaseous phases to form a three-dimensional matrix. The organisms that in­habit this porous, humid and amphibious environ­ment face quite specific constraints due to:

(i) The predominance of poor-quality food resources for species at the lower trophic levels;

(ii) Spatial con­straints in an environment where most organism live within pores that differ broadly in size, shape and inter-connectedness; and

(iii) The rapid alter­nation, in both space and time, of air and water- filled porosities. The adaptive strategies evolved under such conditions are certainly different from those of their above ground counterparts.

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Essay # 2. Components of Soil: (500 Words)

Soil components may be classified in many ways, depending on the intended purpose of the classi­fication. Some common ways of classifying such materials are on their sizes, shapes and origins, the phases they belong to, their chemical or physical characteristics, their mineralogical compositions or on combinations of these.

The classification of soil components employed here is arbitrary and hierarchical and attempts to characterize the indi­vidual components in ways that reflect their eco­logical interrelationships (Fig. 10.1).

The division of soil materials into separate components in no way implies that associations and interactions between the various components do not occur, or are unimportant. Indeed, nearly all-solid and liquid phase entities that occur within soils include both organic and inorganic compo­nents, or are influenced by them.

The formation and functioning of soil depends on the myriad interactions that occur between the organic and inorganic, and the living and non-living components of soils. An enormous variety of organic and in-organic components are available within soils to interact in the synthesis and development of the unique substance which is soil.

Table 10.1 presents the indicative value of concentrations of selected major and minor ele­ments in surface soils together with the ranges that may be expected to occur in areas where mineralization has occurred or which have been polluted by human activities.

The most important of the inorganic com­ponents in terms of soil behaviour is undoubt­edly the highly-diverse group of substances characterised generally as clays. The term clay may refer to three distinct entities. In the textural sense, clay refers to the important colloidal particles less than 0.002 mm (2 µm) in diameter.

More loosely, it may also refer to a class of soils with a high proportion of such particles. It may also be ap­plied to the phyllosilicate clay minerals discussed below. Here, the term clay will refer to clay-sized particles unless otherwise qualified.

The phyllosilicate and other mineral particles in the clay size range have large surface areas relative to their masses and, in soils with appreciable clay contents, they control many reactions important to biologi­cal processes. Surface area is also closely corre­lated with a range of other properties that regu­late the physical and chemical characteristics of the soil mass and influence plant growth.

Organic materials found in soils may be di­vided into the living organisms and non-living materials of biological origin. The later comprise a diversity of materials including roots and other plant and animal remains in all stages of subdivision and decomposition.

In addition, dead fungal hyphae, spores, bacteria and other larger constructs of microbial (e.g., the sporocarps of mycorrhizal fungi) and faunal (e.g., termitaria) origin are frequently present. During the later stages of decomposition of plant materials, less resistant materials are lost leaving only cell wall outlines and fragments which, in the terminal stages of decomposition, may appear amorphous under microscopic examination.

Other directly-derived biological materials include such diverse products as plant root lysates and exudates, the faeces, excreta and secreta of animals, the cutane­ous mucus secreted by earthworms and gels pro­duced by microorganisms.

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Essay # 3. Functions of Soil: (400 Words)

i. Mechanical Support:

The role of the soil in providing a mechanical support for plants is clear.

All the rooted plants get such support from soil in varying degree to resist the wind pressure on aerial parts of plant.

ii. Habitat Provision:

The vast majority of soil organisms are too small and too weak to cre­ate their own habitats and these latter must exist within the soil to sustain the organisms essential to its functioning. Small organisms must live within existing voids formed through physical processes or through the activities of such larger organisms as plant roots and soil-dwelling invertebrates and vertebrates.

In par­ticular, decomposing roots comprise an im­portant habitat for many soil organisms.

iii. Storage of Organic Matter:

The soil is an important store of dead organic materials present in all stages of decomposition from freshly-fallen litter and recently-dead plant roots to highly humified materials of great age and chemical complexity.

Dead biological materials are important energy sources for many soil organisms and the more decom­posed fractions interact with inorganic mate­rials to form organo-mineral structures cen­tral to the organisation and stability of the soil matrix.

iv. Element Release:

In addition to such ele­ments as silicon and aluminium which nor­mally dominate the soil mass, the soil also con­tains stores of elements that are of biological and pedological importance. Certain forms of such elements as iron, aluminium and silicon are important in both these respects.

These and other elements may he held within the organic materials (living and dead) considered above but are also present in the soil solution, retained at and near the surfaces of the or­ganic and inorganic soil colloids and, in less accessible forms, within the mineral soil par­ticles.

Decomposition of organic materials lib­erates the contained elements in inorganic forms (mineralization) in a controlled or ‘slow-release’ way for uptake by plant roots and other soils organisms, or for involvement in ped­ological processes.

v. Water Storage:

Soils possess a store of water that supports the growth of plants and other organisms. The magnitude of this store dif­fers substantially between soils depending on soil depth, the size distribution and organisation of the soil particles and location in the landscape.

Within soils, plants may have access to stores of different sizes, depending on their individual capacities to extract water from the smaller pores and on such factors as their rooting depths, mycorrhizal associations and salt tolerances.

Other soil organisms also have their own characteristic physiological tol­erance ranges beyond which they become in­active, or die. By and large, soil harbour a variety of organ­isms which can be categorized as producer, con­sumer and decomposer.

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Essay # 4. Microclimate of Soil: (300 Words)

The microclimate of the soil is defined in terms of its internal temperature and hydrological re­gimes and is therefore generally determined by the external climate. Since this varies broadly with lati­tude, elevation, rainfall distribution and, to a lesser degree, with vegetation type and cover, aspect and a range of other factors, it is clear that soil micro­climates are likely to differ almost as widely as those of the surface, albeit increasingly buffered from rapid change with greater depth in the profile.

The soil properties that determine water en­try and movement within the soil are clearly im­portant in defining the microclimate. Soil tempera­ture regimes and the factors governing their varia­tion in time and space form the remaining part of its definition.

The temperature regimes that per­tain in soils influence many processes that occur therein and play a part in controlling the rates and processes of soil development and the composi­tion and activities of the biota. Agricultural pro­ductivity is frequently limited by either low or ex­cessively high temperatures, although both effects are often related to moisture status.

In the pedogenetic sense, water is a major agent of physi­cal weathering through expansion: contraction processes and particularly the frequency with which the 0°C boundary is crossed. The rates of many biological and chemical weathering processes are also temperature dependent.

All species have required minimum and maxi­mum temperatures for growth and survival and, in the arthropods, these may differ between de­velopment stages. However, most species usually have a somewhat narrower preferred range.

The species present in extreme environments usually possess specific adaptations to permit their survival therein. However, a minimum requirement for the persistence of most species is that tem­perature and moisture regimes be regularly within a favourable range for a sufficient period to per­mit successful reproduction and development.

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Essay # 5. Formation of Soil: (300 Words)

The formation and evolution of soils involve a series of physical, chemical and biological pro­cesses which act progressively over time, are con­trolled by climatic variables and are greatly influ­enced by topography. Simonson (1978) divides such processes into additions from without the soil system, removals or losses, translocations (or transfers) within the system and transformations of Contained materials.

The original parent material is transformed by in situ weathering into a mixture of stable mineral components which blend intimately with organic materials to form the soil. The parent material is first broken down into its primary minerals whose decomposition products may be partially trans­formed into secondary or neo-formed minerals (Table 10.2).

From this early stage, the nutrients necessary for plant production an such other essential com­ponents as A1 and Fe accumulate progressively in the upper parts of the incipient soils. Clay frac­tions are formed firstly through alteroplasmation i.e., transformation of primary minerals into clays with no subsequent modification of rock struc­ture.

Pedoplasmation is a subsequent transforma­tion whereby clay minerals acquired a pedological structure and such specific properties as swelling and shrinkage.

The initiation of biological activities within the developing substrate leads to the accumula­tion of organic matter. This organic matter mixes with the weathering mineral components to form an A horizon that becomes an active source of further physicochemical changes in the underly­ing parent material to develop a C horizon. CO₂ evolved from decomposing organic matter also participates in the process.

With further develop­ment, weathering and downward transport of materials progressively modify the deeper strata of the parent material and, depending on the pro­cesses operating, E and B horizons may form. At this stage, translocation, biological transport and erosion become the dominant processes in the evolution and differentiation of the soil profile.

Nutrient and other elements (e.g., Si and Al) and organic materials are continually lost in solu­tion and suspension through erosion and by trans­port in subsurface water flows. Such losses are expected to be greatest in incipient soils with ju­venile ecosystems and to diminish with ecosystem and soil development.

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Essay # 6. Types of Pollutants of Soil: (700 Words)

i. Pesticides:

Whether pesticides are applied to plant foliage, to the soil surface, or are incorporated into the soil, a high proportion of the chemicals eventually moves into the soil. These chemicals then move in one or more of six major directions (Fig. 10.4).

a. They may vaporize into the atmosphere with­out chemical change.

b. They may be absorbed by humus and clay par­ticles.

c. They may move downward through the soil in liquid or solution form and be lost from the soil by leaching.

d. They may undergo chemical reactions within or on the surface of the soil.

e. The may be broken down by soil microorgan­isms.

f. They may be absorbed by plants and detoxi­fied within the plants.

ii. Toxic Inorganic Compound Sources and Accumulation:

There are many sources for the inorganic chemi­cal contaminants that can accumulate in soils. The burning of fossil fuels, smelting, and other pro­cessing techniques release into the atmosphere tons of these elements, which can be carried for miles and later deposited on the vegetation and soil.

Lead, nickel, and boron are gasoline additives that are released into the atmosphere and carried to the soil through rain and snow. Borax is used as a detergent and in fertilizer, both of which com­monly reach the soil. Superphosphate and lime­stone usually contain small quantities of cadmium, copper, manganese, nickel, and zinc.

Cadmium and chromium are used in plating metals, and cadmium in the manufacture of batteries. Arsenic, for many years used as an insecticide on cotton, tobacco, and fruit crops, is still being used as a defoliant or vine killer and on lawns. Some of these elements are found as constituents in specific organic pesti­cides and in domestic and industrial sewage sludge.

The quantities of most of the products in which these inorganic contaminants are used have increased notably in recent years, enhancing the opportunity for contamination. They are present in the environment in increasing amounts and are daily ingested by people either through the air or through food and water.

The domestic and industrial sewage sludge are considered to be the major sources of potentially toxic chemicals, and at least one-third of these wastes in the United States is being applied to the soil. Sewage sludge commonly carry significant quantities of inorganic as well as organic chemi­cals that can have harmful environmental effects.

iii. Organic Waste Contamination:

Soils have long been used as disposal “sinks” for municipal refuse. “Sanitary landfills” are widely employed to dispose of a variety of wastes from our towns and cities. These wastes include paper products, garbage, and non-biodegradable mate­rials such as glass and metals.

The sites are often located in swampy lowland areas that eventually become built up by the dumping to create upland areas for such uses as city parks and other facili­ties.

iv. Soil Salinity and Irrigation:

Contamination of soils with salts is one form of soil pollution primarily agricultural in origin. Fur­thermore, it is not a new problem. Ancient civilizations in both the New and Old Worlds crumbled because salts built up in their irrigated soils. The same principles govern the management of irri­gated soils today and the same dangers exist of salt build up and concomitant soil deterioration.

Salts accumulate in soils because more salts move into the plant rooting zone than move out. This may be due to application of salt-laden irri­gation waters or it may be caused by irrigating poorly drained soils. Salts move up from the lower horizons and concentrate in the surface soil lay­ers.

v. Acid Rain Impact on Soil:

Effects of acid rain are more pronounced on the acidity of water than on soil acidity. Soils gener­ally are sufficiently buffered to accommodate acid rain with little or no increase in soil acidity on an annual basis. But continued inputs of acid rain at pHs of 4.0-4.5 would have significant effects on the pH of soils, especially those that are weakly buffered.

This is also serious for soils that are al­ready quite acid, since increased acidity could well make them even less fertile.

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Essay # 7. Fertiliser Contamination: (200 Words)

Fertiliser applications that supply nutrients in quan­tities far in excess of those taken up by plants can result in contamination of both surface and drain­age waters. Nitrates and phosphates are the chemi­cals most often involved. Nitrate contamination can occur in both surface runoff and drainage waters, while excessive levels of phosphates gen­erally occur only in surface runoff.

The loss of nitrogen and phosphorus from the soil has adverse effects on soil fertility, but the effect on water quality is even more serious. Ni­trate levels in drinking water above about 10 mg per liter are considered a human health hazard. In some heavily fertilised areas, the drainage waters are sufficiently high in nitrates to be a problem. Some rural wells have been found to contain ni­trates significantly above this safe limit.

A second problem stemming from high nu­trient-bearing waters coming from soils is the “over fertilisation” of lakes. Nitrogen and phosphorus in lake waters stimulate the growth of algae and other water-loving plants in the lakes.

Algal growth depletes the water of oxygen, which is essential for fish. Other aquatic plants (weeds) are stimu­lated and produce heavy mats near the shoreline interfering with recreational uses of the lakes.

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Essay # 8. Greenhouse Effects and Soil Fertility: (400 Words)

Widespread concern has been expressed that the Earth is warming up. Furthermore, it is predicted that this warming trend will continue and even accelerate in the future. The cause of this warm­ing is thought to be the so-called greenhouse ef­fect.

When certain gases are emitted from the earth, they move into the upper atmosphere, capture, and return to the earth radiant heat that would ordi­narily escape into space. In so doing the gases serve the same purpose as the glass in a greenhouse.

With the advent of modern industrialisation the content of these greenhouse gases in the at­mosphere has increased markedly. For example, the carbon dioxide content of the upper atmosphere is thought to have been about 280 parts per mil­lion (ppm) in pre-industrial times.

It has increased to about 350 ppm in the past 30 years. Significant increases have also occurred in nitrogen and sulphur-bearing compounds and in organic synthetics such as the chlorofluorocarbons (CFCs) used as aerosol propellants, refrigerants, and solvents.

The soil is a source of several of the gases involved in the greenhouse phenomenon includ­ing carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄). The breakdown of soil or­ganic matter as land is cleared and put under culti­vation has been a major source of released CO₂. This loss is small however, in comparison with the much larger releases from the combustion of fos­sil fuels (petroleum and coal) and the burning of tropical forests.

Nitrous oxide (N₂O) content of the atmo­sphere has increased about 25% in this century. About two thirds of this increase is thought to be due to the combustion of coal and oil, the remain­der to agricultural practices. The N₂O is released during the process of de-nitrification, which is fuelled by the presence of nitrates in the soil and in the organic matter cover.

Although heavy nitro­gen fertilisation may be a source of part of the nitrates that undergo de-nitrification, much nitrate reduction occurs in un-fertilised areas. For example, tropical forests have been found to be a signifi­cant year-round source of N₂O.

The methane (CH₄) content of the upper at­mosphere has essentially doubled in the past hun­dred years, and there is no consensus as to the reasons for this increase. However, the soil is a known source of methane. It is released under anaerobic conditions such as are common in swamps or in a rice paddy.

Also, termites can and do produce methane, and some investigators have suggested that 20-40% of the methane reaching the atmosphere may come from this source.

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Essay # 9. Weathering of Soil: (300 Words)

Weathering is the sum of the processes involved in the alteration of materials and not the surface through complex interactions between the lithosphere, the atmosphere, the hydrosphere and the biosphere that occur over time.

It may extend far below the surface and includes all the physical and chemical processes responsible for soil fragmen­tation and the production of dissolving ions.

Weathering in the upper part of the parent mate­rials has been considered separately as pedochemical weathering because of the consid­erable influence of the biomass in the production of, inter alia, complexing agents, substances that attack clay minerals or form organo-mineral com­plexes.

However, biological weathering influences may extend well into the underlying regolith al­though increasingly attenuated with depth.

While physical weathering is often considered separately from chemical weathering, in reality they operate together, often in a synergistic way. Weath­ering involves the simultaneous activities of a range of processes including physical fragmentation, inorganic chemical processes (hydrolysis, oxida­tion, hydration and dissolution) and biologically- mediated processes (e.g., acidolysis and acidocomplexolysis).

The weathering processes predominating at a site are determined by climatic, biological and lithological factors and the degree of evolution of the soil.

In all weathering systems, water plays domi­nant physical and chemical role. In the solid phase it is a major agent of landscape sculpting and trans­port while as a liquid, it is also an important agent for the diffusion and transport of materials.

It is a potent medium of physical disruption through volume change, both as a consequence of phase change and through involvement in hydration and related reactions. Chemically, it is an effective sol­vent, a component of many reactions and of neo-formed products, and an important buffering agent. Little chemical weathering occurs in very dry and frozen environments.

Thus, studies of soil throughout the world have shown that the kinds of soil that develop are largely determined by major factors:

1. Climate (particularly temperature and precipi­tation)

2. Living organism (especially native vegetation, microbes, soil animals and human beings)

3. Nature of parent material

4. Topography of the site

5. Time that parent materials are subjected to soil formation

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Essay # 10. The Soil Profile: (400 Words)

The layering or horizon development described in the previous section gradually gives rise to natu­ral bodies called soils. Each soil is characterised by a given sequence of these horizons. A vertical ex­posure of the horizon sequence is termed a soil profile. Attention now will be given to the major horizons making up soil profiles and the termi­nology used to describe them.

For convenience in study and description, five master soil horizons are recognised. These are des­ignated using the capital letters O, A, E, B, and C. Subordinate layers or distinctions within these master horizon are designated by lowercase let­ters.

O Horizon (Organic):

The O group is com­prised of organic horizons that form above the mineral soil. They result from litter derived from dead plants and animals. O horizons usually occur in forested areas and are generally absent in grass­land regions.

A Horizon:

The A horizons are the topmost minerals horizons. They contain a strong mixture of partially decomposed (humified) organic mat­ter, which tends to impart a darker color than that of the lower horizons.

E Horizon:

E horizons are those of maxi­mum leaching or eluviation’s of clay, iron and aluminium oxides which leaves a concentration of resistant mineral, such as quartz, in sand and silt sizes. E horizon is generally lighter in color and is found under the A horizon.

B Horizon:

The subsurface B horizon include layers in which evolution of materials has taken place from above and even from below. In humid regions the B horizons are the layers of maximum accumulation of materials such as iron and aluminium oxides and silicate clays. In arid and semi- arid regions, Calcium carbonate, calcium sulfate and other salts may accumulate in the B horizons.

C Horizon:

The C horizon is the unconsoli­dated material underlying the solum (A and B) it may be or may not be the same as the parent ma­terials from which the solum form. The C hori­zon is outside the zone of major biological activi­ties and is generally little affected by the processes that formed in horizons above it. Its upper-layers may be with time become apart of the solum as weathering and erosion continues.

R Layers:

Underlying consolidated rock, with little evidence of weathering.

Transition Horizons:

These horizons are transitional between the master horizons (O, A, E, B and C). They may be dominated by proper­ties of one horizon but have prominent charac­teristics of another. Both capital letters are used.

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Essay # 11. Textural Classes of Soil: (100 Words)

To convey an idea of the textural make-up of soils and to give an indication of their physical proper­ties, soil textural class names are used. Three broad groups of these classes are recognised-sands, loams and clays (Table 10.3, Fig. 10.2).

Silts:

The silt group includes soils with at least 80% silt and 12% or less clay. Naturally the properties of this group are dominated by those of silt. The sand group includes all soils in which the sand separates make up at least 70% and the clay separate 15% or less of the material by weight. The properties of such soils are therefore charac­teristically those of sand in contrast to the stickier nature of clays included.

Clays:

To be designated a clay, a soil must contain at least 35% of the clay separate and, in most cases, not less than 40%. In such soils the characteristics of the clay are separated.

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Essay # 12. Cation Exchange Capacity of Soil: (100 Words)

The cation exchange capacity (CEC) of a given soil is determined by the relative amounts of dif­ferent colloids in that soil and by the CEC of each of these colloids. Thus, sandy soils have lower CECs than clay soils because the coarse-textured soils are commonly lower in both clay and humus content.

Likewise, a clay soil dominated by 1: I- type silicate clays and Fe, Al oxides will have a much lower CEC than will one with similar humus content dominated by smectite clays.

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Essay # 13. Soil Reaction: Acidity and Alkalinity: (100 Words)

Perhaps the most outstanding characteristic of the soil solution is its reaction—that is, whether it is acidic, alkaline, or neutral. Microorganisms and higher plants respond markedly to soil reaction because it tends to control so much of their chemi­cal environment.

Soil acidity is common in all regions where precipitation is high enough to leach appreciable quantities of exchangeable base-forming cations (Ca²⁺, Mg²⁺, K⁺, and Na⁺) from the surface lay­ers of soils. The condition is so widespread and its influence on plants is so pronounced that acid­ity has become one of the most discussed properties of soils.

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Essay # 14. Economy of Essential Elements of Soils: (200 Words)

i. Nitrogen and Sulphur Economy of Soils:

Of the various essential elements, nitrogen prob­ably has been subjected to the most study, and for many good reasons still receives much attention. The amount of this element in available forms in the soil is small, while the quantity withdrawn an­nually by crops is comparatively large.

When there is too much nitrogen in readily soluble forms, it is lost in drainage and may become a water pollut­ant. Nitrogen can be added to the soil by some microbes that “fix” it from the atmosphere, and can then be released back to the atmosphere by still other organisms. Nitrogen can acidify the soil as it is oxidised. Most soil nitrogen is unavailable to higher plants. All in all, nitrogen is an impor­tant nutrient element that must be conserved and carefully managed.

ii. Phosphorous and Potassium Economy of Soils:

Next to nitrogen, phosphorus and potassium are most critical essential elements in influencing plant growth and production throughout the world. Un­like nitrogen, these elements are not supplied through biochemical fixation but must come from other sources to meet plant requirements.

The sources include:

(a) Commercial fertiliser;

(b) Animal manures;

(c) Plant residues, including green manures;

(d) Human, industrial, and domestic wastes; and

(e) Native compounds of potassium and phospho­rus, both organic and inorganic, already present in the soil.