CHEMISTRY FORM FOUR TOPIC 3: SOIL CHEMISTRY


TOPIC 3: SOIL CHEMISTRY | CHEMISTRY FORM 4

Soil Formation

Soil is formed by the process of weathering. All types of weathering (physical, chemical or biological) result to disintegration of rocks into smaller particles. Air and water enter the space between these particles and chemical changes take place, which lead to the production of chemical substances.

Bacteria and plant life soon appear. When plants and animals die, they decay and produce humus. Bacteria and other decomposers play a vital role in the decomposition of plant and animal substrata. The end product of these mechanical, chemical and biological processes is soil.

 Therefore, soil can be defined as unconsolidated mineral (inorganic) and organic material on the immediate surface of the earth‟s crust that serves as the medium for plant growth.

 

All soils contain mineral matter, organic matter, water, air and living organisms, especially bacteria. If any one of these is substantially reduced in amount or is removed from the soil, then the soil deteriorates.

There are many types of soil and each has specific characteristics related to the climate, the vegetation and the rock of the region in which it forms. The weathering processes of a region also
play an important part in determining soil characteristics. The relationship of these factors is as shown in figure 3.1.

The Factors Influencing Soil Formation

Information about soil formation can lead to better soil classification and more accurate interpretation of soil properties.There are several factors responsible for soil formation.

The factors include climate, living organisms, relief (topography),parent material and temperature. All the factors, except time,depend to a greater or lesser extent upon each other, upon the soil itself or upon some other factor. None of the factors can be considered more important than any other, but locally one factor may exert a particular strong influence. These factors are explained in details below.

1. Parent material

Parent materials are made up of mineral material or organic matter or a mixture of both. The organic matter is usually composed predominantly of unconsolidated, dead and decaying plant remains. The mineral material, which is the most widespread type of parent material, contains a large number of different rock– to form Climate which decays to form results in weathering of influences the type of climate rocks vegetation humus Climate Soil mineral soil Climate forming minerals and can be in either consolidated or unconsolidated state.

 

Some rocks are more easily weathered than others. Acidic rocks are more resistant to weathering than basic rocks. The parent rock affects soil texture and water permeability.

Parent  rock with fine particles is more resistant to chemical weathering than mechanical weathering. Very compact parent rocks like sandstone are very much resistant to weathering. Porous rocks weather easily by chemical processes. This is because they have large surface areas for weathering agents to act upon.

2. Climate

Climate is the principal factor governing the rate and type of soil formation as well as being the main agent determining the distribution of
vegetation. The dead vegetations decay to form humus as one of the components of the soil.

 

To understand well the influence of climate on soil formation let us have a look at its components and how each of these components affects soil formation.

3. Temperature

The main effect of temperature on soil is to influence the rate of reactions; for every 10°C rise in temperature, the speed of a chemical
reaction increases by a factor of 2 or 3 (twice or thrice). Temperature, therefore, influences the speed of disintegration and decomposition of the parent materials and its consolidation to form the soil.

4. Rainfall (water)

The water in soils includes all forms of water that enter the soil system and is derived mainly from precipitation as rain. The water entering
soils contains appreciable amounts of dissolved carbodioxide, forming a weak carbonic acid. This dilute, weak acid solution is more reactive
than pure water. It thus reacts with unconsolidated minerals and organic matter, breaking them down into mineral (clay, sand) and organic debris (humus) respectively.

5. Organisms

The organisms influencing the development of soils range from microscopic bacteria to large mammals including man. In fact, nearly every organism which lives on the surface of the earth or in the soil affects the development of soils in one way or another. More important soil
organisms of interest to soil formation are as follows:

Higher plants.
Higher plants (particularly grasses) extend their roots into the soil and act as binders. So they prevent soil erosion. The roots also assist in
binding together small groups of particles hence developing a crumby or granular structure. Large roots are agents of physical weathering as they open and widen cracks in rocks and stones. When plants die they contribute organic matter to the soil, which acts as a binder of the
soil particles. Higher plants intercept rain and they shelter the soil from the impact of raindrops. They also shade the soil and hence reduce
evaporation.
 Vertebrates
Mammals such as moles, ground squirrels and mice burrow deeply into the soil and cause considerable mixing up of the soil, often by bringing up subsoil to the surface, and creating burrows through which the top soil can fall and accumulate within the subsoil.

Micro ogarnisms
These include bacteria, fungi, actinomycetes, algae and protozoa. These organisms act as decomposers of organic and even mineral matter.

Mesofauna

These include earthworms, nematodes, millipedes, centipedes and many insects, particularly termites and ants. Activities of mesofauna include:
  • ingesting organic mineral materials e.g. earthworms and millipedes;
  • transportation of materials e.g. earthworms, millipedes, termites, beetles, etc; and
  • improvement of soil structure and aeration.
Man Activities  are too many and too diverse. Man‟s roles include:
  • Cultivation of soils for production of food and tree crops, which in many cases has negative effects causing impoverishment of the soil and erosion.
  • Indiscrimate grazing, casual burning, cutting of trees, manure and fertilizer use, all of which alter the soil characteristics.

6. Relief (Topography)

This refers to the outline of the earth‟s surface. All land surfaces are constantly changing through weathering and erosion. It may take millions of years, in the case of Himalayas and the Andes, to be worn down to flat undulating surfaces. The soils on steep mountain slopes are shallow and often stony and contain many primary minerals. In areas where the difference in elevation between the highest and the lowest point is great, then climatic changes are introduced. These differences in elevation, slope, slope direction, moisture and soil characteristics lead to the formation of a number of interesting soil sequences.

7. Time
Soil formation is a very slow process requiring thousands and even millions of years. Hence, it is impossible to make definite statements about the various stages in the development of soils.This is because it takes a considerable period of time for a particular soil type to be formed and categorized.

PLANT NUTRIENTS IN THE SOIL 
MACRO AND MICRONUTRIENTS
Macro nutrients are those mineral nutrients that are required by plants in greater amounts. They constitute about 99% of plants‟ requirements.
Macro nutrients, also referred to as major nutrients are further divided into primary and secondary macronutrients. Primary macronutrients are the elements that are required by plants in relatively large quantities (60% of the plant‟s requirements). They are nitrogen, phosphorus and potassium. The secondary macro nutrients are calcium, magnesium and sulphur. These elements contribute the remaining 39% of the plant‟s needs Micro nutrients, also referred to as trace elements, are those mineral nutrients that are required by plants in smaller amounts. They constitute about 1% of plants‟ requirements.
Plants use the sugars for energy supply and to produce cellulose (for the cell walls) and starch (for storage). Proteins are made from sugar and nitrogen. They are used to synthesize protoplasm which is vital for the plant cells.
INTAKE OF NUTRIENTS BY PLANTS
Except for some gaseous carbon dioxide, oxygen and sulphur dioxide, all nutrients enter the plants in the form of ions usually from the soil
solution through the roots.
The metals are absorbed by the plant roots in the form of their cations.
Nitrogen is taken in as ammonium or nitrate (V) ions; phosphorus as dihydrogenphosphate (V) ions; sulphur as sulphate (VI) ions and chlorine as chloride ions.
The nutrient ions move towards the plant roots either with the flow of the soil solution (since the roots also take in water) or by diffusion. The ions diffuse from areas of high concentration to those of low concentration caused by the intake by the roots.
Plant roots either absorb equal numbers of positive and negative charges in the form of ions from the soil solution or they exchange one ion against another one with the same charge, usually H+ or OH–. Thus, roots possess electrical charges on their surfaces which can hold and exchange ions.
The Functions of Each of the Primary Macronutrients in Plant Growth
AVAILABILITY, FUNCTION AND DEFICIENCY OF PLANT NUTRIENTS
All essential plant nutrients perform specific functions to aid plant growth or reproduction. They must all be available in the right
proportion to facilitate optimum plant growth.
Carbon, Hydrogen and Oxygen
Availability: carbon and oxygen are taken in by plants through the stomata in the form of carbon dioxide from the air. Thus, they are never in scarce supply. Hydrogen (and oxygen) is supplied by water
Use in the plant: in the presence of sunlight, the green parts of the plant synthesize carbohydrates (sugars) from carbon dioxide and water.
Deficiency symptoms: plants wilt and die if they do not obtain sufficient water.
Nitrogen
Availability: only ammonium and nitrate (V) ions are available to plants. Usually about 98% of a soil‟s nitrogen is available (in organic matter). Organic matter decomposes by microbial action to form NH4+. This process is called ammonification. Almost any microbe can carry out ammonification.
There is another process called nitrification, which is carried out only by a few specific bacteria e.g. nitrosomanas. Nitrosomanas bacteria oxidize ammonium ions to nitrate (V) ions:
A few living organisms can fix nitrogen from the air. The best known are rhizobia (legume bacteria) and the free living bacteria such as azotobacter and clostridium, also the blue-green algae.
Plant needs: plants need more nitrogen than any other nutrient. However, their needs vary greatly: plants with high vegetative growth (stems, leaves, etc) have high N needs (maize, sorghum, rice, sugar cane, pasture grasses, most vegetables). Root crops (cassava, sweet potatoes, taro) have lower N needs. Legumes (alfalfa, desmodium, kudzu, all types of beans and peas, ground nuts, etc) may not need N fertilization if the proper strain of rhizobia is fixing N for them.
Use in the plant

It is used to build amino acids, nucleic acids, many enzymes, chlorophyll, generally speaking: all proteins.

It promotes the vegetative growth in plants. It is therefore important in the growth of plants in which leaves are harvested, such tobacco and vegetables.

It is an essential element in cell division. It is therefore needed for plant growth.

It increases grain size and protein content in cereals.

It promotes root growth.

Deficiency symptoms: leaves turn yellow and finally die, because chlorophyll can not be build up. Then due to lack of chlorophyll the plant grows slowly, because the chlorophyll is needed for carbohydrate production. The shortage of chlorophyll is called chlorosis.
Excess nitrogen: excess nitrogen causes dark green succulent vegetation with weak stems, often at the expense of seed or fruit production, e.g. in grain crops, in tomatoes and beans.
It causes the potatoes to be watery. It delays crop maturity, and makes plants more vulnerable to attack by diseases and pests. Thus,
fertilizers must be carefully dosed.
Available N is easily leached since NH4+ is rapidly nitrified in a warm climate, and NO3– is not adsorbed by soil colloids. Thus, it is important to apply fertilizer at the right time in order to avoid leaching.
Phosphorus Availability: only H2PO4– and to a lesser extent HPO42– are available to plants.
Plant needs: plants need less phosphorus than nitrogen or potassium.
Use in plant

Phosphorus is an essential component of the genetic material of the cell nucleus (RNA, DNA); also in ADP and ATP, which play a vital role in photosynthesis, amino acid and fat metabolism, etc.

It increases the grain yield e.g. of millet, sorghum and rice because it promotes the formation of tillers.

It promotes root growth.

It strengthens the resistance of plants to diseases.

Also rhizobia bacteria need it in order to fix nitrogen from the air.

It hastens plant maturity.

Deficiency symptoms

Dark green colouration

Purple spots or streaks.

Stunting, delayed maturity.

Fertilizers: superphosphates, triple phosphates, ammonium phosphates.
Potassium
Availability: only the K+ ions of the soil solution are available to plants. Hydrated potassium ions attached to soil colloids are readily available because they are not bonded strongly to the surface of the colloids.
After nitrogen, potassium is the second-most element needed by plants. Starch and sugar crops (cassava, sweet potatoes, banana, sugarcane) have relatively high needs of K.
Use in plants.
Potassium is present in plants in the form of it ions only.
It does not form any integral part of the structure of any known organic compounds in plants.

Potassium is an activator of a number of enzymes involved amino acid synthesis and several enzymes concerned with carbohydrate and nucleic acid metabolism.

Potassium aids in the uptake of other nutrients and in their movements within the plant e.g. potassium ions and nitrate (V)
ions may move together.

Potassium is also important in the metabolism of carbohydrates and translocation of food. Thus, it promotes starch and sugar formation.

It regulates osmosis in cells, improves tissue formation and assists in protein synthesis.

It strengthens plant stalk, hence preventing lodging and microbial attack.

Deficiency symptoms.

Stunting: First the edges of the older leaves and then areas between veins turn yellow and finally brown. Small, brown necrotic spots develop while the veins are still green.

Leaf curling and premature leaf fall.

Fertilizers: potassium chloride, potassium sulphate, potassium nitrate. Wood ashes and their aqueous extract (potash) contain potassium carbonate. Tobacco stems contain about 5% potassium, and cocoa shell meal about 3%.
Calcium
Availability: only Ca2+ ions in the soil solution are available to plants. Calcium comes from CaCO3 (calcite), gypsum (CaSO4·2H2O), apatite and other minerals.
Use in plants.

Calcium is a constituent of cell walls and hence makes the straw stiff and resistant to lodging.

It is essential for cell division.

It promotes early root and seed development.

It regulates the intake of potassium by plants.

It neutralizes harmful organic acids like ethanedioic (oxalic) acid in plants, thus detoxifying them:

Magnesium
Availability: plants take in Mg2+(aq) through roots and leaves.
Use in plant;

Magnesium is vital to the production of chlorophyll, because every molecule of chlorophyll contains a magnesium ion at the core of its complex structure. Most of the magnesium in plants is found in either chlorophyll or seeds. A lesser part is distributed in other parts.

Aids in the translocation of carbohydrates.

Regulates the uptake of other nutrients.

Part of the distributed magnesium functions in the enzyme system involved in carbohydrate metabolism.

Fertilizers: Mostly dolomitic limestone (CaCO3·MgCO3). The principal magnesium fertilizer is magnesium sulphate (VI), sometimes known as epsom salt. It is soluble in water and can be sprayed onto the leaves.
Sulphur
Availability: Plants take in sulphate (VI) ions (SO42–) from the soil solution and sulphur dioxide from the air.
Its Use in plants.

Sulphur is a vital part of plant proteins since cystine and methionine are sulphur-containing amino acids.

Sulphur is also essential for the action of enzymes involved in nitrate (V) production.

The Deficiency symptoms,
Sulphur deficiency symptoms resemble nitrogen deficiency symptoms because both are related to protein and chlorophyll deficiency.
Fertilizers: gypsum and elemental sulphur, both of which are also used to lower soil pH; ammonium sulphate, superphosphate potassium sulphate (VI).
Iron
Occurrence:
In igneous rocks, iron occurs in the Fe2+ form. The iron in water-logged soils tends to remain in this form and contributes to the
bluish-grey colours that indicate wetness. Much of the iron in well drained soils is in the Fe3+ form and is associated with humus and
mineral particles.
Availability: Plants absorb iron in the form of Fe2+ and Fe3+.
Use in plants;
Iron is an essential catalyst in the formation of chlorophyll and functions in some of the enzymes of the respiratory system. Iron is needed in larger quantity than all other micronutrients.
Its Deficiency symptoms,
An iron deficiency results in the young leaves being small and pale green or yellow in colour.
Fertilizers:
Iron (II) sulphate (FeSO4) which is soluble in water. Application of iron is generally ineffective to calcareous soils.
THE OTHER MICRONUTRIENTS
Availability:
Micronutrients are taken in by plants as Bo2–, Co2+, Cu2+, Mn2+, MoO2– and Zn2+. The micronutrients in the soil usually originate from the parent material of the soil. Plant needs of these micronutrients are very small.
Use in plants: Most micronutrients are used as catalysts in plant metabolism:

Manganese is a catalyst in the formation of chlorophyll and in many redox reactions, e.g. metabolism of nitrogen, iron, copper, zinc and in vitamin C synthesis.

Boron aids protein synthesis, regulates the K:Ca ratio in plant tissues and is required for the formation of roots and fruits.

Copper is involved in respiration and in the nitrogen and iron metabolism.

Molybdenum is essential in the protein synthesis and for the nitrogen fixation by rhizobia on the roots of legumes.

Zinc catalyses the formation of growth hormones and promotes the synthesis of RNA and chloroplasts. Thus it is essential for normal growth.

Chlorine seems to be essential in photosynthesis and is required for plant growth.

Cobalt is essential for nitrogen fixation by rhizobia and hence aids growth of legumes. However, it is not clear whether it is essential for growth of higher plants.

Fertilization:
Large amounts of micronutrients are usually toxic to plants. The best method of application is usually foliar spraying.
Preparation of Plant Nutrient Cultures in the Laboratory
Nutrient cultures are prepared in the laboratory by using chemicals such as CaSO4, Ca3(PO4)2, MgSO4 and KNO3. These salts are dissolved in water to make cultures containing ions of plant mineral elements. Different cultures lacking some mineral nutrients are made and used to grow plants. The health of plants in various cultures is compared with those grown in a culture with all elements.
Mangaging the Loss of Plant Nutrients from the Soil
Manage the loss of plant nutrients from the soil
Crop plants take up nutrients from the soil continuously. To maintain the soil fertility, the nutrients taken by plants must be replenished
(replaced). There are several methods that when combined at least in some aspects can help raise or maintain soil fertility. These are:
Addition of inorganic fertilizers and manure
Inorganic (industrial) fertilizers
Fertilizers are mostly inorganic compounds which contain one or more plant nutrients in a concentrated form. They help to increase or maintain fertility if used carefully with a good background of knowledge.
However, if used without proper knowledge or advice by agricultural officers they can be harmful to the soil, crops, animals and humans.
It should also be noted that without reasonable humus content, the soil may have such a low cation exchange capacity that most of the applied fertilizer is leached from the soil instead of being available to plants.
Thus, just adding fertilizer on a field without good cropping system and, advisably, with addition of manure is often a waste of money, time and energy.
Organic fertilizers (manures)
There are different kinds of manures that can be applied to the soil. These include:

biogas manure – from biogas plants

farm yard manure – from wastes of farm animals such as cattle, sheep, goats, poultry, pigs, donkeys, etc;-

compost manure – from decomposed organic matter; and-

leguminous green manures, like sunhemp, beans, cowpeas, groundnuts, peas, etc.
These young plant materials when ploughed and incorporated into the soil provide organic matter and nitrogen.

Prevention of soil erosion
Most plant nutrients are concentrated on the top soil. If this soil is eroded the nutrients are lost too. This can be stopped by taking soil
conservation measures which include mulching terracing/ridging, deep tillage, contour ploughing, strip cropping, planting shelter belts or
windbreaks, reforestation, avoiding overgrazing and overstocking, etc.
Crop rotation
This refers to the practice of planting different crops in a field in successive growing seasons. A good crop rotation is that which include
leguminous crops (which fix nitrogen in the soil) followed by non-leguminous crops and vice-versa.
Intercropping
Intercropping refers to the act of planting two different crops (preferably legumes with non-legumes) on the same field. Legumes provide nitrogen to non-legumes and the non-legumes help to cover the soil to prevent erosion.
Agroforestry
This refers to mixed cropping of e.g. cereals and usually leguminous trees like Leucaena leucocephala, which provide nitrogen to the field. The trees take up nutrients from the deeper layers of the soil while the cereals take up their nutrients from the top layers. Leguminous trees
provide the cereals with the humus when their leaves fall and rot on the soil. They also provide forage for animals, and firewood. Agroforestry is also one of the protections against soil erosion.
Good harvesting practices
Crop remnants harvest should not be burnt down but they must be ploughed and incorporated into the soil. This will help to maintain adequate levels of organic matter in the soil and hence reduce soil erosion. Also by burning crop residues, valuable plant nutrients (N, P and S) are lost and the chance to increase humus level of the soil is reduced.
Prevention of leaching
Leaching refers to loss of plant nutrients from the top to the bottom soil layers, following heavy rains or overirrigation. This can partly be
stopped or reduced by maintaining adequate levels of soil organic matter to trap the nutrients and also by avoiding too much irrigation. It can also be stopped by avoiding overcultivation, a fact which makes the soil too loose that the nutrients are easily percolated with the soil
solution to the bottom soil layers making these nutrients unavailable to plants.
Following /bush fallow
This refers to leaving the land idle to rest, a fact which allows the land time to regain its lost fertility. However this practice is only
possible for farmers with plenty of land. It was an equally good practice in the past when human population was low as compared to
vastness of the land at that time. It is not widely practiced today except in areas with low population density and abundant arable land.
Soil conservation
Taking soil conservation measures such as terracing, contour ploughing, mulching, deep tillage, etc. will help maintain soil fertility.
Cultivation of the appropriate crops for the soil
Different crops are suited to different soil types. If you plant crops in a wrong soil you are likely to weaken that soil and destroy its fertility
status. But if appropriate crops are planted in the right soil type, chances of maintaining or sustaining the fertility of that soil is also
 very high.
Good farming practices
Adoption of appropriate soil amendment practice such as liming, acidification, conversion, etc helps to maintain soil fertility. For example, liming is good as it corrects soil acidity but if too much lime (overliming) is applied it results to another problem of setting on soil alkalinity.
Likewise, an acid soil, no matter how much beneficial nutrients it contains, is of no use unless its acidity is corrected
Avoidance and/or control of soil pollution
Avoid dependence and overuse of agrochemicals such as pesticides, herbicides and inorganic fertilizers unnecessarily. These chemicals must be used with care and only where agricultural production is impossible without their application. This is because they contribute a great deal to soil pollution and toxification of beneficial soil organisms.
Nutrient balance maintenance
The balance of nutrients in the soil must always be maintained. Plants usually require specific quantities of different nutrient elements.
These nutrients must be maintained in the soil by good cropping systems, application of appropriate fertilizers and manure and adopting good soil management practices.