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CHEMISTRY FORM FOUR TOPIC THREE: SOIL CHEMISTRY
Soil Formation
Soil Formation
Describe 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.
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The Factors Influencing Soil Formation
Describe the factors influencing soil formation
Information about soil formation can lead to better soilclassification and more accurate interpretation of soil properties.There are several factors responsible for soil formation. Thefactors 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 soilitself or upon some other factor. None of the factors can beconsidered more important than any other, but locally one factormay exert a particular strong influence. These factors areexplained in details below.
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.
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.
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 conso
lidation to form the soil.
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.
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.
Microogarnisms
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 of man 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.
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.
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.
Soil Reaction
The Concept of Soil Reaction
Explain the concept of soil reaction
Soil reaction refers to how acidic or alkaline a soil is. It isexpressed as a pH value. The soil can be acidic, neutral or alkaline. Extremely acidic soils can have pH values below 4.5 and on the other hand, very alkaline soils can have pH values up to,and even higher than 9.0.
Nearly all soils have pH values between 4 and 8. Soils with pH< 4 generally contain sulphuric (IV) acid, while those with pH <8 contain a high percentage of Na+ ions and thus they are alkaline.
pH is defined as the negative logarithm (to base 10) of the hydrogen ion activity (concentration):
pH = -log10[H+] where [H+] denotes the concentration of H+ ionsin grams/litre. This is the same as saying that pH is the logarithmof the reciprocal of [H+]:
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The greater the [H+], the lower the pH and the more acidic the soilis. Acidic soils are common in humid regions, particularly thetropics, where rainfall is sufficiently high to leach theexchangeable bases from the top soil. Alkaline soils, on the otherhand, are characteristic of the arid regions of the world where,because of low rainfall, there is a high concentration of basiccations (Ca2+, Mg2+, Na+, etc) in the surface soil layer. Generally,acidic soils occupy a large area of arable land than alkaline soilsdo. Because of this, acidic soils are considered to be moreimportant, at the practical level, as compared to alkaline soils.
Causes of soil acidity
Having known that the soil can be acidic and that the acidity is aresult of great concentration of H+ ions in the soil, let us knowlook at the causes of soil acidity. The causes of soil acidity includethe following:
  1. Leaching: Heavy rains may leach bases like Ca2+, Mg2+, K+ andNa+ from the soil to the ground water table, leaving a surplus ofH+(aq) in the soil.
  2. Soil Microorganisms and root respiration produce carbondioxide which forms weak carbonic acid with the soil solution.
  3. Near industrial regions, acid rain (often pH 2–4) may bringsulphuric (IV) acid and nitric (V) acid to the soil.
  4. Acid mineral fertilizers, like ammonium sulphate (VI) andammonium chloride make the soil solution more acidic due to oxidation and hydrolysis:
Oxidation:
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Hydrolysis:
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Also the NH4OH produced by oxidation and hydrolysis dissociates further to give NH4+ and OH– ions. The NH4+ ions produced undergoes microbial and enzymatic oxidation (as show above) to release more H+ ions to the soil.
5. Nitrification of ammonium ions by bacteria produces H+(aq):
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6. Organic acids produced during the decomposition of organic matter also contribute to soil acidity. Due to such reasons, most soils in the humid tropics are acidic.
7. The Al3+ ions present in soil solution contribute to soil acidity indirectly when they are hydrolysed:
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For simplicity and easy understanding, the equation for hydrolysis of Al3+ ions is sometimes represented in a single equation as:
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8. Small amounts of ions such as NO3‾, NO2‾, H2PO4‾, SO42‾and Cl‾ present in the soil solution also contribute to soil acidity.
The above causes to soil acidity can be categorized as either natural or artificial. The artificial causes are acid rain and acid mineral fertilizers which are a result of deliberate human actions. The rest of the causes are classified natural because they occur naturally.
The pH of a Given Soil Sample
Measure the pH of a given soil sample
The pH of a soil can be tested by using Soil pH Kit. The kitcomprises of equipment and dyes (pigments) that are employed insoil pH determination.
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Soil pH kit is simple to use and can be used many times. Colour indicator dye and chart easily helps
to find out pH. Knowledge of soil pH is very important to a farmer. Most plants grow well in soils with pH ranging between 5.5 and 7.5.
The following is a quick and accurate way to find soil pH using a soil pH kit:
1. Getting prepared
First collect your equipment
You will need a clean trowel and a clean container for each sample you take. You can use clean boxes, plastic bags, or any convenient container to collect your sample. For powder-based kits you will need distilled water for the test itself.
Decide where to take the samples
Soil pH can vary in different parts of your garden either naturally or through different types and levels of cultivation. You may be able to see clear differences in colour, texture and humus content. Therefore, aim to take a number of samples from different areas and test each one separately.
2. Collecting the soil samples
For each area you are sampling, scrape away the top soil to a depth of about 5 cm. This prevents the reading being affected by any top dressings or mulches you have applied or any accumulation of leaf litter or pine needles.
Now break up the soil to a depth of about 12 cm and take the sample from the bottom level. Collect more than you think you will need as you will be picking out all the lumps, stones, twigs etc. Make sure you label each sample so that you know where they have come from.
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3. Sample preparation
– Pick out any stones, roots and twigs and leave the sample to dry.
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– Break up the dry sample with the back of a clean teaspoon or the tip of a clean trowel, and place the specified amount in the test tube or test chamber provided.
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Crop
Preferred pH
Irish potato
4.5 – 6.0
Chicory, parsley
5.0 – 6.5
Carrot, sweet potato
5.5 – 6.5
Cauliflower, garlic, tomato
5.5 – 7.5
Broad bean, onion, cabbage and many others
6.0 – 7.5
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EcoleBooks | CHEMISTRY O LEVEL(FORM FOUR) NOTES - CHEMISTRY FORM FOUR TOPIC THREE: SOIL

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1 Comment

  • EcoleBooks | CHEMISTRY O LEVEL(FORM FOUR) NOTES - CHEMISTRY FORM FOUR TOPIC THREE: SOIL

    Stephanie Pastory, December 12, 2023 @ 5:01 am Reply

    That’s notes are very we

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