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This document provides an overview of soil, including its definition, formation, profile, components, types, properties, and human impacts. It defines soil as the organic and inorganic material on the surface of the earth that provides the medium for plant growth. The key factors that influence soil formation are climate, parent material, topography, biota, and time. The four main soil components are minerals, air, water, and organic material. Common soil types include sand, silt, clay and loam. Human activities like erosion, desertification, acidification, deforestation, and salinization can degrade soils.
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- 1. Unit 6: The Soil Learner Notes
- 2. 1 Unit 6: The Soil Introduction One of the first things you have probably learned about plants in previous grades was that they needed certain things to live and grow like water, sunlight, and soil. Plants use these to live and grow and each has its own job in the larger system. Man is dependent on soils as they are key to sustaining life- affecting air and water quality, the growth of plants and crops ant the health of the entire planet and soils are in turn dependent on man and the use he makes on them (Puri, 2010). 1. What is soil? Soil can be defined as the organic and inorganic material on the surface of the earth that provides the medium of plant growth. It is the extremely thin but precious skin covering our planet that sustains all terrestrial life forms and contributes nutrients and filters contaminants to aquatic environments. 2. Soil Formation All soils initially come from rocks, this is termed the ‘parent material’. Geological factors such as hard rock, lakes, river and glacier deposits, windblown silt or sand are the starting point from which soil formation begins and they are the parent material for soil. Soils are key part of the geological cycle, which includes rock formation, land scape evolution and weathering to produce soil followed by erosion, deposition, and additional soil formation (Singer & Munns, 1992). 2.1. Factors in Soil Formation The factors influencing soil formation are: Parent material Parent material is rock or material from which soil is formed. The minerals found in the parent material contribute to the soil’s fertility, the kind and amounts of minerals that will be found in the soil. Climate Climate is one of the most important factors affecting the formation of soil .Temperature and precipitation play a major role in soil formation. Water is the main agent in weathering. The amount of water entering the soil depends on how much falls and runs off the surface. The amount of precipitation that falls influences the kind of soil formed and the rate at which it is formed at. Precipitation and temperature also determine the kind and amount of vegetation that grows in an
- 3. 2 area and temperature determines how much precipitation falls as rain or snow. Warmer temperatures and an abundance of water speed up the formation of soil, in some cases rather dramatically. Whereas cooler temperatures and less precipitation slow down soil formation (Puri, 2010). Biota (Organisms) The biotic factors include both plants and animals which is organic matter. Plant fix atmospheric pressure and add it to the soil. Organic matter is added to the soil surface by plant tops and to the surface of the plant roots. These factors influence the rate of soil formation and the kinds of products formed (Singer & Munns, 1992). Topography Topography is a characteristic of landscape. Slope angle and length are features of topography that govern the amount of water that runs off or enters soil. Topography basically changes the development of soil on a local or regional scale depending on the slope. Soil that develops on moderate to gentle slope is often better drained than soil found at the bottom of valleys while steep topographic gradients inhibit the development of soils because of erosion (Singer & Munns, 1992). Time It is difficult to answer how old soil is but all soils are not the same age because the factors of soil formation act continuously over time (Singer & Munns, 1992). These factors are combined to produce an endless range of soils. Physical, biological and chemical processes transform the parent material into soils, reducing the parent material size and convert the individual minerals into soluble products that can be carried by the soil (Singer & Munns, 1992). Figure 1: The development of a soil is influenced by five interrelated factors.
- 4. 3 2.2. The Soil Profile If you look in a soil pit, you will see various layers in the soil. These layers are called soil horizons. The arrangement of these horizons in a soil is known as a soil profile. Soil scientists, who are also called pedologists, observe and describe soil profiles and soil horizons to classify and interpret the soil for various uses. Figure 2: The Soil Profile Soil horizons differ in a number of easily seen soil properties such as color, texture, structure, and thickness. Other properties are less visible. O HORIZON- also known as humus, this is the utmost top layer of soil that is made up of living and decomposed materials like leaves, plants, and bugs (organic materials). This layer is thin and usually dark in color (Puri, 2010). A HORIZON- This is the layer that we call “topsoil” and it is located just below the O Horizon. This layer is made up of minerals and decomposed organic matter and is also very dark in color. This is the layer that many plants roots grow in and where biological activity occurs. Soil organisms such as earthworms, arthropods, nematodes fundi and many other bacteria are found in this layer (Puri, 2010). B HORIZON- We call this layer “subsoil” and it is located just below the A Horizon. This layer has clay and mineral deposits such as iron and aluminum and less organic materials than the layers above it. Plant roots penetrate through this layer but it has very little humus. This layer appears to be brownish or reddish in color due to the clay and iron oxides(rust) washed down from the A horizon (Puri, 2010) . C HORIZON- This is the layer is called “regolith” and it is located just below the B Horizon. This layer is made up of lumps or more likely unbroken and un-weathered rock and only a little bit of organic material is found here. Plant roots are not found in this layer. Parent material may also be found here (Puri, 2010).
- 5. 4 R HORIZON- This layer is called the “bedrock” or “parent rock” and is located below the C Horizon, at the base of the soil profile. Unlike other horizons, the R horizon is comprised of large continuous masses of rock which cannot be excavated by hand (Singer & Munns, 1992). 3. Components of the Soil Soils are made up of four basic components: air, water, minerals and organic material. In most soils, minerals represent 45% of the total volume, water and air about 25% each and 5% of organic material. The minerals portion consists of three distinct particle sizes that can be classified as sand, silt and clay. Figure 3: A pie chart representing the Components of the Soil 4. Types of Soil 4.1. Sand This type of soil has the biggest particles and the size of these particles does determine the degree of exposure to air and drainage that the soil allows. Sand is granular and consists of rock and mineral particles that are very small. Sandy soil is formed by the breakdown and weathering of rocks such as limestone, granite, quartz and shale. It is easier to cultivate if it is rich in organic material but then it allows drainage more than is needed, thus resulting in over-drainage and dehydration of the plants in summer. Sandy soil retains moisture and nutrients. The sand soil properties The fertility of the sand soil is low. It is well ventilated soil that has low absorption of the water. The size of its particles is large. The color of the sand soil is yellow. It is loosely packed and has the fastest and the greatest drainage of the water.
- 6. 5 4.2. Silt Silt soil is made up of fine particles. Like clay the soil holds water but doesn’t have good ventilation around the roots. Some people think that adding sand to clay soil will open it up but adding sand to clay creates a low grade concrete and most plants don't do well with this hardened soil. The silt soil properties The color of the silt soil is grey. The size of its particles is medium (between the sand soil particles and the clay soil particles). It is moderately ventilated soil that has medium absorption of the water. It is moderately compacted and has the medium drainage of the water It is also highly fertile. 4.3. Clay Clay soil particles are very small and compact. Gardens with these types of soil particles don’t work well because the air has a hard time getting to the roots. The soil absorbs and holds water and creates a drainage problem. This badly affects healthy root and plant growth. Clay is very fine, small inorganic particles, worn down from rock. The clay soil properties The color of the clay soil is dark (black). The size of its particles is small. It is fertile. It has highly compacted (hard). It is poorly ventilated soil that has high absorption of the water. It has the lowest drainage of the water. 4.4. Loam Loam soil is a mixture of sand, clay and silt. This type of soil normally has very fine particles making it one of the most ventilated. It also contains humus which is ideal for supporting plant growth. Loam soil contains plant nutrients and allows animals like earthworm to thrive in it. This is the most commonly used soil for agriculture throughout the world. The loam soil properties It has well drainage and good ventilation levels. Loam soils have the ability to maintain nutrient levels.
- 7. 6 5. Soil Properties 5.1. Soil Texture Textures range from clay, sand, and silt at the extremes, to a loam which consists of the other types of soil. The main influence of texture is on the permeability which generally decreases with decreasing particle size. 5.2. Soil Air According to (Buckman & Brady, 1960), soil air is found in those spaces between the soil particles that are not filled with soil water. The amount of air in a soil depends on how firmly the soil is compacted. It differs from that of the atmosphere as it is not continuous because it is located in a maze of soil pores separated by soil solids. Soil air also generally has a higher moisture content than the atmosphere with a relative humidity approaching 100% when soil moisture is optimum. The content of carbon dioxide found in the soil air is higher and that of oxygen is lower than that found on the atmosphere. In well-aerated soil at least 20% of its volume is made up of air. 5.3. Soil Water Soil water can be classified into three types, namely hygroscopic, capillary and gravitational water. Hygroscopic water occurs as a thin film of water around each soil particle. Capillary water is that water held in the small spaces between the soil particles and gravitational water is the water which drains downwards through the soil. There are two major factors concerning soil water that are important to obtain an idea of the significance of water as a component in water. These are: Water is held within soil pores with different degrees of tenacity depending on the amount of water present Together with the soil’s dissolved Salts, soil water makes up a solution (soil solution) which is important as a medium for supplying nutrients to growing plants (Buckman & Brady, 1960). 5.4. Soil Temperature Chemical and biological activities in the soil are influenced by temperature for example nitrification does not begin until soil temperatures reach about 4°C, the most favorable limits being 8°C to 9°C. Soil temperatures for seed germination may vary thus making soil temperature an important ecological factor. At low temperature there is little decay by decay causing micro- organisms (Buckman & Brady, 1960). 5.5. Soil Acidity vs. Alkalinity Acidity or alkalinity of soil (the pH of the soil) influences the biological activity in soil and the availability of certain minerals. Thus the pH of soil has a greater influence on the growth and development of plants. Soil p.H is an important consideration for several reasons like: many plants and life forms prefer either alkaline or acidic conditions, some diseases thrive when soil is acidic or alkaline and the p.H level can affect the availability of nutrients in the soil (Puri, 2010).
- 8. 7 Figure 4: An image showing the different p.H levels in the soil when planting different vegetables 6. Human Impact on the Soil 6.1. Erosion Erosion removes mineral particles, organic material and nutrients form the soil reducing its thickness and water holding capacity. Eroded soil then becomes a pollutant in streams and reservoirs. The time required to form new soil is so long that from a person’s point of view soillost during erosion is lost forever. Soil erosion is a natural geologic process however, humans can accelerate the process by removing cover. Accelerated erosion occurs at 10-1000 times the natural rate. Erosion can happen in all of the biomes on earth, and can be caused by removing trees or grasses. This can decrease local water quality and contribute to poor health in populations in the area. Removing the soils generally leads to other types of degradation and reduced food production (Singer & Munns, 1992). Human Causes (Seshoka, Keats, Earle, Dilley, & Rampou, 2002) Monoculture Ploughing on marginal land Lack of crop rotation Planting crops against the gradient of slopes Overgrazing Use of chemicals e.g. pesticides Deforestation Animal Causes (Seshoka, Keats, Earle, Dilley, & Rampou, 2002) Mass migrations e.g. wildebeest Imbalance and /or overpopulation of one type of species
- 9. 8 Physical Causes Erosion by water Erosion by wind Effects of Soil Erosion on People Reduction in the availability of farmland Drop in food production Rise in food prices Increase in unemployment Increase in poverty People migrate to urban areas – rural depopulation Effects of Soil Erosion on the Environment Loss of vegetation Increase in run-off Silting of dams Desertification Management Strategies to Prevent and Control Soil Erosion (Seshoka, Keats, Earle, Dilley, & Rampou, 2002) Political and Economic Changes Land distribution and land reform Education and training More scientific approaches to farming Technical Changes Contour ploughing Strip farming methods Avoiding overgrazing and over cropping Biological diversity Conserving Wetlands Figure 5: An example of a piece of land that has undergone soil erosion
- 10. 9 6.2. Desertification Desertification is the extreme degradation of productive land in arid and semi-arid areas. This is most common in the tropical savannah and grasslands. This can create poor quality vegetation, and the spreading of deserts to areas that weren't deserts before (Singer & Munns, 1992). Figure 6: An image showing desertification 6.3. Acidification Acidification occurs when the basic cations (like Calcium and Magnesium) leach from the soil, leaving the acidic cations in the soil (Hydrogen, Aluminum, iron and manganese). The pH decreases and soil becomes more acidic. This is a natural process in weathering. However, the use of certain fertilizers to provide food, like anhydrous ammonia, causes soil to become more acidic much faster. This can occur in all biomes (Singer & Munns, 1992). Figure 7: An image showing soils contaminated by Iron and Sulphur
- 11. 10 6.4. Deforestation Deforestation is the permanent destruction of forests in order to make the land available for other uses like housing and urbanization, to harvest timber to create commercial items such as paper, furniture and homes, to create ingredients that are highly prized consumer items, such as the oil from palm trees, to create room for cattle farming. Deforestation is considered one of the most common causes of global climate change as it has an impact on the global carbon cycle. Gas molecules that absorb thermal infrared radiation are called greenhouse gases. If greenhouse gases are in large quantity, they can force climate change, according to studies done. While oxygen (O2) is the second most abundant gas in our atmosphere, it does not absorb thermal infrared radiation, as greenhouse gases do. Carbon dioxide (CO2) is the most prevalent greenhouse gas. The deforestation of trees not only lessens the amount of carbon stored, it also releases carbon dioxide into the air. This is because when trees die, they release the stored carbon. (Seshoka, Keats, Earle, Dilley, & Rampou, 2002) Common methods of deforestation are burning trees and clear cutting. It is a very severe problem. Figure 8: An image showing Deforestation 6.5. Salinization This build-up of salt on the soil surface is called salinization. This is a very big problem in the desert and savannah biomes. In arid or semi-arid regions, too much or too little irrigation water can lead to an increase of soluble salts, reducing plant growth. Salts such as chlorides, sulfates and bicarbonates of Na, Ca, and Mg accumulate as water evaporates form the soil. This can cause physical soil damage, and the ability to grow plants thus only the most salt tolerant species can be grown (Singer & Munns, 1992).
- 12. 11 Figure 9: An image showing salt surface accumulation
- 13. 12 References Buckman, H. O., & Brady, N. C. (1960). The Nature and Properties of Soil (Sixth ed.). New York: The Macmillan Company. Puri, S. (2010). Soil Chemistry. New Delhi: Mehra Offset Press. Seshoka, A., Keats, G., Earle, J., Dilley, L., & Rampou, S. (2002). Focus on Geography Grade 11. Cape Town: Maskew Miller Longman. Singer, M. J., & Munns, D. N. (1992). Soils, An Introduction (Second ed.). (P. Corey, Ed.) New York: Macmillan Publishing Company.