Introduction to Soil Science PDF - University of Mpumalanga

INTRODUCTION TO SOIL SCIENCE 1A Module code RMGT 151 Program name DIPLOMA IN AGRICULTURE IN PLANT PRODUCTION School name AGRICULTURAL SCIENCES Faculty name AGRICULTURE AND NATURAL SCIENCES Topic Introduction to Soil Science Compiled by: Lecturer’s Name...

INTRODUCTION TO SOIL SCIENCE 1A Module code RMGT 151 Program name DIPLOMA IN AGRICULTURE IN PLANT PRODUCTION School name AGRICULTURAL SCIENCES Faculty name AGRICULTURE AND NATURAL SCIENCES Topic Introduction to Soil Science Compiled by: Lecturer’s Name PHASHA MORUFANE Building No BLD 6 (12) Office No 127 Email address [email protected] Telephone 013 002 0172 1 Page INTRODUCTION TO SOIL SCIENCE Overview: At the end of the module students will be able to: Define soil in different terms Explain the two approaches to learning soils. Discuss the various components of soils Outline the difference between the organic and inorganic components of soils Part 1: THE COMPOSITION OF SOIL What is soil? Introduction Chances are that you have not thought a lot about the soil under your feet, but you may be surprised at the complexity of soil. The soil varies in its composition and the structure of its particles, and these factors are closely examined by farmers, who need appropriate soil for planting crops, as well as engineers who may need to understand how soil is going to hold up under different demands. Soil is also vitally important to the sustainability of an ecosystem because it serves as the natural medium for the growth of vegetation. In this lesson, you will discover just what soil is and which factors are looked at when determining the structure and the types of soil. So, what exactly is soil? Soil is a loose material, which forms a thin layer covering the earth’s surface. The word “soil” means different things to different people. In crop production, the soil is a medium for plant growth, which supplies nutrients, water, and air for roots and support for plants. It forms on the earth’s surface as a result of weathering rocks and minerals. Soil can be defined as the organic and inorganic materials on the surface of the Earth that provides the medium for plant growth. Soil develops slowly over time and is composed of many different materials. Inorganic materials, or those materials that are not living, include weathered rocks and minerals. Weathering is the mechanical or chemical process by which rocks are broken down into smaller pieces. Where the underlying rock has weathered in place to the degree that it is loose enough to be dug with a spade, the term saprolite is used. As rocks are broken down, they mix with organic materials, which are those materials that originate from living organisms. For example, plants and animals die and decompose, releasing 2 nutrients back into the soil. Page Whitney (1982) Hilgard (1892) Dokuchaiev (1900) Joffe (1936): Soil is a natural body of mineral and organic constituents differentiated into horizons usually unconsolidated, of variable depth which differs among themselves as well as from the underlying parent material in morphology, physical makeup, chemical properties and composition and biological characteristics. This unconsolidated layer is called regolith, and varies in thickness from virtually nonexistent in some places to tens of meters in other places. Where the underlying rock has weathered in place to the degree that it is loose enough to be dug with a space, the term saprolite is used. In any ecosystem (backyard, farm, forest, or regional watershed) soils play five key roles: Soil supports the growth of higher plants mainly by providing a medium for plant roots and supplying nutrient elements that are essential to the entire plant. Soil properties are the principal factor controlling the fate of water in the hydrologic system. Water loss, utilization, contamination, and purification are all affected by soil. Soil functions as nature’s recycling system. Within the soil waste products and dead bodies of plants and animals are assimilated and their basic elements are made available for reuse by the next generation of life. Soil provides habitats for a myriad of living organisms (from small mammals and reptiles to tiny insects and microscopic cells). Soil plays a role in human–built ecosystems as an Engineering medium. (Building materials e.g. bricks provide the foundation for roads and all types of buildings). SIGNIFICANCE OF SOIL AND ITS STUDY All life depends on the soil, often synonymous with "dirt," and is perhaps the most undervalued of all natural resources. All human food is obtained from crops grown on soil. All livestock and herbivores graze on grasses and other plants that grow in the soil. Most fibers used in clothing, such as flax and cotton, are grown in soil. Building materials, such as brick, adobe, aluminum, and glass come from soil materials. The timber used for construction and furnishings is grown in soil. NATURE AND USES OF SOIL Soil is a medium in which plants are grown for food and fiber. Soil gives mechanical support for 3 plant roots so that even tall trees stand for decades against strong winds. Soil also physically Page supports structures such as houses, buildings, sidewalks, streets, and highways. Soil is involved in several processes in the hydrologic cycle. Water in the form of rain, dew, fog, irrigation, or snowmelt may move into the soil (infiltrate) or evaporate or run off of the soil surface into the area drainage systems into lakes or streams. Soil is an air-storage facility. Plant roots and billions of other organisms living in the soil need oxygen. SOIL AS A NATURAL BODY Scientists sometimes refer to “soil” as “the soil”, sometimes to “a soil,” and sometimes to “soils”. These variations of the word “soil” refer to two distinct concepts- soil as a material or soils as natural bodies. Soil is a material composed of minerals, gas, water, organic substances, and microorganisms. Some people also refer to this material as dirt, especially when it is found where it is not welcome. APPROACHES TO STUDYING SOIL TWO CONCEPTS: One treats soil as a natural body, a weathered and synthesized product in nature (Pedology) while the other treats soil as a medium for plant growth (Edaphology). PEDOLOGICAL APPROACH: The origin of the soil, its classification, and its description are examined in Pedology. (From the Greek word pedon, which means soil or earth). Pedology is the study of soil as a natural body and does not focus on the soil’s immediate practical use. A pedologist study examines and classifies soil as they occur in its natural environment. EDAPHOLOGICAL APPROACH: Edophology (from the Greek word edaphic, which means soil or ground) is the study of soil from the standpoint of higher plants. Edaphologists consider the various properties of soil concerning plant production. They are practical and have the production of food and fiber as their ultimate goal. They must determine the reasons for variation in the productivity of soils and find means for improvement. 4 Page COMPONENTS OF SOIL The four major components of soil are shown: inorganic minerals, organic matter, water, and air. Inorganic mineral matter, about 40 to 45 % of the soil volume Organic matter, about 5 % of the soil volume Water, about 25 % of the soil volume Air, about 25 % of the soil volume The amount of each of the four major components of soil depends on the quantity of vegetation, soil compaction, and water present in the soil. Good, healthy soil has sufficient air, water, minerals, and organic material to promote and sustain plant life. The organic material of soil, called humus, is made up of microorganisms (dead and alive), and dead animals and plants in varying stages of decay. Humus improves soil structure, providing plants with water and minerals. The inorganic material of soil is composed of rock, slowly broken down into smaller particles that vary in size. Soil particles that are 0.1 to 2 mm in diameter are sand. Soil particles between 0.002 and 0.1 mm are called silt, and even smaller particles, less than 0.002 mm in diameter, are called clay. Some soils have no dominant particle size, containing a mixture of sand, silt, and humus; these soils are called loams. 5 THE SOLID PHASE Page The solid phase of soil consists of mineral particles and pieces of rock. These are the inorganic component that forms the framework of soil. Inside this framework is the organic component, known as humus. THE INORGANIC COMPONENT OF SOIL (Mineral Particles, 45 %) Soil normally consists of pieces of rock and mineral particles of varying sizes. The rock particles are the remains of rocks that have been transformed by erosion into loose material (called regolith). While the rock particles are all relatively coarse, the mineral particles vary considerably in size. Some are as large as the smallest rock particles, while others are so small that they can only be seen through an electron microscope. The table below explains this: SIZE FRACTION CROSS-SECTION Stone Greater than 75 mm Gravel 2-75 mm Sand 0,05-2,0 mm Silt 0,002-0,05 mm Clay Less than 0,002 mm Some of the minerals in the soil are known as primary minerals because they did not undergo any chemical change during the various weathering processes. Quartz is a primary mineral. Minerals such as clay silicates and iron oxides, which have been formed by chemical changes that have taken place during the weathering of the parent rock, are known as secondary minerals. THE ORGANIC COMPONENT OF SOIL Organic materials are compounds that consist of carbon compounds. They are usually found in the following forms in soils: Decaying materials: this is dead plant material in which the original material, such as leaves or twigs, is still identifiable. Non-humus substances: these are all forms of plant and animal material that have decomposed to such an extent that is no longer possible to identify the original material. Humus: this is part of organic material remaining after the decomposition of the original decaying material, and which is termed humus. NB: the organic content of South African soils is relatively low as a result of favorable temperature 6 conditions which promotes the microbiological breakdown of dead plant material. The organic Page material content will also vary enormously from soil to soil. Although the organic component of soil is relatively small, its influence on the soil and therefore on plant growth is very significant for the following reasons: ▪ Organic matter is almost the only source of nitrogen, which is essential for strong plant growth. Organic matter improves the structure of the mineral component. Soil that is rich in humus has a crumbly consistency. Improving the texture of the soil also improves the ability of the soil to retain water. ▪ It is the only source of energy-rich nutrients that are required by the microorganisms in the soil. These micro-organisms are responsible for breaking down organic matter. Humus colloids, with their large cation adsorption capabilities, play a large role in preventing the leaching of nutrients, particularly cations. THE LIQUID PHASE (SOIL WATER) Soil water is extremely important for higher plants, for two reasons: ▪ Soil water is one of the essential plant growth factors and is therefore extremely important. Not all soil water is equally accessible to plants. Its accessibility is determined, inter alia by water content, clay content, and other factors. ▪ Soil water together with dissolved salts forms the soil solution which serves as an extremely important medium for the provision of nutrients to plants. THE GAS PHASE (SOIL AIR) The composition of the soil air, which occurs particularly in the macro pores of the soil, differs considerably from the composition of the atmosphere. The content and composition of the soil air are mainly determined by the ratio of micro to macro pores in the soil. The soil air usually occurs in the macro pores and the soil water is usually held up in the micro pores. If the micro pores predominate, the soil may have a good water retention capacity, but be poorly aerated. If the macro pores predominate, the soil may be well aerated but have poor water retention capacity. The speed of gaseous exchange is also largely determined by a favorable macro/micropore ratio. Soil compaction and unfavorable structural conditions have a big influence on the micro/macro pores ratio of soils, and thus on soil air content and composition. In addition, encrustation and flooding in soils play an important role in the gaseous exchange speed. 7 Page The oxygen content of soils is particularly important for root activities, and also for micro-organism life. It is thus important for the oxygen content of the soil to be replenished constantly and for the carbon dioxide to be able to diffuse or move out of the atmosphere. Soil air differs from atmospheric air in the following ways: Soil air does not occur evenly and continuously in the soil-it is found only in the pore spaces. Between the pores, there are solid ‘walls’ of soil particles, and for this reason, the composition of the soil air varies from one place in the soil to the other. The composition of atmospheric air varies much less. Soil air usually contains much more moisture than atmospheric air. In very moist soil, the air maybe 100 % saturated with moisture. The percentage of carbon dioxide in soil air is very much higher than in atmospheric air, while the percentage of oxygen is much lower than in atmospheric air. Like the atmospheric air we breathe, soil air contains: Oxygen: approximately 20, 5 % O2 Nitrogen: approximately 79 % N2 Carbon dioxide: 0, 5 % CO2. 8 Page PART 2: SOIL AS A GROWTH MEDIUM FOR PLANTS FUNDAMENTALS OF PLANT GROWTH Growth is a characteristic of a living organism. It is a permanent change that increases the size of the plant. Just like other living organisms, plants also show growth. Growth is an essential property of plants that helps them gain nutrients from places that are far from their position. Growth helps plants compete with each other and also protects their important organs. The growth of plants is determined in the first place, by the generic composition of a specific plant species. This determines the degree to which a plant will develop and yield a crop. No environmental factors, however favorable, can increase the growth of plants and their crop production any further than the generic potential allows. Some of the most important environmental factors influencing plant growth are: ❖ Temperature ❖ Water provision ❖ Radiant energy ❖ Provision of nutrient elements ❖ Gas content of the atmosphere and soil ❖ pH ❖ Biotic factors Factors that determine the growth of plants Plant growth and distribution are limited by the environment. If anyone environmental factor is less than ideal it will become a limiting factor in plant growth. Limiting factors are also responsible for the geography of plant distribution. For example, only plants adapted to limited amounts of water can live in deserts. Most plant problems are caused by environmental stress, either directly or indirectly. Therefore, it is important to understand the environmental aspects that affect plant 9 growth. These factors are light, temperature, water (humidity), and nutrition. Page  LIGHT Light has three principal characteristics that affect plant growth: quantity, quality, and duration. Light quantity refers to the intensity or concentration of sunlight and varies with the season of the year. The maximum is present in the summer and the minimum in winter. The more sunlight a plant receives (up to a point), the better capacity it has to produce plant food through photosynthesis. As the sunlight quantity decreases the photosynthetic process decreases. Light quantity can be decreased in a garden or greenhouse by using shade-cloth or shading paint above the plants. It can be increased by surrounding plants with white or reflective material or supplemental lights. Light quality refers to the color or wavelength reaching the plant surface. Sunlight can be broken up by a prism into respective colors of red, orange, yellow, green, blue, indigo, and violet. On a rainy day, raindrops act as tiny prisms and break the sunlight into these colors producing a rainbow. Red and blue light has the greatest effect on plant growth. Greenlight is least effective to plants as most plants reflect green light and absorb very little. It is this reflected light that makes them appear green. Blue light is primarily responsible for vegetative growth or leaf growth. Red light when combined with blue light, encourages flowering in plants. Fluorescent or cool-white light is high in the blue range of light quality and is used to encourage leafy growth. These lights are excellent for starting seedlings. Incandescent light is high in the red or orange range but generally produces too much heat to be a valuable light source. Fluorescent "grow" lights have a mixture of red and blue colors that attempts to imitate sunlight as closely as possible. They are costly and generally not of any greater value than regular fluorescent lights. Light duration or photoperiod refers to the amount of time that a plant is exposed to sunlight. When the concept of photoperiod was first recognized it was thought that the length of periods of light-triggered flowering. The various categories of response were named according to the light length (i.e., short-day and long-day). It was then discovered that it is not the length of the light period but the length of uninterrupted dark periods that is critical to floral development. The ability of many plants to flower is controlled by a photoperiod. Temperature Temperature influences most plant processes, including photosynthesis, transpiration, respiration, germination, and flowering. As temperature increases (up to a point), photosynthesis, 10 transpiration, and respiration increase. When combined with day length, temperature also affects Page the change from vegetative (leafy) to reproductive (flowering) growth. Depending on the situation and the specific plant, the effect of temperature can either speed up or slow down this transition. Temperature affects the productivity and growth of a plant depending on whether the plant variety is a warm-season or cool-season crop. If temperatures are high and the day length is long, cool- season crops such as broccoli and spinach will bolt rather than produce the desired flower. Temperatures that are too low or high for a warm-season crop will prevent fruit set. Temperatures that are too high for warm-season crops such as pepper or tomato can cause pollen to become inviable and not pollinate flowers. Adverse temperatures also cause stunted growth and poor quality. For example, the bitterness in lettuce is caused by high temperatures. Thermoperiod refers to daily temperature change. Plants produce maximum growth when exposed to a day temperature that is about 5.5 to 8°C higher than the night temperature. This allows the plant to photosynthesize and respire during an optimum daytime temperature and to curtail the rate of respiration during a cooler night. High temperatures cause increased respiration sometimes above the rate of photosynthesis. This means that the products of photosynthesis are being used more rapidly than they are being produced. For growth to occur photosynthesis must be greater than respiration. Low temperatures can result in poor growth. Photosynthesis slows at low temperatures. Since photosynthesis is slowed, growth is slowed and this results in lower yields. Not all plants grow best in the same temperature range. However, in some cases, a certain number of days of low temperatures are needed by plants to grow properly. This is true of crops growing in cold regions of the country. Water (humidity) Water is a primary component of photosynthesis. It maintains the turgor pressure or firmness of tissue and transports nutrients throughout the plant. In maintaining turgor pressure, water is the major constituent of the protoplasm of a cell. Using turgor pressure and other changes in the cell, water regulates the opening and closing of the stomata, thus regulating transpiration. Water also provides the pressure to move a root through the soil. Among water’s most critical roles is that of a solvent for minerals moving into the plant and for carbohydrates moving to their site of use or storage. Its gradual evaporation of water from the surface of the leaf, near the stomata, helps 11 stabilize plant temperature. Page Relative Humidity is the ratio of water vapor in the air to the amount of water the air could hold at a given temperature and pressure expressed as a percent. Warm air can hold more water vapor than cold air. If the amount of water in the air stays the same and the temperature increases the relative humidity decreases. Water vapor will move from an area of high relative humidity to one of low relative humidity. The greater the difference in humidity the faster water will move. The relative humidity in the air space between the cells within the leaf approaches 100%. When the stomata are open water vapor rushes out. As the vapor moves out, a cloud of high humidity is formed around the stomata. This cloud of humidity helps slow down transpiration and cool the leaf. If air movement blows the humid cloud away transpiration will increase as the stomata keep opening to balance the humidity. NUTRITION Many people confuse plant nutrition with plant fertilization. Plant nutrition refers to the needs and uses of the basic chemical elements in the plant. Fertilization is the term used when these materials are supplied to the environment around the plant. A lot must happen before a chemical element supplied in a fertilizer can be taken up and used by the plant. Plants need 18 elements for normal growth. Carbon, hydrogen, and oxygen are found in air and water. Nitrogen, phosphorus, potassium, magnesium, calcium, and sulfur are found in the soil. The latter six elements are used in relatively large amounts by the plant and are called macronutrients. Nine other elements are used in much smaller amounts; these are called micronutrients or trace elements. The micronutrients, which are found in the soil are iron, zinc, molybdenum, nickel, manganese, boron, copper, cobalt, and chlorine. All 18 elements, both macronutrients, and micronutrients are essential for plant growth 1. Nutrient uptake processes Imagine you are a tiny creature trying to move around in the soil. You are surrounded by millions of pores of all sizes and shapes, shaped and blocked by particles of organic matter and minerals. The surfaces of these particles are chemically active, adsorbing ions and organic molecules all around you. You start to learn your way around, but your microenvironment changes with each wet-and-dry cycle and freeze-and-thaw cycle. Sometimes it is not a physical process but a biological one that rearranges the structure of your 12 little world, like a burrowing animal that tunnels through. In short, you live in a constantly changing Page soil ecosystem that has numerous barriers to the movement of organisms and chemicals. In terms of soil fertility, we are particularly interested in the physical component of the soil ecosystem. For a nutrient to be available for the plant to take up it must meet two criteria: 1) it must be in the proper chemical form to pass the root membrane 2) it must be available at the root surface. PLANT NUTRIENT REQUIREMENTS Although it is easier to consider one nutrient at a time, it is important to think of plant needs holistically. Supplying one nutrient while ignoring other plant needs, including other nutrients and environmental factors such as temperature, water, and light, may have little benefit or even be detrimental to the crop. The principle of Limiting Factors developed from a law of agricultural science called Liebig’s Law of the Minimum. Liebig’s Law of the Minimum was formulated in 1843 by a German scientist called Justus von Liebig. As important as Liebig’s contributions are, they do not address the situation holistically. In the above example, for instance, nitrogen that is applied more than what the crop will consume is in danger of being leached into the groundwater, where it will become a pollutant. Also, applying too much of any one nutrient can be injurious. For example, if too much nitrogen is supplied to tomatoes relative to the amount of phosphorus supplied, you may end up with vigorous plants that don’t produce any fruit. One advantage of organic farming and gardening is that natural and organic soil amendment, unlike many synthetic ones, frequently supplies many nutrients at once, including micronutrients. One of the ways we speak to growers about nutrient deficiencies is by using Liebig’s Barrel, which is an illustration based on Liebig’s Law of the minimum. Liebig’s Law of the Minimum was formulated by German scientist Justus von Liebig. It states that if one essential plant nutrient is deficient, plant growth will be poor even when all other essential nutrients are abundant. Liebig’s Law of the Minimum is the scientific underpinning for Liebig’s Barrel visualization below. 13 Page Each stave of the barrel represents a different, essential nutrient. The length of each stave equals the amount of each nutrient in the soil available for plant uptake. If the limiting factor isn’t remedied, then your stave stays shorter and prevents your barrel from holding more water. The barrel shows just how important every nutrient is for a plant’s potential yield. LIEBIG’S BARREL HELPS US TO UNDERSTAND: Identifying the limiting factors of your soil is an essential step in improving yield potential. Increasing the number of plentiful nutrients will not increase potential plant growth. Increasing the amount of the most limited nutrient can improve the potential plant growth. Protect the plentiful nutrients so they don’t become limiting factors. DON’T SHORT YOUR CROP’S POTENTIAL The Law of the Minimum takes on added importance when input costs are high, and when growers may be tempted to reduce or even eliminate applications of micro- or macronutrients. For example, if the soil is deficient in zinc, yields will be depressed regardless of how much other macro or micronutrients you apply. “Determining which element of plant development is the limiting factor can be challenging, so taking soil samples is an important step for growers to make,” said Suarez. “The findings will help to reduce operating costs and improve crop health and productivity.” PART OF THE 4R NUTRIENT STEWARDSHIP Liebig’s Barrel is further proof of the importance of the 4R Nutrient Stewardship framework. By helping determine the right source, rate, time, and place of a plant’s needed nutrients, growers can fine-tune their inputs to protect the nutrients that are already abundant and add only the amounts needed for the deficient nutrients. Practicing good stewardship keeps the staves of the barrel even, leading to increased yield and helping to promote a crop reaching its full potential. Liebig’s Law of the minimum states: “Growth is controlled not by the total of resources available, but by the scarcest resource ( limiting 14 factor)”. Page The principles or laws that help explicate limiting factors in an ecosystem are Liebig’s law of the minimum, Blackman’s law of limiting factors, and Shelford’s law of tolerance. In the law of the minimum, the growth of the population could be regulated by the scarcest resource, not by the resources in abundance. In the law of limiting factor, a biological or an ecological process that depends on multiple factors will tend to have a rate limited by the slowest factor. In the law of tolerance, the survival success of an organism is suggested to depend on a complex set of environmental factors. 15 Page References Eash, N.S., Sauer, T.J., O’Dell B., Odoi, E. 2016. Soil Science Simplified.6th Ed. John Wiley & Sons. New Jersey. Weil, R.R. and Brady, N.C. 2017. The nature and properties of soils.. 15th Ed. Pearsons Education. Thomas M. Smith., Robert Leo Smith. 2009. Or simply, Limiting factors are things that prevent a population from growing large. Elements of Ecology. Pearson International Edition. 7th Ed. 16 Page