ITEL Week 8 RT10303 Chemistry for Agriculture PDF

# RT 10303 CHEMISTRY FOR AGRICULTURE ## The Chemical Properties Of Soil On completion of this chapter, students will be able to understand the soil composition and the chemical properties of soils. ## Soil Composition * Soil chemistry is a branch of soil science that deals with the chemical com...

# RT 10303 CHEMISTRY FOR AGRICULTURE ## The Chemical Properties Of Soil On completion of this chapter, students will be able to understand the soil composition and the chemical properties of soils. ## Soil Composition * Soil chemistry is a branch of soil science that deals with the chemical composition, chemical reactions and chemical properties in soils. * Abiotic phases of soil include solids (organic matter and inorganic minerals), liquids (soil water), and gases (soil air), while the biotic phase consists of living organisms. ### Soil Composition * Soil essentially consists of three phases: a solid phase, a solution phase and a gas phase. * The solid phase usually includes an intimate mixture of mineral material, originating from rock, sediment or till, and organic material arising as a consequence of biological activity. * The solution phase, this interacts continuously with the solid phase. It originates infiltrating the soil or from rising water or water moving laterally. * The gas phase or soil atmosphere composition depends upon biological activity. ### Soil Component * **SAND** * Made up of weathered primary rock minerals. * The particles are irregular in outline. * They are large and so do not pack together easily. * Large pore spaces in between. * Air gets in very easily and water flows rapidly through it. * **SILT** * Silt has smaller particles compared to sand. * When dry, silt feels floury and has a smooth texture. When wet, it feels slippery or greasy. * Silt soil is prone to washing away with rain because of its fine particles. * **CLAY** * Clay particles are the smallest of the soil types, measuring less than 0.002 mm. * Clay soil is sticky when wet and smooth when dry. * Clay soil stores water well and can remain wet for long periods. * Clay soil is the densest and heaviest type of soil. ### Soil Component * **JAR TESTING FOR SOIL TYPE** * **SAND:** 0-10% clay; 0-10% silt; 80-100% sand. * **LOAM:** 10-30% clay; 30-50% silt; 25-50% sand. * **CLAY:** 50-100% clay; 0-45% silt; 0-45% sand. ### Soil Component * **WATER** * Water can comprise approximately 25% of the soil volume. * Important for transporting nutrients to growing plants and soil organisms and facilitating biological and chemical decomposition. * The capacity of soil to hold water is mainly dependent on soil texture. * The more small particles in the soil the more water the soil can retain. Thus, clay soils have the greatest water-holding capacity and sands the least. * Permanent wilting point (PWP) or wilting point (WP) is defined as the minimum amount of water in the soil that the plant requires not to wilt. ### Soil Component * **AIR** * Oxygen is essential for root and microbe respiration, which helps support plant growth. * Carbon dioxide and nitrogen are important for belowground plant functions such as for nitrogen-fixing bacteria. * If soils remain waterlogged, it can prevent root gas exchange leading to plant death, which is a common concern after floods. ### Soil Compaction | Normal Soil | Compacted Soil | | ----------------- | --------------------- | | Air & Water | No Moisture and Air | | Percolation Allow | No Percolation Allow | ### Soil Component * **ORGANIC MATTER** * Organic matter is derived from dead plants and animals, and as such, has a high capacity to hold onto and provide the essential elements and water for plant growth. * Soils high in organic matter also have a high CEC and are, therefore, generally some of the most productive for plant growth. * Organic matter also has a very high “plant-available” water-holding capacity, which can enhance the growth potential of soils. * Benefits: Nutrient supply, water holding capacity, soil structure aggregation, erosion prevention ### Soil Layers | Layer | Description | | ------------- | ----------------------------------------------------------------------------------------- | | **A:** Surface | Organic matter | | **B:** Subsoil | Organics mixed with mineral matter | | **C:** Parent rock | Mixture sand, silt or clay | | **R:** Bedrock | Unweathered parent material | ## Physico-Chemical Properties In Soil Soil quality may include a capacity for water retention, carbon sequestration, plant productivity, waste remediation, and other functions or it may be defined more narrowly. | Feature | Description | | ------------- | ----------------------------------------------------------------------- | | **Texture** | Proportion of the soil particles and the fineness or coarseness of a soil | | **Temperature** | How warm or cold the soil is | | **SOM** | Soil Organic Matter | | **pH** | How acidic or alkaline the soil is | | **Moisture** | How much water the soil holds | | **EC** | Electrical conductivity of the soil water | | **Nitrogen** | Amount of nitrogen in the soil | | **Phosphorus** | Amount of phosphorus in the soil | | **Potassium** | Amount of potassium in the soil | ### Soil Horizons | Horizon | Description | | --------- | ------------------------------------------------------------------------------------------------------------------------------- | | **O** | Organic Layer (humus) - The top layer of soil, composed of decayed plant and animal matter, providing nutrients and structure. | | **A** | Topsoil - A mineral layer mixed with organic matter, often darker in color than the B horizon. | | **E** | Eluviation Layer - Minerals and organic matter leached from the A horizon, creating a lighter-colored layer. | | **B** | Subsoil - Minerals deposited from the A horizon, often denser and compacted than the A horizon. | | **C** | Parent Rock - Partially weathered bedrock, providing the source material for the soil above. | | **R** | Bedrock - Unweathered bedrock, the base of the soil profile. | ## The O Horizon The O horizon, also known as the organic horizon, has many functions in the soil profile including: * **Nutrient cycling:** The O horizon is a site where nutrients like carbon, nitrogen, phosphorus, and sulfur are recycled back into the soil. * **Organic material decomposition:** The O horizon is home to a thriving ecosystem of decomposing bacteria, worms, and other organisms that break down organic material. * **Plant growth:** The O horizon provides nutrients for plant growth and creates the conditions for seed germination and root development. * **Habitat for microorganisms:** The O horizon provides a habitat for microorganisms like earthworms and fungi. ## The A Horizon The A horizon, also known as topsoil, has many functions in a soil profile including: * **Providing nutrients:** The A horizon is rich in organic matter and microorganisms, making it one of the most productive and nutrient-rich layers in soil. * **Supporting plant growth:** The A horizon is where most plant roots grow, and it provides a favorable environment for root development. * **Facilitating seed germination:** The A horizon is soft and porous, allowing for enough air and water for seeds to germinate and new roots to grow. * **Supporting microorganisms:** The A horizon is home to microorganisms such as bacteria, fungi, and earthworms. ## The B Horizon The B horizon, also known as the subsoil, has several functions in the soil profile including: * **Accumulation:** the B horizon is a zone of accumulation or illuviation, where materials like clay, iron, and soluble salts are deposited . * **Water storage:** the B horizon holds more water than the topsoil due to the presence of clay and iron minerals. * **Erosion:** Erosion can sometimes expose the B horizon at the surface, especially in sloping, agricultural landscapes. Bulldozers and other landscaping can also expose the B horizon. ## The R Horizon The R horizon, also known as bedrock, is the bottom layer of a soil profile and serves several functions: * **Represents original material:** The R horizon is the original geological material from which the soil formed. It's relatively unaffected by biological activity and is often unweathered. * **Influences soil composition:** The type of bedrock in the R horizon can affect the mineral composition of the soil above it. * **Marks the boundary:** The R horizon is the boundary between the soil and what lies beneath it. ## pH * Soil pH is a measure of soil acidity or alkalinity. * It is an important soil property that affects plant suitability, nutrient availability, soil microorganism activity, chemical cycling, and mobility of pollutants such as metals. * If the pH is less than 6, then it is said to be acidic soil, the pH range from ­6­-­7.5, it's normal soil and greater than 7.5, then it is said to be alkaline soil. * When the pH is outside a desirable range, the soil pH can be altered through amendments such as lime to raise the pH. Ammonium sulfate, iron sulfate, or elemental sulfur can be added to soil to lower pH. ## Moisture * Soil moisture is the amount of water in the active layer of the soil typically the top 1-2 m. * It is extremely important because it is the main water source for agriculture and natural vegetation. * The amount of soil moisture in a particular area depends on several factors including topography, land cover, land surface temperature and other climatic parameters. ## Temperature * Soil temperature depends on the ratio of the energy absorbed to that lost. * Soil has a temperature range between ­-­20 to 60 °C. * The temperature of the soil is the most important property because it shows its effect on the chemical, physical and biological processes related to growth of plants. * Soil temperature changes with season, time of day, and local conditions of climate. ## Electrical Conductivity (EC) * Soil EC is a measure of how well soil water can conduct an electrical current. * It's a soil quality indicator that can be used to estimate a variety of soil properties. * Soil contains ions: cations (positively charged ions) and anions (negatively charged ions). * The attraction of these opposingly charged particles allows soil to transmit a measurable electrical current. * EC is expressed in deciSiemens per meter (dS/m) or milliSiemens per meter [mS/m]. * It is a measure of ions present in solution. * The electrical conductivity of a soil solution increases with the increased concentration of ions. ## **Cation Exchange Capacity (CEC)** * CEC is a measure of the amount of negative charge capable of adsorbing cations on mineral or organic surfaces, usually expressed in centimoles of charge [cmol[+]] per kg of soil. * CEC is an important soil property that indicates a soil's fertility and ability to supply nutrients to plants. * Cations are held by negatively charged particles of clay and humus called Colloids. Colloids consist of thin, flat plates and for their size have a comparatively large surface area. For this reason they are capable of holding enormous quantities of cations. They act as a storehouse of nutrients for plant roots. * As plant roots take up cations, other cations in the soil water replace them on the colloid. If there is a concentration of one particular cation in the soil water, those cations will force other cations off the colloid and take their place. * The stronger the colloid’s negative charge, the greater its capacity to hold and exchange cations, hence the term cation exchange capacity (CEC). ## Relationship Between CEC And Fertilization Practices * Soils with high CEC retain more nutrients than soils with low CEC. * Soils with high CEC change pH more slowly than soils with low CEC. This means that high CEC soils don’t need to be limed as often, but they do need higher rates of lime to reach the right pH. * Soils with low CEC are more likely to lose nutrients through leaching, especially when large amounts of fertilizer are applied in one go. * The timing and frequency of fertilizer applications depends on the soil’s CEC. For example, on low CEC soils, it’s better to apply fertilizer in the spring to avoid leaching. On high CEC soils, it’s possible to apply fertilizer in the fall. ## Essential Plant Nutrients | Macronutrients | Secondary Macronutrients | Micronutrients | | ---------------- | ------------------------- | --------------- | | Nitrogen (N) | Calcium (Ca) | Boron (B) | | Phosphorus (P) | Magnesium (Mg) | Chlorine (Cl) | | Potassium (K) | Sulfur (S) | Manganese (Mn) | | | | Iron (Fe) | | | | Nickel (Ni) | | | | Copper (Cu) | | | | Zinc (Zn) | | | | Molybdenum (Mo) | | | | Hydrogen (H) | | | | Carbon ( C) | | | | Oxygen (O) | ## **Soil Organic Matter (SOM)** Also known as Soil Organic Carbon (SOC) * Organic material is the fraction of the soil that includes approximately by weight, according to USDA-NRCS, 2014: * 5% of living organisms * 10% crop residues * 33-50% decomposing organic matter (the active fraction) * 33-50% stable organic matter (humus) * Improves soil structure, making it more friable and improving workability/trafficability * Improves infiltration, water holding capacity and drainage * Reduces the risk of capping, compaction, and erosion * Enhances drought resistance * Adds to cation exchange capacity when well broken down (humus) * Buffers pH during decomposition and stabilisation * Contains and supplies nutrients * Acts as a long-term store for carbon added via the plant-soil system and organic matter applications * Provides a food source for soil organisms ## Soil Salinity * Soil salinity refers to the accumulation of soluble salts in the soil to levels that negatively affect plant growth, soil structure, and overall agricultural productivity. * A saline soil is generally defined as one in which the electrical conductivity (EC) of the saturation extract (ECe) in the root zone exceeds 4 dS m-1 (approximately 40 mM NaCl) at 25 °C and has an exchangeable sodium of 15%. ### **Impact:** * Excess of salts adversely affects soil structure and fertility, plant growth, crop yield, and microorganisms. ### Salinity: Causes * **Natural causes:** * Weathering: The breakdown of parent rocks. * Rainwater: Salt in rainwater can come from the ocean. * Prehistoric flooding: Oceans have flooded in prehistoric times, but only in certain areas. * Dry climates: Low precipitation and high evaporation rates can contribute to soil salinization. * **Human-made causes:** * Irrigation: Using saline water for irrigation, or over-irrigating. * Drainage: Poor drainage systems. * Fertilizers: Excessive use of chemical fertilizers. * Deforestation: Removing trees from an area. * Groundwater: Extracting too much groundwater. ### Salinity and Plant Growth * Excess soil salinity causes poor and spotty stands of crops, uneven and stunted growth and poor yields, the extent depending on the degree of salinity. * Soil salinity has a significant impact on crop production by making it difficult for plants to access water and nutrients, which can lead to impaired growth and decreased yields: * **Dehydration:** When there's too much salt in the soil water, water can flow out of the plant roots and back into the soil. This can lead to dehydration, which can cause a decline in yield or even death. * **Toxic ions:** Some ions, especially chloride, can be toxic to plants. As the concentration of these ions increases, the plant can become poisoned and die. * **Nitrogen uptake:** Salinity can interfere with nitrogen uptake, which can reduce growth and stop plant reproduction. * **Oxidative and osmotic stress:** Salinity can cause oxidative and osmotic stress. * **Increased susceptibility to diseases:** Salinity can increase a plant’s susceptibility to diseases. ## Soil Salinity: Solutions * **Leaching:** Apply large amounts of water to dilute and wash out salts from the soil profile. This method is especially effective in post-mining landscapes. * **Water management:** Improve drainage, lower the water table, and promote downward movement of salts. * **Phytoremediation:** Grow salt-tolerant plants in the saline soil. * **Microbes:** Use microbes to promote crop development and production. * **Biochar:** Apply biochar to improve stomatal conductance and reduce abscisic acid (ABA) levels in crops.