AGRICULTURE O LEVEL(FORM FOUR) NOTES – SOIL AND ITS UTILIZATION IN AGRICULTURE

THEME 4.0: SOIL AND ITS UTILIZATION IN AGRICULTURE

Soil fertility is the capacity or ability of the soil to supply the plant nutrients required by crop plants in available and balanced forms. It is also the capacity of soil to produce crops of economic value to humans while maintaining the health of the soil for future use.

Soil is considered fertile when it contains all the required nutrients in the right proportions for luxuriant plant growth. Plants, like animals and humans, require food for growth and development. This food is composed of certain chemical elements often referred to as plant nutrients or plant food elements. These nutrients are obtained from the soil through the roots.

Plants need 16 elements for their growth and completion of their life cycle. In addition to these, four more elements—sodium, vanadium, cobalt, and silicon—are absorbed by some plants for special purposes.

Classification and Source of Nutrients

Four more recognized nutrients are Na, Co, V, and Si.

Basic nutrients (C, H, and O) constitute 96% of the total dry matter of plants. Macro (major) nutrients include primary nutrients—nitrogen (N), phosphorus (P), potassium (K)—and secondary nutrients—calcium (Ca), magnesium (Mg), and sulfur (S)—which are required in large quantities. Micronutrients (trace elements) such as iron (Fe), zinc (Zn), copper (Cu), boron (B), molybdenum (Mo), chlorine (Cl), and manganese (Mn) are required in small quantities.

These trace elements are very efficient; minute quantities produce optimum effects. However, even a slight deficiency or excess of these elements can be harmful to plants.

Soil Reaction

Soil Reaction (pH)

From an agricultural standpoint, pH is important because it strongly affects plant growth, nutrient availability, elemental toxicity, and microbial activity. Soil pH indirectly affects plant growth by influencing the availability of mineral nutrients.

Various mineral nutrients are readily available in varying concentrations depending on the soil pH. At certain pH levels, some mineral nutrients combine with other minerals and become unavailable to plants.

For example, cotton grows best in soil with a pH range of 5.5 to 7.0. If the pH exceeds 7, the availability of some nutrients such as zinc may become limited. This situation often occurs in arid and semi-arid cotton-growing areas where the soil is moderately alkaline (pH 7–8.5) to strongly alkaline (pH > 8.5).

Conversely, if the pH is less than 5, the availability of nutrients such as phosphorus, calcium, magnesium, and molybdenum is very low, limiting plant uptake.

Additionally, some generally insoluble cations, such as iron and aluminum, may be released into the soil solution, reducing plant vigor due to aluminum toxicity, which many plant roots are sensitive to.

Adjusting soil pH often results in the re-adsorption or release of nutrients back into the soil solution. Therefore, pH is considered the single most important diagnostic chemical measurement of soil.

The term pH stands for potential hydrogen, indicating the concentration of H⁺ ions in the soil solution. It is expressed as:

pH = -log (H⁺)

Soil reaction or pH describes the acidity or alkalinity of soil. The pH scale ranges from 0 to 14. Values between 0 and 7 are acidic, with 1.0 being very acidic and 6.0 slightly acidic.

Values between 7 and 14 are basic or alkaline, with higher numbers indicating stronger alkalinity. A pH of 7 is neutral, which is the pH of pure water.

Because the pH scale is logarithmic, each unit change represents a tenfold change in acidity. For example, soil with pH 6 is ten times more acidic than neutral soil (pH 7), and soil with pH 5 is 100 times more acidic than neutral soil.

The typical soil pH range for most soils (about 95%) is between 4 and 10. The distribution of acidic and alkaline soils generally depends on climate.

Acidic soils are most common in areas with high rainfall and good drainage, which favor leaching and biological production of acids. This occurs because exchangeable cations such as calcium, magnesium, potassium, and sodium are leached from the soil.

This leaching process is driven by the introduction of weak carbonic acid into the soil profile in two ways. In the atmosphere, as raindrops form and fall, CO₂ dissolves in rainwater to form carbonic acid:

H₂O (aq) + CO₂ (g) ↔ H₂CO₃ (aq)

This weak acid is harmless to plants and animals but can dissolve rocks like feldspar and limestone over time. In soil solution at pH values above 6, carbonic acid breaks down to release H⁺ ions:

H₂CO₃ ↔ H⁺ (aq) + HCO₃^– (aq)

Excess H⁺ makes the solution acidic, with further release of H⁺ occurring as:

HCO₃^– (aq) ↔ H⁺ (aq) + CO₃^2- (aq)

This explains why water in equilibrium with the atmosphere has a slightly acidic pH of about 5.6 and why naturally acidic soils are found in high rainfall areas.

Carbonic acid is a weak acid and only partially dissociates to release H⁺. It has a significant acidifying effect in alkaline (pH > 7) and neutral soils by releasing plant nutrients and promoting mineral weathering.

Soil organic matter (OM) production is generally greater in high rainfall areas, and carbonic acid formation is catalyzed by microorganisms. Bacteria mediate the oxidation of decaying OM to CO₂. Representing OM as a simple carbohydrate (CH₂O), the overall reaction is:

CH₂O (s) + O₂ (g) → H₂O (aq) + CO₂ (g) ↔ 2H⁺ (aq) + CO₃^2- (aq)

Soil processes such as root respiration and decomposition of soil OM by microbes push these reactions to the right, producing high levels of CO₂.

As a result, percolating water becomes slightly acidic, gradually acidifying the soil by replacing and leaching soluble exchangeable cations (calcium, magnesium, potassium, sodium) with hydrogen ions (H⁺).

Conversely, alkaline soils (e.g., Calcarosol, Vertosol, and Sodosol) are common in arid and semi-arid regions where evapotranspiration exceeds rainfall, favoring retention and accumulation of exchangeable cations.

In these climates, soil alkalinity is influenced by calcium carbonate present in subsoil horizons. Carbonate accumulates from carbonate-rich dust (CaCO₃) initially deposited on the soil surface. Water in precipitation (H₂O) combines with atmospheric and soil CO₂ to form weak carbonic acid (H₂CO₃).

Calcium carbonate at the soil surface reacts with carbonic acid, dissolving CaCO₃ and releasing Ca²⁺ ions, which move deeper into the soil with percolating water:

H₂CO₃ + CaCO₃ (s) ↔ Ca²⁺ (aq) + 2HCO₃^– (aq)

Dry conditions at depth lead to precipitation of secondary carbonates:

Ca²⁺ + 2HCO₃^– + H₂O ↔ CO₂ + CaCO₃ (s)

High pH indicates soil is saturated with exchangeable cations and contains free CaCO₃. Soil profiles rich in carbonate typically have pH values around 8.3.

Carbonates may appear as concretions or diffuse areas. Effervescence with hydrochloric acid (HCl) indicates the presence of carbonates.

Soil pH is measured in the laboratory using a glass electrode pH meter in a soil-liquid system. The liquid is either distilled water or a salt solution (e.g., 0.01 M calcium chloride, CaCl₂) with soil-to-liquid ratios varying from 1:1 (pH1:1) to 1:5 (pH1:5).

CaCl₂ solution is preferred because water dilutes the soil solution excessively, leading to overestimation of pH by about 0.5 units for most soils. In Australia, pH1:5 is commonly determined. Data shown in terraGIS uses the pH1:5 method.

The pH meter measures the electrical potential difference between a glass electrode containing a solution of known pH and the test solution.

To determine soil pH1:5 using the electrometric method, follow these steps:

1. Place 5 grams of soil into a pop-top tube;
2. Add 25 ml of distilled water;
3. Place the sealed pop-top tube on a spinning wheel;
4. Remove the tube after 20 minutes;
5. Measure and record soil pH using a pH meter.

When soil pH is low to moderate, lime is commonly added as it dissolves to form acid-neutralizing constituents (Ca²⁺) and provides a source of calcium.

However, in neutral and alkaline soils, lime is very stable and dissolves slowly. Adding lime in these conditions has little effect on nutrient availability and may reduce the solubility of phosphorus and some micronutrients.

Cation Exchange

Essential soil cations for plant growth include ammonium, calcium, magnesium, and potassium. Three additional soil cations—sodium, aluminum, and hydrogen—are not essential but affect soil pH.

Soil Cations Essential to Plant Growth

• Ammonium
• Calcium
• Magnesium
• Potassium

Soil Cations That Affect Soil pH

• Sodium
• Aluminum
• Hydrogen

Cations have a positive charge and are attracted to negatively charged soil particles. Soils with negative charges attract and retain cations, a property known as cation exchange capacity (CEC). Most soils are negatively charged, though some Hawaiian soils are exceptions.

Soil cations are divided into two categories: base cations (ammonium, calcium, magnesium, potassium, sodium) and acid cations (aluminum and hydrogen).

Base Cations

• Ammonium
• Calcium
• Magnesium
• Potassium
• Sodium

Unlike other base cations, sodium is not essential for all plants. Soils high in sodium can develop salinity and sodicity problems.

Acid Cations

• Aluminum
• Hydrogen

The terms “base” and “acid” refer to the cations’ influence on soil pH. Soils with many acid cations tend to have low pH, while alkaline soils predominantly contain base cations.

Cations compete for sites on the cation exchange capacity. Some cations are held more strongly than others. The order of decreasing holding strength is: aluminum, hydrogen, calcium, potassium and nitrate, sodium.

CEC values vary among soil types, media, and minerals. Soils with high organic matter and moderately weathered clays tend to have high CECs. Highly weathered soils and sandy soils generally have lower CECs due to smaller surface area and reduced nutrient retention.

Source: Brady and Weil. 2002. Elements of the Nature and Properties of Soil. Prentice Hall, New Jersey.

Anion Exchange

In tropical regions, many highly weathered soils have an anion exchange capacity, meaning they attract and retain anions rather than cations. Anions are negatively charged and include phosphate, sulfate, nitrate, and chlorine (in order of decreasing strength).

Soils with anion exchange capacity have a net positive charge and typically contain weathered kaolin minerals, iron and aluminum oxides, and amorphous materials. Anion exchange capacity depends on soil pH and increases as pH decreases.

Base Saturation

Base saturation measures the relative amounts of base cations in soil. It is the percentage of calcium, magnesium, potassium, and sodium cations occupying the total cation exchange capacity. For example, a base saturation of 25% means that 25% of the CEC is occupied by base cations.

If the soil lacks anion exchange capacity, the remaining 75% of CEC is occupied by acid cations such as hydrogen and aluminum. Moderately weathered soils formed from basic igneous rocks, like Hawaiian basalts, tend to have high base saturation. Soil pH increases as base saturation increases.

Highly weathered or acidic soils tend to have low base saturation.

Movement of Nutrients from Soil to Root

Nutrients reach the root surface by three basic methods: root interception, mass flow, and diffusion.

• Root interception: Occurs when a nutrient physically contacts the root surface. Root interception increases with root surface area and mass, allowing plants to explore more soil. Mycorrhizal fungi can enhance root interception by colonizing roots and increasing soil exploration. Root interception contributes significantly to calcium uptake and some magnesium, zinc, and manganese.
• Mass flow: Nutrients are transported to roots by water movement in soil (percolation, transpiration, evaporation). The rate of water flow governs nutrient transport, so mass flow decreases as soil water decreases. Most nitrogen, calcium, magnesium, sulfur, copper, boron, manganese, and molybdenum reach roots by mass flow.
• Diffusion: Nutrients move along a concentration gradient from higher to lower concentration. Diffusion delivers phosphorus, potassium, zinc, and iron to roots. It is slower than mass flow.

Nutrient Uptake into Root and Plant Cells

Before water and nutrients are incorporated into plants, they must first be absorbed by roots.

Uptake of Water and Nutrients by Roots

• Root hairs and the root surface are major sites of water and nutrient uptake.
• Water enters roots by osmosis and capillary action.
• Soil water contains dissolved particles called solutes, including nutrients. Osmosis is the movement of water from areas of low solute concentration to high solute concentration, essentially diffusion of water.
• Capillary action results from water’s adhesion to solids and cohesion to other water molecules, enabling water to move upward against gravity into plants.
• Nutrient ions enter roots by diffusion and cation exchange.
• Diffusion moves ions along concentration gradients.
• Cation exchange occurs when nutrient cations are attracted to negatively charged root cortex cells. The root releases hydrogen ions during this process, lowering soil pH near the root.
• Water and nutrients move through spaces between root cells.
• They are transported into the xylem, which conducts water and nutrients throughout the plant.

Once in the xylem, water and nutrients are transported to parts of the plant where they are needed.

Absorption of Nutrients into Plant Cells

• Plant cells have barriers (plasma membrane and tonoplast) that selectively regulate water and nutrient movement.
• These barriers are permeable to oxygen, carbon dioxide, and certain compounds.
• They are semi-permeable to water.
• They are selectively permeable to inorganic ions and organic compounds like amino acids and sugars.
• Nutrient ions cross these barriers by active or passive transport.
• Passive transport is diffusion along a concentration gradient and requires no energy.
• Active transport moves nutrients against concentration gradients and requires energy.