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THEME 4.0: SOIL AND ITS UTILIZATION IN AGRICULTURE
Soil fertility: is the capacity/ability of the soil to supply the plant nutrients required by the crop plants in available and balanced forms. Or it is the capacity of soil to produce crops of economic value to man and maintain the health of the soil for future use.
The soil is said to be fertile when it contains all the required nutrients in the right proportion for luxuriant plant growth. Plants like animals and human beings 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 soil through roots.
Plants need 16 elements for their growth and completion of life cycle. In addition to these, 4 more elements viz. sodium, vanadium, cobalt and silicon are absorbed by some plants for special purposes.
Classification and source of nutrients:
Class | Nutrient | Source |
Basic | Air and water | |
Macro | N, P, K, Ca, Mg, S | Soil |
Micro | Fe, Mn, Zn, Cu, B, Mo & CI | Soil |
Four more recognized nutrients are NA, Co, VA & SI.
Basic nutrients (C, H, and O) constitute 96% of total dry matter of plants. Macro (Major) nutrients (primary-N, P, K, and secondary-Ca, Mg, S) are required in large quantities while
Micro nutrients (Trace elements-Fe, Zn, Cu, B, Mo, Cl, and Mn) are required in small quantities.
These trace elements are very efficient and minute quantities produce optimum effect. On the other hand, even a slight deficiency or excess is harmful to plants.
Soil reaction
Soil Reaction (pH)
From an agricultural standpoint p
H is important because it strongly affects plant growth, nutrient availability, elemental toxicity and microbial activity. In an agricultural sense, soil pH indirectly affects plant growth.
H is important because it strongly affects plant growth, nutrient availability, elemental toxicity and microbial activity. In an agricultural sense, soil pH indirectly affects plant growth.
This is because various mineral nutrients are readily available in varying concentrations depending on the pH of soil. At certain pH levels, certain mineral nutrients remain with other minerals and are unavailable to the plant.
For example and with respect to cotton, soil pH should be in the range of 5.5 – 7.0. If the pH is greater than 7, the availability of some nutrients such as zinc may become limiting. This may be the case in the arid and semi-arid cotton growing areas where the soil is moderately- (i.e. pH 7-8.5) to strongly-alkaline (i.e. > 8.5).
Conversely, if pH is less than 5, the availability of some nutrients such as phosphorus, calcium, magnesium and molybdenum is very low and so plant uptake is limited.
In addition, some generally insoluble cat ions (e.g. iron and aluminum) may be released into the soil solution. The result will be reduced plant vigor owing to the sensitivity of many plant roots to aluminum toxicity.
Adjustment of soil pH will often result in the re-adsorption or release of the nutrient back into soil solution. It is therefore argued that pH is the single most important diagnostic chemical measurement of soil.
The term pH is short for potential hydrogen. This is because pH indicates the concentration of H+ activity in the soil solution. It is expressed as follows:
pH = -log (H+)
Soil Reaction or pH describes the acidity or alkalinity of a soil. The pH scale ranges from 0 -14. Values between 0 and 7 are said to be acidic with a pH value of 1.0 being very acid and a pH value of 6.0 said to be slightly acid.
Values between 7 and 14 are said to be basic or alkaline; whereby a larger number indicates stronger alkalinity. The value of 7 is the midpoint of the scale and is neutral. The pH of pure water is neutral.
Because the pH scale is logarithmic, going down the scale from a pH value of 7, each number is ten times (x10) more acid than the one before. For example, soil pH of 6 is x10 more acid than neutral (i.e. pH 7). Further, a soil with a pH of 5 is x100 more acid than neutral (pH 7). In other words the more hydrogen ions there are the more acid is the soil.
The range of soil pH (i.e. 95 % of most soil) is generally between 4 and 10. The distribution of acid and alkaline soil is in general a function of climate.
Acidic soil is most common where rainfall is high and free drainage favors leaching and biological production of acid. This is because most of the exchangeable cat ions of calcium, magnesium, potassium and sodium are leached. This process occurs because of the introduction of a weak (i.e. carbonic acid) into a soil profile in one of two ways.
In the atmosphere, as raindrops form and fall through the air, CO2 dissolves in the rainwater to form carbonic acid:
H2O (aq) + CO2 (g) ↔ H2CO3 (aq)
This weak acid is harmless to plants and animals, but over a prolonged period of time it is able to dissolve rocks, like feldspar and limestone. This is because in soil solution and at pH values above 6, the carbonic acid quickly breaks down to liberate H+ ions as follows:
H2CO3 ↔ H+ (aq) + HCO3- (aq)
Now there is an excess of H+ so this dilute solution is acidic with additional release of H+ occurring as follows:
HCO3- (aq) ↔ H+ (aq) + CO32- (aq)
This is the reason why in equilibrium with the atmosphere water has a slightly acidic pH of 5.6. This is also the reason why naturally acidic soil types are found in high rainfall areas.
Carbonic acid is a weak acid is only partially dissociated to release H+, whilst a strong acid is almost completely dissociated. Carbonic acid is too weak to dissociate much at pH < 5 but has significant acidifying effect in alkaline (pH > 7) and neutral soil where it can release plant nutrients and promote mineral weathering.
In addition and because soil organic matter (OM) production is generally greater in high rainfall areas, carbonic acid is catalyzed by micro-organisms.
Here bacteria mediate oxidation of decaying OM to CO2. This oxidation reaction can be considered by representing OM by a simple carbohydrate (CH2O). The overall reaction is as follows:
CH2O (s) + O2 (g) → H2O (aq) + CO2 (g) ↔ 2H+ (aq) + CO32- (aq)
Soil processes that push these reactions to the right include root respiration and decomposition of soil OM by microbe
s. In both cases high levels of CO2 are produced.
s. In both cases high levels of CO2 are produced.
As a consequence, the percolating water is slightly acidic and this gradually results in the acidification of soil. This is because the percolating water replaces and leaches soluble exchangeable cat ions (i.e. calcium, magnesium, potassium and sodium) out of the soil profile. The exchangeable cat ions have been replaced with hydrogen ions (i.e. H+).
Conversely, alkaline soil (e.g. Calcarosol, Vertosol and Sodosol) is synonymous with arid and semi-arid landscapes where evapotranspiration exceeds rainfall, which favors retention and accumulation of exchangeable cations.
In these climatic regions the alkaline nature of the soil is a function of the calcium carbonate present in subsoil horizons.
The carbonate accumulates from carbonate rich dust (CaCO3) which is initially blown onto the soil surface. As described above water in precipitation (H2O) combines with atmospheric and soil carbon dioxide (CO2) to make weak carbonic acid (H2CO3).
Calcium carbonate in and at the soil surface reacts with the carbonic acid. This dissolves CaCO3 releasing Ca2+ (aq) making it mobile with both ions translocates deeper into the soil profile 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 therefore indicates the soil is fully saturated with exchangeable cations and free CaCO3 is present in the soil. Soil profiles high in carbonate have pH values of approximately 8.3.
Carbonates (e.g. CaCO3) may be in the form of concretions or diffuse areas. Effervescence with hydrochloric acid (HCl) suggests the presence of carbonates.
In order to measure pH, laboratory methods require the use of a glass electrode pH meter in a soil/liquid system of varying proportions. The liquid is either distilled water or a range of salts (e.g. 0.01 M Calcium Chloride – CaCl2) with the soil: water ratio varying from 1:1 (i.e. pH1:1) to
1:5 (i.e. pH1:5).
A CaCl2 solution is preferred (i.e. pHCaCl2) as water excessively dilutes the soil solution and therefore leads to an overestimation of pH by about 0.5 for most soil types. In Australia, however, soil pH1:5 is usually determined. All data shown in terraGIS has been determined using the pH1:5 methods.
Regardless of the method used, the extract is measured using a pH meter, which displays the electrical potential difference between the glass electrode containing a solution of a known pH and the test solution.
In order to determine soil pH1:5, using the electrometric method, the following steps are undertaken:
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 onto spinning wheel;
4. Remove pop-top tube after 20 minutes; and,
5. Measure and record soil using a pH meter.
Soil Unit Very low Low Moderate High Very High pH 8.3
When soil pH is very low to moderate, lime is commonly added since it will dissolve to form acid-neutralizing constituents (i.e. Ca2+) and also provide a source of Ca.
However, in neutral and alkaline soils, lime is very stable and will not rapidly dissolve. Adding lime in these conditions will do very little to improve nutrient availability and may even further reduce the solubility of phosphorus (P) and some micro nutrients.
Cation exchange
The “soil cations” essential for plant growth include ammonium, calcium, magnesium, and potassium. There are three additional “soil cations”, which are not essential plant elements but affect soil pH. The additional “soil cations” include sodium, aluminum and hydrogen.
Soil cations that is essential to plant growth
∙ Ammonium
∙ Calcium
∙ Magnesium
∙ Potassium
Soil cations that affect soil pH
∙ Sodium
∙ Aluminum
∙ Hydrogen
The major distinguishing characteristic of cations is their positive charge. Just like a magnet, a positive charge is strongly attracted to a negative charge. When soil particles have a negative charge, the particles attract and retain cations. These soils are said the have a cation exchange capacity. Although most soils are negatively charged and attract cations, some Hawaii soils are exceptions as we will see.
The “soil cations” are further divided into two categories. Ammonium, calcium, magnesium, potassium, and sodium are known as the “base cations”, while aluminum and hydrogen are known “acid cations”.
Base Cations
∙ Ammonium
∙ Calcium
∙ Magnesium
∙ Potassium
∙ Sodium
Unlike the other base cations, sodium is not an essential element for all plants. Soils that contain high levels of sodium can develop salinity and solidity problems.
Acid Cations
∙ Aluminum
∙ Hydrogen
The words “base” and “acid” refer to the particular cation’s influence on soil pH. As you might suspect, a soil with a lot of acid cations held by soil particles will have a low pH. In contrast, a highly alkaline soil predominately consists of base cations.
Cations in the soil compete with one another for a spot on the cation exchange capacity. However, some cations are attracted and held more strongly than other cations. In decreasing holding strength, the order with which cations are held by the soil particles follows: aluminum, hydrogen, calcium, potassium and nitrate, and sodium.
CEC values of various soil type, media, and minerals. Soils which have high amounts of organic matter and moderately weathered clays tend to have high CECs. As soils become highly weathered, the CEC of the soil decreases. Sandy soils, too, generally have lower CEC values. This is due to the lesser surface of sandy particles in comparison with clay minerals, which decreases the ability of sand particles to hold and retain nutrients.
Source: Brady and Well. 2002. Elements of the Nature and Properties of Soil. Prentice Hall, New Jersey.
Anion exchange
In the tropics, many highly weathered soils can have an anion exchange capacity. This means that the soil will attract and retain anions, rather than cations. In contrast to cations, anions are negatively charged. The anions held and retained by soil particles include phosphate, sulfate, nitrate and chlorine (in order of decreasing strength). In comparison to soils with cation exchange capacity, soils with an anion capacity have net positive charge. Soils that have an anion exchange capacity typically contain weathered kaolin minerals, iron and aluminum oxides, and amorphous materials. Anion exchange capacity is dependent upon the pH of the soil and increases as the pH of the soil decreases.
Base Saturation
Base saturation is a measurement that indicates the relative amounts of base cations in the soil. By definition, it is the percentage of calcium, magnesium, potassium and sodium cations that make up the total cation exchange capacity. For example, a base saturation of 25 % means that 25 % of the cation exchange capacity is occupied by the base cations. If the soil does not exhibit an anion exchange capacity, the remainder 75 % of the CEC will be occupied by acid cations, such as hydrogen and aluminum. Generally, the base saturation is relatively high in moderately weathered soils that formed from basic igneous rocks, such as the basalts of Hawaii. The pH of soil increases as base saturation increases.
In contrast, highly weathered and/or acidic soils tend to have low base saturation.
Movement of nutrient from soil to root
There are three basic methods in which nutrients make contact with the root surface for plant uptake. They are root interception, mass flow, and diffusion.
∙ Root interception: Root interception occurs when a nutrient comes into physical contact with the root surface. As a general rule, the occurrence of root interception increases as the root su
rface area and mass increases, thus enabling the plant to explore a greater amount of soil. Root interception may be enhanced by mycorrhizal fungi, which colonize roots and increases root exploration into the soil. Root interception is responsible for an appreciable amount of calcium uptake, and some amounts of magnesium, zinc and manganese.
rface area and mass increases, thus enabling the plant to explore a greater amount of soil. Root interception may be enhanced by mycorrhizal fungi, which colonize roots and increases root exploration into the soil. Root interception is responsible for an appreciable amount of calcium uptake, and some amounts of magnesium, zinc and manganese.
∙ Mass flow: Mass flow occurs when nutrients are transported to the surface of roots by the movement of water in the soil (i.e. percolation, transpiration, or evaporation). The rate of water flow governs the amount of nutrients that are transported to the root surface. Therefore, mass flow decreases are soil water decreases. Most of the nitrogen, calcium, magnesium, sulfur, copper, boron, manganese and molybdenum move to the root by mass flow.
∙ Diffusion: Diffusion is the movement of a particular nutrient along a concentration gradient. When there is a difference in concentration of a particular nutrient within the soil solution, the nutrient will move from an area of higher concentration to an area of lower concentration. You may have observed the phenomenon of diffusion when adding sugar to water. As the sugar dissolves, it moves through parts of the water with lower sugar concentration until it is evenly distributed, or uniformly concentrated. Diffusion delivers appreciable amounts of phosphorus, potassium, zinc, and iron to the root surface. Diffusion is a relatively slow process compared to the mass flow of nutrients with water movement toward the root.
Nutrient Uptake into the root and plant cells
Before both water and nutrients are incorporated into plants, both must first be absorbed by plant roots.
Uptake of water and nutrients by roots
∙ Root hairs, along with the rest of the root surface, are the major sites of water and nutrient uptake.
∙ Water moves into the root through osmosis and capillary action.
∙ Soil water contains dissolved particles, such as plant nutrients. These dissolved particles within soil water are referred to as solute. Osmosis is the movement of soil water from areas of low solute concentration to areas of high solute concentration. Osmosis is essentially the diffusion of soil water.
∙ Capillary action results from water’s adhesive (attraction to solid surfaces) and cohesion (attraction to other water molecules). Capillary action enables water to move upwards, against the force of gravity, into the plant water from the surrounding soil.
∙ Nutrient ions move into the plant root by diffusion and cation exchange.
∙ Diffusion is the movement of ions along a high to low concentration gradient.
∙ Cation ion exchange occurs when nutrient cations are attracted to charged surface of cells within the root, called cortex cells. When cation exchange occurs, the plant root releases a hydrogen ion. Thus, cation exchange in the root causes the pH of the immediately surrounding soil to decrease.
∙ Once water and nutrient ions enter the plant root, they move though spaces that exist within the root tissue between neighboring cells.
∙ Water and nutrients are then transported into the xylem, which conducts water and nutrients to all parts of the plant.
Once water and nutrients enter the xylem, both can be transported to other parts in the plant where the water and nutrients are needed. The basic outline of how nutrient ions are absorbed by plant cells follows.
Absorption of nutrients into plant cells
∙ Plant cells contain barriers (plasma membrane and tonoplast) that selectively regulate the movement of water and nutrients into and out of the cell. These cell barriers are:
∙ Permeable to oxygen, carbon dioxide, as well as certain compounds.
∙ Semi-permeable to water.
∙ Selectively permeable to inorganic ions and organic compounds, such as amino acids and sugars.
∙ Nutrient ions may move across these barriers actively or passively
∙ Passive transport is the diffusion of an ion along a concentration gradient. When the interior of the cell has a lower concentration of a specific nutrient than the outside of the cell, the nutrient can diffuse into the cell. This type of transport requires no energy.
∙ Active transport is the movement of a nutrient ion into the cell that occurs against a concentration gradient. Unlike passive transport, this type of movement requires energy.
