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CHEMICAL CHARACTERISTICS OF SOIL:
- A general idea of Soil
- Chemical Composition of Soil
- A general idea of Silica Sesquioxide ratio and its influence
- Cation Exchange and Cation Exchange Capacity
- Soil pH / Soil Acidity
- Saline soil or Salinity
- Alkaline soil or Alkalinity
(from Latin “Solum” which means “ground/ soil”)
A natural body on the surface of the earth, in which plants grow, composed of organic and inorganic matter. It consists of fragmented and partly weathered rocks and minerals, organic matter, water and air in varying proportions. It has more or less distinct layers due to the influence of climate and living organisms.
SOIL SCIENCE (PEDOLOGY):
(From Greek “Pedo” means “soil”)
It the scientific study of soil, its properties, formation and geographic distribution and the classification of soil types.
GENERAL IDEA OF SILICA AND SESQUIOXIDE RATIO AND ITS INFLUENCE:
- The term Silica means the number of Sand particles in a soil
- The term Sesquioxide show the amount of Clay particles in a soil
- The composition of clay fraction varies considerably depending upon the characteristics of the materials and conditions of soil formation.
- Silica and Sesquioxide ratio is the convenient method of expressing the approximate indication of the general soil characteristics.
Influence of Silica:
- The soil tends to be more porous
- Less power of moisture retention
- Contains least capillary and hygroscopic water
- Contains little nutrients
Influence of Sesquioxide:
- The soil tends to be sticky
- Little permeability
- Contains more capillary and hygroscopic water
- Not good for plant growth in case of heavy clay.
- Thus, an intimate mixture of these two in a good proportion is very essential for plant growth.
- From Greek kata “down” + ienai “moving thing”
- Cation (an ion that has a Positive Electric Charge)
- The surfaces of certain soil particles, particularly the clays, hold groupings of atoms known as ions.
- These ions carry a negative charge, called anions including OH– hydroxide, Cl– chloride, Br– bromide, I– iodide, , NO3– nitrate, NO2– nitrite, SO42- sulfate, SO32- sulfite, PO43- phosphate, HPO42- hydrogen phosphate; attract positive ions (called cations).
- Cations, including those from Na+ sodium K+ potassium Fe2+ ferrous Fe3+ ferric Cu+ cuprous Cu2+ cupric NH4+ ammonium H3O+ hydronium, then become attached to the soil particles, in a process known as cation exchange
- A soil’s cation exchange capacity is an important measure of its fertility.
- The chemical reactions in cation exchange make it possible for calcium and the other elements to be changed into water-soluble forms that plants can use for food.
CATION EXCHANGE CAPACITY (CEC):
- It is the sum of the total number of exchangeable cations.
- Its unit is Centimoles of charge per kg (Cmolckg-1)
- (from French pouvoir hydrogène, “hydrogen power”)
- pH = – log10[H+]; the negative logarithm of the concentration of H+ ions
- A measure of acidity or alkalinity in which the pH of pure water is 7, with lower numbers indicating acidity and higher numbers indicating alkalinity.
- The pH of a soil will often determine whether certain plants can be grown successfully
- Blueberry plants, for example, require acidic soils with a pH of roughly 4 to 4.5
- Chir (Pinus roxburghii) requires acidic soil
- Eucalyptus spp, Acacia modesta, Olea ferrugenia, Acacia nilotica, Tamarix spp, Salvadora spp, etc require alkaline soil
Determination of Soil pH:
- Soil pH can be determined by two methods:
- Color dyes / Dye color indicator
- Potentiometric meter
SALINITY and ALKALINITY:
Saline and Alkali soils:
- Soils contain an excess of soluble salts or exchangeable sodium or both
- Such soils occur throughout the arid plains of Pakistan and are important for forestry because all the new irrigated plantations are being raised on them.
- Such soils may automatically be reclaimed by repeated heavy irrigation unless the groundwater is close to surface.
- In initial stages of establishment, however, the choice of spp has at times to be confined to more tolerant species such as Acacia nilotica, Tamarix aphylla, Albizzia lebbek, etc.
- Saline soil can be classified on the basis of electrical conductivity; it is expressed in millimhos/cm.
- Saline and alkaline soils can be classified in the following categories:
- Saline soils/ White Alkali Soils
- Saline Alkali soils/ Saline-Sodic Soils
- Non-saline alkali soils (Sodic soils)
Saline soils/ White Alkali Soils
- Saline soils are soils containing sufficient amount of soluble salts.
- Electrical conductivity (ECe) @ 25OC is ≥ 4 millimhos/cm.
- Exchangeable sodium percent (ESP) is < 15.
- pH is < 8.5.
- Chlorides and Sulphates are the basic anions.
- The defect of such soil is that they contain an excess of soluble salts.
- Such soils can be easily reclaimed by leaching out the salts with irrigation.
Saline Alkali soils/ Saline-Sodic Soils
- Saline Alkali soils are soils containing both soluble salts and exchangeable sodium.
- Electrical Conductivity (ECe) @ 25 OC is = 4.
- Exchangeable sodium percent (ESP) is ≥ 15.
- pH seldom above 8.5 (Depending upon the dominance of salts or sodium)
- They are converted into non-saline alkali soils by leaching out of soluble salts.
Non-saline alkali soils (Sodic soils)
- Non-saline alkali soils are soil containing sufficient amount of exchangeable sodium ions.
- ECe is < 4 millimhos/cm at 25O
- ESP is ≥ 15.
- pH value b/w 8.5 and 10.
- Also called Black Alkali (the dissolved organic matter present in the soil solution often deposited on the soil surface by evaporation giving it the dark color and accounting for the name “black alkali”)
- Due to the adverse effect of excessive Na, the permeability of such soils is very low and low aeration as well.
- This can be reclaimed by deep plowing and establishment of any vegetation that will grow.
- Since the soil of Pakistan contains a large amount of Calcium, so artificial amendments are not necessary, because in the course of time the calcium replaces sodium, and the automatic reclamation of soil occur.
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