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Chemical Properties of Soil |
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Chemical Properties of Soil
Soil is the medium from which plants normally derive their nutrients. The nutrients are carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), sulphur (S), potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), molypdenum (Mo), boron (B), and chlorine (Cl). Off these sixteen elements, Fe, Cu, Mo, B and Cl are considered as micronutrients since they are required in trace amounts for plan growth and the remaining ones are classed as macronutrients because they are required in large quantities.
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The three main components of the soil which provide nutrients for plants growth are the organic matter, the derivatives of parent rock materials and the clay fraction. The nutrients are first released into the soil solution (soil water) before they get transferred into the root system of plants.The organic matter in soils is a potential source of N, P and S for plant growth. Microbiological decomposition of organic matter is an essential step to release the bound nutrients in organic residues into an available form.
The inorganic minerals come from rocks (sand and silt) which are made available to plants by mineral decomposition. On the other hand, the clay fraction of the soil provides secondary minerals and amorphous materials which are different form those derived fro sand and silt. They are clay minerals and hydrous from those derived from sand and silt. They are clay minerals and hydrous oxides.
The clay minerals are composed of three mineralogical types―kaolinite, Montmorillonite and illite. The hydrous oxides are compounds of iron and aluminium. Kaolinite has a crystalline structure composed of two-layer uni cells, one layer of which is made of aluminium and oxygen octahedral (eight-sided) and the other layer made of silicon and oxygen tetrahedra (four-sided) with silica and aluminium in a 1 : 1 ratio.
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The two layers are help together by common oxygen-hydroxyl linkage into a rigid structure that does not expand when wet and hence prevent the exchange of available cations. Montmorillonite has a units comprising of one layer of silicon and oxygen tetrahedra, another layer of aluminium and oxygen tetrahedra and one more layer of silicon and oxygen and hydroxyl bonds. Together, the silica and alumina are in a 2 : 1 ratio.
Between the two units lies a space that expands when wet accommodating cations in an exchangeable dorm that are available for absorption of plants. Illite is similar in structure to Montmorillonite and does not expand when wet but has the capacity to fix potassium (K+) between nonexpanding plates.
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The colloidal clay in soil acts as a large anion and absorbs cations. Having absorbed the cations, the clay particles act as reservoirs of exchangeable ions and release them for plant nutrition as and when required. Thus, the cation exchange or the base exchange properties of soil determine soil fertility and hence plant nutrition. The exchangeable cations are hydrogen, calcium, magnesium, potassium, sodium, ammonium, manganese, zinc, copper and aluminium.
These ions possess different energies by which they are held to the solid phase of soil. The binding energy of ions also determines the ease wuth which the ions may be exchanged in soil. In fact, the property of base exchange in soils enables ions to be held in a readily available state for plant nutrition and at the same time prevents the loss of nutrients by leaching.
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There are both positively and negatively charged ions in the soil solution. The positively charged ions are known as cations (K+, Mg++, Ca++, Fe+++, Mn++, Zn++ and Cu++) and the negatively charged ones are known as anions (NO3-, H2PO4-, SO4--, CI-, HB44O7-and HMoO4-). The ions by roots and the liberation of ions from soil particles are two independent processes whose common of ions from soil particles are two independent processes whose common poll is the soil solution.
Transfer of ions into roots involves the exchange of cations. Likewise, anions are absorbed by roots in exchange of OH-and HCO3- ions. Further roots exhibit selective action in absorbing several ions from soil solution which is referred to as the internal regulation of absorption or selective absorption. External factors governing absorption by roots are the concentration of the soil solution and the relative preponderance of individual ions in the root region.
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Structure and composition of three mineralogical types of Clay; A-Kaloinite; B- Montmorillonite; C- Illite (From Donehue et al., 1971)
A-Kaloinite |
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B- Montmorillonite |
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C- Illite |
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Fixation of Ions in Soil
It has been estimated that approximately 5% of the total nitrogen in surface soils and 60% of total nitrogen in sub-soils are held as non-exchangeable (fixed) ammonium. This property of soils to retain NH4+ ion is known as ammonium fixation, the mechanism of which is highly complicated. The ammonium ion replaces other cations in soil such as Ca, Mg, Na and H in the expanded lattice of clay minerals. During this process, the lattice of the clay minerals contracts and traps ammonium ions in a non-exchangeable (fixed) form.
Potassium fixation also takes place by a similar mechanism. Most of the phosphorus added to soil is also fixed as insoluble compounds of iron, aluminium, orthocalcium, phosphates and apatite. This situation can be remedied if soils receiving fertilizer phosphorus are amended with organic matter. The decomposing organic residues form citrates, tartarates, acetates, oxalates, malates, malonates and other organic anions which release fixed phosphorus.
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