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This article throws light upon the six main factors that affect phosphate fixation in soil. The factors are: 1. Nature and Amount of Soil Components 2. pH 3. Other Ions 4. Organic Matter 5. Temperature 6. Over Liming.
Factor # 1. Nature and Amount of Soil Components:
Adsorption and desorption reactions are affected by the type of surfaces contacted by phosphorus in the soil solution. However, there are various other following soil components that affects phosphate fixation.
(a) Hydrous Oxides of Iron and Aluminium:
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These substances have the ability to fix phosphates through adsorption on their surfaces. Aluminium and iron oxides can occur as discrete particles in soils or as coatings or films on other soil particles. They occur as their both amorphous and crystalline hydroxy compounds in soil.
When iron and aluminium oxide compounds is soil are less crystalline, the phosphate fixing capacity of the soil be more because of greater surface areas. However, crystalline hydrous oxides are usually capable of fixing more phosphorus than layer silicates.
Fixation of phosphorus by oxides of Fe and Al takes place over a wide pH range that is shown below:
(b) Type of Clay:
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The clay fraction of soil is likely to be the main site of phosphate fixation. The nature of the clay mineral is also important with regards to phosphate fixation. Phosphorus is retained or fixed to a greater extent by 1: 1 than by 2: 1 clays and this may be due to the higher amounts of hydrated oxides of Fe and Al associated with 1: 1 type clays. The phosphate fixing capacity of clay minerals may be found in the following order:
Montmorillonite > Vermiculite > Kaolinite > Muscovite.
(c) Amount of Clay:
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Soils containing large quantities of clay will fix more phosphorus than that of soils containing small amount of clay.
(d)Amorphous Colloids:
Amorphous aluminosilicate minerals like allophane (Si—Al— Fe—O—OH—OH2 gel) have a large negative charge which is partly or entirely balanced by complex aluminium cations. Phosphorus becomes adsorbed by reacting with this aluminium.
(e) Calcium Carbonate:
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Calcium carbonate (CaCO3) exerts significant influence on phosphate fixation. Usually higher the content of CaCO3 in soil, the higher is the fixation of phosphate. Phosphate may be adsorbed on the surface of calcium carbonate particles. The solubility of phosphatic minerals has also been found to be depressed in the presence of CaCO3 (due to formation of less/soluble tricalcium phosphate and latter carbonate apatite).
Exchangeable calcium has also found to be effective in phosphate fixation (due to linkage between phosphate and soil colloid via calcium).
Factor # 2. pH:
Soil pH has a profound influence on the amount and manner in which soluble phosphorus becomes fixed. Between pH 4.0 and 8.0, H2PO4 and HPO4– are the principal phosphate ions in soil. Above pH 9.0, PO43- dominates but H2PO4– is still present. Different mechanisms of phosphate fixation operate at different pH levels.
At pH 2-5 the fixation is chiefly due to the formation of Fe and Al-phosphates. Al pH 4.5-7.5, phosphates are fixed on the surface of the clay colloids and at pH 6-10 phosphate is precipitated usually by Ca and Mg. So the fixation of phosphate in relation to different soil pH is presented in Fig. 17.5.
Factor # 3. Other Ions:
Both cations and anions affect fixation of phosphorus.
Cations:
Divalent cations enhance phosphate adsorption than that of monovalent cations. The concentration of exchangeable Al also influences the phosphate fixation indirectly through precipitation and adsorption as follows:
Cation exchange:
Hydrolysis:
Al3+ + 2H2O DAl(OH)2+ + 2H+
Precipitation and/or Adsorption:
Al(OH)2+ + H2PO4–DAl (OH)2H2PO4(Ksp = 2.8 × 10-29)
Anions:
Both inorganic and organic anions can compete in varying degrees with phosphate anion for the same adsorption sites resulting in some cases in a decrease in the adsorption of added phosphorus or a desorption of fixed phosphorus. Anions are hydroxyl, silicic acid, sulphate and molybdate etc.
Factor # 4. Organic Matter:
During decomposition of organic matter various organic acids are produced which solubilize phosphates and other phosphate bearing minerals and thereby lower phosphate fixation.
The decrease in phosphate fixation due to presence of sufficient organic matter in soils may be described by the following reactions:
(a) By the formation of phosphohumic complexes that are easily assimilated by plants.
(b) Through the replacement of the phosphate by the humate ion (anion exchange reaction).
(c) Forming coating on sesquioxide particles by humus to form a protective cover and thus reduce the phosphate fixing capacity of the soil.
Organic matter plays an important role in influencing the phosphate fixation in soils.
It is evident that organic matter decreases the fixation of phosphate in soils as follows:
(i) In adsorption reactions the organic matter containing various organic compounds, being dominantly anionic in nature, compete with phosphate anion in polar adsorption phenomenon and thereby decrease phosphate fixation in soils.
(ii) Generally phosphate fixation does not occur with the organic anions (because of not well fitting into clay mineral lattices) through isomorphorus replacement, if happens so, they (organic anions) would be competitive with phosphate anions and thereby decrease fixation.
(iii) In double decomposition reactions involving Fe and Al, the acids produced during organic matter decomposition could decrease the pH and thus increase fixation (phosphate) by solubilizing larger amounts of Fe and Al.
On the contrary, hydroxy acids like tartaric, citric, malonic etc. are formed in soils and those such acids can form insoluble chelates of Fe and Al and preventing them (Fe and Al) from reacting with phosphate to form insoluble precipitates and as a result phosphate fixation decreases. In general organic matter decreases the phosphate fixation in most of the soils.
Factor # 5. Temperature:
Temperature affects the speed of chemical reaction. High temperatures are expected to slightly increase the solubility of phosphatic compounds like apatites (hydroxy and carbonate), octacalcium phosphate, variscite, strengite etc.
Mineralization of phosphorus from soil organic matter, crop residues and other organic wastes etc. are enhanced with the increase in temperature by stimulating the activity of appropriate microorganisms. Phosphorus fixation generally increases with the increase in temperature.
Factor # 6. Over liming:
Over timing increases the fixation of phosphorus by forming more insoluble Ca—P compound in soil. Besides phosphate, molybdate (MoO42-) and borate (BO33-,HBO3–,B(OH)4–) anions also fix in the soils of mostly acidic reaction. Majority of the factors discussed earlier for the fixation of phosphates in soil also affects the fixation of molybdenum and boron in soils.
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