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In this article we will discuss about:- 1. Distribution of Phosphorus in Soil 2. Availability of Phosphorus 3. Soil pH and Phosphate Ions 4. Fixation 5. Losses 6. Phosphorus Supply and Plant Behaviour 7. Function.
Contents:
- Distribution of Phosphorus in Soil
- Availability of Phosphorus
- Soil pH and Phosphate Ions
- Fixation of Phosphorus in Soil
- Losses of Phosphorus from Soil
- Phosphorus Supply and Plant Behaviour
- Function of Phosphorus
1. Distribution of Phosphorus in Soil:
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The distribution of phosphorus in soil profile and in different sized fraction of soil are as follows:
(a) Soil Profile:
Generally phosphorus content in soil profile is minimum either in lower A horizon or upper ‘B’ horizon. The minimum phosphorus percentage apparently result from the combined action of absorption of phosphorus by plant and leaching of phosphorus from the soil. The sub soil contains more inorganic phosphorus than organic phosphorus. Organic phosphorus concentration is more in surface soil than in subsoil.
(b) Different Sized Fraction of Soil:
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When soluble phosphorus is added either by weathering or by application of fertilizer, they first combine with finer fraction of soil particles. So the finer fraction of soil contains more phosphorus than coarse fraction of soil.
2. Availability of Phosphorus:
(i) Inorganic Phosphorus:
Generally the plant absorbs phosphorus either as primary ortho phosphate (H2PO4–) or secondary ortho phosphate (H2PO4– –) ion from the soil. Out of these, two forms H2PO4– ions is preferred to H2PO4– – ions. Besides these, two forms, metaphosphate and phytophosphate are supposed to be absorbed by plant when they are present in the soil.
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(ii) Organic Phosphorus:
The organic phosphorus such as phytin and nucleic acid is supposed to be absorbed by plants as a source of phosphorus. Apparently the phytin is absorbed directly by plants and the nucleic acids are probably broken down by enzymes at root surfaces and the phosphorus is absorbed in either organic or inorganic form. Pierre and Parkar (1929) are of opinion that plant cannot absorb organic phosphate compound as such unless they are transformed to other form.
Phytin behaves in the soil much as do the inorganic phosphate forming iron, aluminium and calcium phytates. In acid soils, phytin is render insoluble and thus unavailable because of reaction with iron and aluminium. In alkali soil, calcium phytate is precipitated and phosphorus carried is rendered unavailable. In acid condition, nucleic acids are adsorbed by clay, especially montmorillonite and the available phosphorus supply from nucleic acid is low.
3. Soil pH and Phosphate Ions:
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Phosphoric acid (H3PO4) has three types of ions such as primary ortho phosphate (H2PO4–), secondary ortho phosphate (HPO4– –) and tertiary orthophate (HPO4– –).
The kind of phosphate ion present varies with the pH of the soil solution. At a pH of 4.0, primary orthophosphate (H2PO4–) ions tend to dominate. As the pH increases, the concentration of H2PO4– decreases. When the soil is alkaline, secondary orthophosphate (HPO4– –) ion apparently is the most common form. As the pH is lowered and the soil becomes slightly to moderately acid, both H2PO4– and HPO4– – ion prevail. As the pH increases, the concentration of HPO4– – increases. Below pH 6.7, H2PO4– ion is dominant over HPO4– –. Above this pH, HPO4– is dominant over H2PO– –.
The concentration of HPO4– is maximum at a pH between 9.0-10.0. Above this pH, PO4– – – ion is more important than H2PO4–. But even at pH 12.0, concentration of HPO4– – is more than PO4– – – ion. In acid soil, iron, aluminium and manganese remains more soluble form. Under such conditions, soluble phosphates are markedly fixed as very complex and insoluble compounds of these elements. This fixation is more serious when the soil pH is below 5.0.
Our agricultural soil lies between pH 4.0-9.0.
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4. Fixation of Phosphorus in Soil:
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The problem of phosphorus is baffling to the agriculturist because of the fact that the added phosphorus is converted into unavailable form i.e. phosphorus get fixed in the soil. Only 10-20 per cent of added phosphorus can be utilized by succeeding or next growing crop and the rest is supposed to be fix in the soil.
The phenomenon of phosphate fixation can be defined as conversion of soluble phosphorus to insoluble (i.e. solid phase) phosphorus or it may be defined as a process by which easily soluble and easily available phosphorus is converted to insoluble form and thus restricting its mobility and thereby decreasing its availability to crops.
The availability of phosphorus is correlated with soil pH. The maximum availability of phosphorus to plant is obtained when the soil pH is maintained in the range of 6.0-7.0.
The fixation of phosphorus can be discussed under two heads as follows:
1. Reaction of Phosphorus in Acid Soil:
In acid soil, phosphorus becomes unavailable due to the following reasons:
(i) Precipitation of phosphorus from soil solution. In acid soil, the concentration of iron, aluminium and manganese increases in soil solution and exchangeable phase with the increasing of acidity. The chemical reaction occurring between the soluble iron and aluminium and the H2PO4– ions probably results in the formation of hydroxy phosphates. This may be illustrated by the following reaction taking aluminium (Al) as a replacement ion
The insoluble compounds thus formed are precipitated from soil solution. But it does not exclude the possibility that after forming this compound, they are adsorbed by inorganic soil colloids. So it may be said that once this insoluble compounds are formed, they may be precipitated or adsorbed on inorganic soil colloid. In slightly acid soil, little amount of phosphorus may also be converted into insoluble form by calcium as dicalcium phosphate. In acid soil media, phosphorus is represented by the phosphate of iron and aluminium (FePO4, AlPO4, Fe2(OH)3PO4, Al2(OH)PO4)
(ii) Reaction of Phosphorus with Hydrous Oxide:
In acid soil, phosphate ion (H2PO4) reacts with insoluble hydrous oxide of aluminium and iron. The compound formed as a result of fixation by iron and aluminium oxides is likely to be hydroxy phosphate. This can be illustrated taking aluminium hydroxide as hydrous oxide of aluminium as follows –
Though at the initial stage, the reaction is of adsorptive type, the ultimate compound formed is probably the same as it is precipitation reaction. It has been suggested that major fraction of phosphate in acid soil is fixed by this mechanism. The red and yellow podzol soils and red brown latosolic soil containing high percentage hydrous oxide of iron and aluminium are great phosphate fixer.
(iii) Reaction of Phosphorus with Silicate Clays:
The silicate clays such as kaolinite, montmorillonite and illite can retain phosphate through different mechanism as follows:
(a) Phosphate are fixed by silicate minerals as surface reaction between exposed hydroxyl (–OH) group on mineral crystal and the H2PO4– ions. This type of reaction might be expressed as follows –
Phosphate retention by this mechanism suggests that anion exchange phenomenon also takes place in soil. Generally 1 : 1 type of clay mineral can fix more phosphorus by this mechanism than 2 : 1 type of clay mineral.
(b) Clay calcium phosphate linkage (Clay-Ca-H2PO4 linkage). In acid soil phosphorus can also be retained by silicate clay minerals by clay calcium phosphate linkage.
2. Reaction of Phosphorus in Alkaline Soil:
In alkaline soil, phosphates are made insoluble by calcium and magnesium present in soil solution or in exchangeable phase. In alkaline soils, calcium is also present in the form of carbonate. Calcium carbonate also can fix phosphorus to certain extent. When concentrated super phosphate is applied in calcareous soils, the reactions take place as follows –
In alkali soils, the reaction of HPO4– – and PO4– – – ion becomes dominant over H2PO4–. The solubility of different orthophosphoric salt of calcium is in the following order.
Ca(H2PO4)2 > CaHPO4 > Ca3(PO4)2
In alkali soils, when the activity of calcium and magnesium is increased, this high activity of calcium and magnesium in association with high pH causes the precipitation of phosphorus as dicalcium phosphate and tricalcium phosphate. If the high pH of the soil is due to presence of sodium instead of calcium, then the availability of phosphorus will increase instead of decreasing it, because the sodium salts of phosphorus are soluble. It has been found that phosphate becomes more soluble in soil having pH 8.5-9.0.
It has been found that the silicate clays saturated with calcium ion have high capacity to retain phosphorus by clay calcium phosphate linkage.
Naftel (1930) suggested that phosphate can be retained by the silicate clays with this mechanism only at a pH slightly below 6.5 and above this pH, dicalcium and tricalcium phosphates are precipitated. In alkaline soils, aluminium (Al+++) and aluminium hydroxide [Al(OH)3] present in silicate clay surface can retain phosphorus probably by formation of compound like Al(OH)2H2PO4.
Fixation of Organic Phosphorus:
Like inorganic phosphate ion, the phosphate in organic combination like phytin and nucleic acid which can be directly utilized by plants is fixed in acid and alkali soils, as follows:
Phytin – In acid soil, phytins are fixed as iron and aluminium phytate, and this forms become unavailable to plant. In alkali soil, phytins are retained as calcium phytate and phosphorus carried is rendered unavailable.
Nucleic acid – The nucleic acids which are basic in character, are fixed in soil through different mechanism i.e. they are retained by soil colloid through cation exchange reaction and thus become less susceptible to microbial attack. The extent of this type of reaction is more in acid soil than the alkali soil.
5. Losses of Phosphorus from Soil:
(i) Crop removal – Plants absorb phosphorus from soil and store them in their different parts. Lipman and Conybeare (1936) estimated that for the United States as a whole, the average amount of phosphorus removed in the harvested portion of crops in 1930 was approximately 10 kg/ha.
(ii) Losses of phosphorus by leaching – When the soluble phosphatic fertilizers are applied in soil, they react rapidly with soil so that most of added phosphorus remains near the root zone. Due to low solubility and limited movement of phosphorus in soil, the loss of phosphorus by leaching is negligible in most soils. The leaching loss of phosphorus is significant in sand and peat soil as they have little tendency to react with phosphorus and when heavily fertilized.
(iii) Losses of phosphorus by soil erosion – The availability of phosphorus is much higher in the surface portion of the soil. The surface soils are removed by the process of erosion. Consequently removal of phosphorus takes place. Lipman and Conybeare (1936) estimated that the loss of phosphorus by erosion from crop land of United State averages 10.6 kg/ha.
6. Phosphorus Supply and Plant Behaviour:
1. Deficiency Symptoms:
(i) Root and shoot growth is restricted and plants become thin and spindly.
(ii) The leaves of cereal crops become dull greyish green in colour. The deficiency is characterised by slow growth and low yields.
(iii) Leaves may shed prematurely and flowering and fruiting may be delayed considerably.
(iv) Stunted growth even under abundant supply of nitrogen and potassium, premature ripening of crops.
(v) The tillering of cereal crops decreases and as such yield also becomes low.
(vi) Potato tuber shows rusty brown lession.
2. Phosphorus Supply and Root Growth:
Phosphorus stimulates root development and growth in seedling stage and thereby it helps to establish the seedling quickly. Phosphorus affects the root system of plants and encourages the formation of lateral and fibrous root which increases absorbing surface for nutrients. If the root designates the subterranean storage tissue of the root crops, the phosphorus supply does have special effects. If such crop is deficient in phosphorus, phosphatic fertilization increases the yields of roots more than that of above ground portion.
3. Phosphorus Supply and Time of Maturity:
Phosphorus enhances the development of reproductive parts and thus bringing about early maturity of crops particularly the cereals. The young plants absorb phosphorus rapidly when phosphorus availability in soil is high. Rapid absorption of phosphorus early in life of the plant is conductive to rapid development. William (1948) found that the time required for plants to attain maximum rate of phosphorus absorption decreases as the concentration of phosphorus in good medium is increased. In consequence of the difference in rates of development, plants that are deficient in phosphorus mature late with plants that are amply supplied with phosphorus.
4. Phosphorus Supply and Disease Incidence:
Phosphorus develops resistance to certain diseases of plant. In phosphorus deficiency condition, fungal root rot is greater. The importance of phosphorus supply in relation to the incidence of plant diseases apparently is less than that of nitrogen supply.
7. Function of Phosphorus:
(i) Phosphorus stimulates root development and growth in the seedling stage and thereby it helps to establish the seedlings quickly.
(ii) It hastens leaf development and encourages greater growth of shoots and roots.
(iii) It enhances the development of reproductive parts and thus bringing about early maturity of crops, particularly the cereals and counteracts the effect of excess nitrogen. It develops resistance to certain diseases.
(iv) It increases the number of tiller in cereal crops and also increases the ratio of grain to bhusa or straw. As a result, yield is increased. It strengthen the straw of cereal crops and thus helps to prevent lodging.
(w) It stimulates the flowering, fruit setting and seed formation and the development of roots, particularly of root crops.
(vi) Phosphorus has a special action on leguminous crops. It induces nodule formation of this crop and rhizobial activity. Thus it helps in fixing more of atmospheric nitrogen in root nodules.
(vii) It influences cell division and the formation of fat and albumin.
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