Heterotrophic		OH+Energy
R-NH2+HOH		NH3+R

The ammonia so released is subject to several fates in the soil.

1. It may be converted to nitrites and nitrates by the process of nitrification.

2. It may be absorbed directly by higher plants.

3. It may be utilised by heterotrophic organisms in further decomposing organic carbon residues.

4. It may be fixed in a biologically unavailable form in the lattice of certain expanding type clay minerals.

1. Nitrification

Some of the NH₄⁺ (ammonia) released by the process of ammonification is converted to nitrate nitrogen. This biological oxidation of ammonia to nitrate is known as nitrification. It is a two step process in which the ammonia is first converted to nitrite (No^-₂) and then to nitrate (No₃). Conversion to nitrite is brought about largely by a group of obligate autotrophic bacteria known as Nitrosomonas.

2NH4+3O2

2NO2+2H2O+4N+

The conversion from nitrite to nitrate is effected largely by a second group of obligate autotrophic bacteria termed Nitrobacter.

2 No2 +O2

2No3

Some few heterotrophs mostly fungi will also produce nitrates, Nitrosomonas and Nitrobacter are usually referred to collectively as the Nitrobacteria.

Retention of ionic nitrogen in soil

The cationic nature of NH₄+ permits its absorption and retention by soil colloidal material. It is necessary that the soils have a sufficiently high exchange capacity to retain the added ammonium nitrogen or it will be removed in percolating water. Sandy soils with low exchange capacities permit appreciable movement of ammonium N to the soil. Once ammonium is nitrified it is subject to leaching Nitrate N is completely mobile in soils and within limits moves largely with the soil water. Under conditions of excessive rain it is leached out of the upper horizons of the soil. During extremely dry weather nitrates will accumulate in the upper horizons of the soil or even on the soil surface when capillary movement of water is possible. Both ammonium N and No₃N can be immobilized by soil microflora and N is not lost by leaching.

• Ammonium fixation

One of the possible fates of NH₄+ nitrogen in soils is its fixation by clays with an expanding lattice. It comes about by a replacement of NH₄+ for interlayer cations in the expanded lattice or clay minerals. The fixed ammonium can be replaced by cations which expand the lattice (Ca²⁺, Mg²⁺,Na⁺,H⁺) but not by those that contract it (K⁺,Rb⁺,cs⁺). The clay minerals largely responsible for ammonium fixation are montmorillonite, illite and vermiculite. As a rule, fixation occurs to a much greater extent in subsoils than in topsoils. The moisture content and temperature of the soil will affect the fixation of added ammonium compounds. Freezing and drying of soils increase ammonium fixation.

• Nitrogen fixation

Nitrogen fixation is also a biological process. Some species of bacteria, algae actionmycetes can absorb free nitrogen gas N₂ and convert it to ammonium NH₄+ which plants can use. Some nitrogen fixing bacteria are free livings, such as Azotobacter and require carbohydrates in their substrate. Others are symbiotic, such as Rhizobia, that infect root mainly of legumes. The infected roots eventually from nodules in which the free nitrogen is fixed from them it is translocated to other parts of the plant. However the account of nitrogen fixation is reduced when nitrogen is applied as fertilizer. Other microorganisms known to free nitrogen as blue green algae, which flourish in rice, paddy. Among anaerobic bacteria one should mention clostridium.

Phosphorus

Phosphorus is the second most important plant nutrient next to N for plant growth and development. Retention refers to that portion of the P which is loosely held by the soil and which can generally be extracted with dilute acids. This P is largely available to plants.

• Forms of organic phosphorous

Phospholipids
ucleic acids.
Inositol phosphates

• Inorganic soil phosphorus

Orthophosphate ions- H₂Po₄ and HPO₄

1. Precipitation by Fe, Al and Mn

These ions found in strongly acid mineral soils. These ions combine with Phopsphate to form insoluble compounds of Al, Fe and perhaps Mn. The resulting compounds may be precipitated from solution or absorb on the surface of iron and aluminum oxides or on clay particles. As clay becomes more acid they tend to contain more absorbed Al and Fe. Hence in acid soils the products of P fixation are largely complex phosphates of iron and Al.

2. Fixation by silicate clays

This is the another mechanism of P fixation. It is the reaction of phosphates with silicate clays. Soil clays are composed of layer of silica and Al combined to form silica alumina sheets. Phosphate ions may combine directly with these clays by (a) replacing a hydroxyl group from Al atom of. (b) forming a clay ca phosphate linkage. It is known that clays with low Sio2, R2o3 ratio will fix larger quantities of phosphate that will clays with a high ratio.

Inorganic P availability in alkaline soils

In alkaline soils, phosphate precipitation is caused mostly by calcium compounds. Such soils are plentifully supplied with exchangeable ca and in most cases with CaCo₃. Available phosphate will react with both the ca ion and its carbonate.

1. An increase in the pH favours the formation of diphosphate ions. In addition the solubility of the calcium orthophosphates decreases in the order of mono-di-tricalcium phosphate.

2. In alkaline soils that contain CaCo₃ is responsible for decreasing the activity of P. Phosphate ions coming in contact with solid phase CaCo₃ are precipitated on the surface of these particles. Finer the size of CaCo₃, more will be "P" fixation.

3. For P fixation in alkaline soils the retention of phosphate by clays saturated with Ca. Clays saturated with these ions can retain greater amount of P than those saturated with sodium or other monovalent ions. The concentration of phosphorus in the soil solution in alkaline or calcareous soils will be largely governed by three factors as below:-

1. Ca²⁺ activity.

2. The amount and particle size of freeCaCo₃ in the soil.

3. The amount of clay present.

The activity of phosphorus will be lower in those soils, that have a high Ca²⁺ activity, a large amount of finely divided. CaCo₃ and large amount of calcium saturated clay. In order to maintain a given level of phosphate activity in the soil solution, it is necessary to add large quantities of phosphate fertilizers to such soils.

1. Type of clay (1:1) Kaolinite

Phosphorus is retained to a great extent by 1:1 than 2:1 clays. Soils high in Kaolinitic clays such as those found in areas of high rainfall and high temperatures, will fix or retain larger quantities of added phosphorus than those containing the 2:1 type. Soils containing large amounts of clay will fix more P than those containing small amounts. The more the surface are a exposed with a given type of clay, the greater the amount of fixation taking place.

2. Time of reaction

The greater the time the soil and added P are in contact the greater the amount of fixation. The time between application and utilization of P is short in soils with high fixing capacity. This time period will determine whether the fertilizer P should be applied at one time or in split application.

3. Soil reaction

In most soils, P availability is at a maximum in the pH range 5.5 to 7.0 decreasing as the pH drops below 5.5 and decreasing as this value goes above 7.0. E.g. 5.5-7-fixation by hydrous oxides of Fe, Al, Mn, 6-8-fixation by silicate minerals, 6.5-8.5-fixation by the calcium.

4. Temperature

The soil of warmer climates are generally much greater fixers of P than the soils of more temperate region.

5. Organic matter

Addition of organic materialize either through green manuring or decomposed manures increase availability of soil and added P during the decomposition process by the evolution of Co₂. This gas when dissolved in water forms carbonic acid, which is capable of decomposing certain primary soil minerals in calcarious and acid soils. Co₂ production plays important role in increasing P availability.

Potassium fixation

Potassium is the major nutrient, which imparts increased vigour and disease resistance to plants and improves the quality of final products. Relative proportions of the total soil potassium in available, slowly available and readily available forms only 1-2% is rated as readily available of this 90% is exchangeable and only 10% appears in the soil solution at any time.

Potassium fixation: Slowly available forms

The potassium fixation is the result of reentrapment of K⁺ ions between the layers of 2:1 minerals especially illite. Alternate wetting and drying of 2:1 type clay minerals may aid in slow release of fixed potassium. In the presence of vermiculite, illite and other 2:1 type minerals, the potassium of such fertilizers as m/p not only became absorbed but also may become definitely fixed by the soil colloids.

Non exchangeable K		Exchangeable K		soil solution

1. Clay minerals

The soils containing 2:1 type of clay minerals like illite, vermiculite and montmorillonite can fix considerable amounts of potassium. A laterite soil containing kaolinite type or clay mineral fixed very little amount of potassium. The potassium and ammonium ions are attracted between the crystal units by the same negative charges responsible for the internal absorption of these and other cations. The tendency for fixation is grates in minerals where the major source of negative change is in the silica sheet.

2. Potassium concentration

An increase in K concentration is likely to increase K fixation because more K goes into the exchange complex by mass action.

3. Wetting and drying

The K fixation is strongly influenced by wetting and drying of soils. Fixation occurs when initial level of exchangeable and soluble K is high and release occurs when the level of such K is low. Thus, the process of drying favours attainment of equilibrium in distribution of K in soils.

4. Temperature

Higher temperature favours dehydration and contraction of the crystal lattice resulting higher K fixation.

5. Soil pH

Increase in soil pH leads to higher fixation of potassium. But all the soils may not exhibit this phenomenon.

6. Exchangeable cations

The size of K and other ions replacing K is important in K fixation. The cations of smaller size of the hydrated ions can easily enter into clay lattices and replace some of the fixed potassium.

7. Timek fixation proceeds from surfaces and edges to the interior of the soil particles.

8. Texture Finer the texture more is the k fixation.

9. Anions The k fixation from KH₂PO₄ was greater than from kcl and k₂so₄, but there was no difference in k fixation from k₂so₄ and kcl.

10. Organic matter The addition of organic matter decreases the k fixation by inorganic colloids.

Sulphur

It is an essential constituent of many proteins, enzymes and certain volatile compounds and helps in root growth, seed formation and nodule formation. Forms of sulphur: sulphides S^--, sulphates SO₄, organic forms and elemental sulphur.

Role of sulphur compounds in soils

Sulphur acts much like nitrogen as it is absorbed by plants and micro organisms and moves through the sulphur cycle.

1. Mineralisation
   
   

   Organic sulphur		Decay Products		So4 2- + H+
   (protein and other organic combinations)		(H2S and other sulfides)		Sulphates

   
   

2. Immobilisation

Immobilisation of inorganic forms of sulphur occurs when low sulphur, energy rich organic materials are added to soils not plentifully supplied with inorganic sulphur only when the microbial activity subsides does the inorganic sulphate form again appear in the soil solution. These facts suggest that, like N, sulphur in soil organic matter may be absorbed with organic carbon in a reasonably content ration. 130:10:1.3, AC/N/S ratio.

3. Volatilisation

During the microbial breakdown of organic materials, several sulphur containing gases are formed e.g. H₂S,CS₂,Cos,CH₃Sh. All are more prominent in anaerobic soils.

Sulphur oxidation and reduction

During the microbial decomposition of organic sulphur compounds. Sulphides are forms alongwith other incompletely oxidized substances such as elemental sulphur, thiosulphates and polythionates. In poorly drained soils the sulphide ion will react immediately with iron or manganese which in anaerobic conditions would be present in the reduced forms.

Fe2+ + S2-

FeS

Mn2++ S2-

MnS

Sulfites, thiosulphates and elemental sulphur are rather readily reduced to the sulfide form by bacterial and other organisms.