Phosphatic Fertilizer

P fertilizers – Rock phosphate, bone meal, basic slag, SSP, DAP, TSP, 

properties and their reaction in soil

Phosphatic fertilizers:  

​The nutrient phosphorus present in phosphate fertilizers are usually expressed in terms of phosphoric anhydride or simply as phosphorus pentaoxide, P2O5. The availability of phosphorus in which it is present. The amount of phosphorus available to the plants depends upon the extent to which the fertilizer supplies HPO4----or H2PO4 –ions. According to the solubilities, the phosphatic fertilizers are divided in following groups.

a). Water soluble phosphatic fertilizers: These fertilizers contain phosphorus in available form in neutral soils, which can be readily absorbed by young plants. In acid, soils and free iron, aluminium hydroxy phosphates but in alkaline and calcareous soils, water soluble phosphorous is converted into insoluble calcium phosphate. The common examples of these fertilizers are:

b).Citric acid soluble Phosphatic Fertilizers: The citric soluble fertilizers are suitable for acidic soils because at low pH citrate soluble phosphorus is converted to monocalcium phosphate not as water soluble phosphate and therefore, phosphorus is not fixed as iron and aluminium phosphate.

c).Water and citrate insoluble phosphatic fertilizers: These mineral fertilizers contain phosphorus, which is insoluble in water as well as in citric acid. They are suitable in strongly acid soils or organic soils. These fertilizers are given in green manured fields. The phosphorus is very slowly released by microbes at action and remains in soil for long time.

d). Other phosphatic fertilizers: Following are the other phosphatic fertilisers.

Citric acid soluble phosphatic fertilizers

​Citric acid soluble phosphatic fertilizers are not soluble in water but are readily soluble in acidic water or weak acids like 2 per cent citric acid. They also contain phosphorus in available form, i.e., HPO4. The fertilizers are suitable for acidic soils where they can easily dissolve and become available to plants. The examples of  these fertilziers are:

a) Basic slag -18% phosphate (P20S)

b) Dicalcium phosphate -34-39% P20S

c) Rhenania phosphate -23-26% P20S

The first two are very important and are commonly used in a few pockets of the country.

a). Basic slag: 

​Basic slag is a by-product of iron and steel industries. Original iron ore used as a raw material in iron industries contains appreciable amount of phosphorus as impurities. Purer form of iron is exracted from raw material leaving phosphorus and calcium rich basic slag as a by-product. Basic slag produced in India is of low quality. In contains only 3-8 per cent phosphorus in comparison to 14-18% phosphorus in basic slag produced in European countries. This is a grayish  black powder with a very high specific weight. , Its phosphorus is easily soluble in soil water of acidic soils. It exerts alkaline residual effect in the soil, therefore is useful for applying in acid soils. Basic slag is also used as liming material in our country.

 b). Dicalcium  phosphate: 

​Dicalcium phosphate has a excellent physical condition. It is also very rich in phosphorus (34% citrate soluble phosphates), therefore, less costly than basic slag in transport, storage and distribution. This fertilizer is suitable for a wide range of crops and soils because its phosphorus does not so easily convert into unavailable form as that of super phosphates.  Dicalcium phosphate is suitable for acidic soils.

c). Renonia phosphate: 

​This is a citrate solube phosphatic fertilizer which contains alteast 24% available phosphorus (HPO4)' The production and consumption of this ferrtilizer in our country is negligible. However, in foreign countries this fertilizer is used in large amounts.

Water soluble phosphatic fertilizers

​Water soluble phosphatic fertilizers contain phosphate in very easily dissolvable form (-H2PO4) in water and, therefore, are readily available to the plants. These fertilizers are highly prone to fixation, and therefore, their leaching loss from the soil is minimal. But, the fixed form of phosphorus is not available to the plant. The problem of fixation is severe in highly acidic or alkaline soils. The examples of these fertilizers are:

Single superphosphate (SSP)                    16-18% phosphate (P205)

Double superphosphate (DSP                32% (P205:)

Tripple' superphosphate (TSP)               42% (P205:)

Some important characteristics of these fertilizers can be highlighted individually.

There are five major processes that happen to applied fertilizer.

• It is taken up by the crop  

• It reacts with soil minerals and organic matter to become part of the soil reserve 

• It can leach from the root zone with water

• It can be lost to the atmosphere as a gas  

• It can move from the field through soil erosion and water runoff

Reaction in soils

​Phosphate fertilizer quickly reacts in soil to form many new compounds and remains very close to where it is applied. The most common phosphate fertilizers are diammonium phosphate (DAP; 46% P2O5, pH 7.5 to 8) and monoammonium phosphate (MAP; 48 to 61% P2O5, pH 4 to 4.5).

​Phosphate fertilizers are initially soluble in water and thus readily used by plants, but they quickly react with clays and other elements in the soil to become less soluble. These newly formed compounds will slowly dissolve and release soluble P over many months or years. These chemical reactions can be influenced by modifying the fertilizer properties or by minimizing fertilizer contact with soil with banded fertilizer application.

​Phosphorus movement in agricultural soils is quite limited, with diffusion occurring in the range of a few millimeters to less than an inch. In very sandy soils or where application rates greatly exceed agronomic needs, P movement through the soil can be greater.  

​Since P fertilizer is tightly bound to soil particles, erosion from the field in runoff water can be a pathway of loss. Conservation practices should be implemented to minimize erosion losses. Added phosphate fertilizer is incorporated into microbial biomass and soil organic matter, but in smaller amounts than N.

PHOSPHATIC FERTILIZERS

         Phosphate fertilizers are chemical substances that contain the nutrientelement phosphorus in the form of absorbable phosphate ions (anions) or thatyield such phosphate anions after conversion.

Origin and reserves:

         The raw material of P-fertilizers are essentially rock phosphates from phosphate deposits, phosphate ores and other P compounds .The deposits ofphosphate rock exists in nature because of the low solubility of thecompounds present . The composition of rock phosphates consists of variousapatites (Calcium phosphate) which are partly as magmatic and partly asorganogenic origin.

Magma : Weathering and decomposition of primary minerals

Organogenic : Bones ,Teeth of animals ,Guano deposits results in the formationof apatites .

Large deposits of rock phosphates occurs in

i) North Africa (Morocco, Algeria, Tunisia) in the form of organic crystallinephosphate, especially in a soft earth ,finely crystalline form known asgafsa phosphates.

ii) USA (eg.Florida apatite) in the form of pebbles.

iii) USSR in the form of hard earth, coarsely crystalline kola apatite

In India phosphate rock deposits have been found in udaipur in Rajasthanand Mussoorie in Uttaranchal . Some deposits occur in Singbhum district(Bihar) ,Jhabhua district (M.P),Visakhapatnam district (AP) ,Tiruchanapalli (TN) .

Production of phosphate fertilizers

Mineral P-fertilizers are obtained by chemical treatment or fine grinding of phosphates found in nature .Soft earth rock phosphates requiresgrinding where as hard earth raw material require chemical treatment.This isaccompanied by1) Thermal methods 2) Chemical methods using H2SO4 or other acids 3)Combination of above two methods [1 and 2 methods]

Classification of phosphatic fertilizers

Phosphatic fertilizers were classified by two ways .They are

I. Classification of phosphatic fertilizers based on relative solubility of phosphate

II. Classification of phosphatic fertilizers based on the form in which orthophosphoric acid or phosphoric acid is combined with calcium.

I. Classification of phosphatic fertilizers based on relative solubility of phosphate:

Based on the relative solubility, the phosphatic fertilizers are classified intofollowing three types

A) Water soluble phosphorus fertilizers

B) Water insoluble but citrate soluble phosphorus fertilizers

C) Water and citrate insoluble phosphorus fertilizers

A) Water soluble phosphorus fertilizers: Phosphorus in these fertilizers is present in water soluble form. P is present in the form of monocalciumphosphate Ca (H2PO4)2 . This form of P is generally regarded as the mostreadily available to plants.

1. Single super phosphate (16-18% P2O5)

2. Double super phosphate (32 % P2O5)

3. Triple super phosphate (46-48% P2O5)

4. Higher content or concentrated super phosphate

5. Ammonium phosphate ( 20% N and 20% P2O5)

These fertilizers are suitable for neutral to alkaline soils and should be applied at the time of sowing. Immediately after application, phosphorusgets converted into insoluble dicalcium phosphate. Hence the P fertilizerscontaining water soluble P should be applied in granulated form ratherthan powdered form. Contact between soil and fertilizers should be reduced. Hence pocketing of fertilizers is beneficial than broad casting.

         Under acidic conditions, water soluble phosphoric acid gets converted intoun available iron aluminium phosphates.

B) Water insoluble but citrate soluble phosphorus fertilizers: Phosphorus present in these fertilizers is soluble in 2% citric acid or neutral normalammonium acetate solution. P is present as dicalcium phosphate Ca2H2(PO4)2 / CaHPO4

1.Basic slag (14 to 18% P2O5)

2.Dicalcium phosphate (34-39 % P2O5)

3.Raw and steamed bone meal ( part of the P2O5 soluble in citric acid ) – suitable for acid soils and lateritic soils

         The fertilizers of this group are particularly suitable for the acidic soils,because with low pH citrate soluble phosphoric acid gets converted intomonocalcium phosphate or water soluble phosphate ,and there is lesschances of phosphate getting fixed as iron and aluminium phosphate.

Contact between soil and fertilizer should be more to solubilize the citratesoluble P present in the fertilizer. Hence they should be applied as broadcasting to increase contact with soil.

C) Water and citrate insoluble phosphorus fertilizers: Phosphorus present in the fertilizer is not soluble both in water and citrate solution containing insolublephosphoric acid or tri calcium Phosphate (Ca3(PO4)2

1) Rock phosphate (20 to 40 % P2O5)

2) Raw bone meal (20 to 25% P2O5)

3) Steamed bone meal (22% P2O5)

These fertilizers very well suited for acidic soils or organic soils, which requirelarge quantities of phosphatic fertilizers to raise the soil fertility.They should beapplied as broad casting to facilitate intimate contact with soil. They should beapplied one month before taking up the crop so that insoluble-P getssolubilized by the time of sowing of crop.

II. Classification of phosphatic fertilizers based on the form in which orthophosphoric acid or phosphoric acid is combined with calcium.

The phosphatic fertilizers can be classified broadly into three groups, depending on the form in which orthophosphoric acid or phosphoric acid iscombined with calcium . They are:

1. Ortho phosphates Eg. MAP, DAP,UAP,SSP, Nitro phosphates

2.Polyphosphates Eg.Ammonium poly phosphate,Potassium poly phosphate

3.Metaphosphates Eg.Ammonium meta phosphate,Potassium meta phosphate

Available Phosphorus : The sum of the water soluble and citrate soluble values is taken as an estimate of the fraction of the total P which is available toplants.

Total phosphorus : The total P is the sum of the available and citrate insolublefractions. It is determined by treating a sample of the original material with strongacids and analyzing the solution for phosphates.

 I. NORMAL (Or) ORDINARY (OR) SINGLE SUPER PHOSPHATE [16%P]:

         Super phosphate is a term used in reference to phosphates of which is in aform readily available to plantsSingle super phosphate is the oldest artificially produced fertilizer and its

manufacture dates back to 1842, when LAWES J.B prepared for the first time inENGLAND by treating Rock phosphate with sulphuric acid.

Manufacture process of single super phosphate

Raw materials: 1) Phosphate rock 2) Sulphuric acid

Single super phosphate is (SSP) is manufactured by mixing gravimetrically equal parts of

sulphuric acid (75%) and rock phosphate of 0.14 mm [100 mesh seive]There are two methods of preparation of SSP viz.,

i. Den process

ii. Continuous rock phosphate acidulation process

i. Den process:

Weighed quantities of Rock phosphate(0.14mm) and sulphuric acid (75% )are mixed in a mixer, capable of handling 40-50 tonnes per hour, are allowed to react for about a minute and the resultant slurry is dumped in to acompartment known as DEN [100-300 tonnes capacity ] . In a few hours thereaction goes to completion .Water ,carbon dioxide ,fluorine ,volatilize away resulting in the reduction in the bulk of the material by retaining the material in the Den becomes a hard block and removal is accomplished by means of mechanical excavators equipped with revolving knifes which cut into the block and the disintegrated SSP is stored for 2 to 6 weeks to cure and attain the desired physical condition .

ii. Continuous rock acidulation process:

         This is a popular and latest process in which acidulation of rock phosphate with sulphuric acid is done continuously in mixer provided with indigenous metering (H2SO4) and weighing (RP) devices. The mixer is agitated for 2-3minutes and is then discharged into an endless conveyer belt on which it solidifies .The belt conveyer moves the blocks of hardened super phosphate towards a revolving cutter which disintegrate the material .It is then transferred to a storage bin and stored for 2-6 weeks to cure and attain the desiredphysical condition .

         The chemical reaction (Exothermic) involved in both processes are same as represented below

         Ca 10 [PO4]6 F2 +7 H2SO4 +3HO                3 CaH4(PO4)2 H2O + 7CaSO4 +2HF

                                                                     [Super phosphate i.e., Mono calcium phosphate ]

Three points are suggestive of the above equation viz.,

1) Phosphate originally present as apatite is converted into water soluble Monocalcium phosphate

2) The by-product reaction is gypsum which is initially mixed with the monocalcium phosphate

3) The reaction releases toxic hydro fluoric acid gas

Physical properties of SSP:

1) SSP is in granular form has bilk density 961.10 kg m-3 and is easy to handle

2) SSP is also available in powder form, it is not free flowing and being slightly hygroscopic has a tendency to cake . Hydration of monocalcium phosphate may be the cause for hardening SSP  .It has grey colour and an acidic odour.

3) Free acid in the SSP, will usually rot the jute fiber bags and hence the fertilizer has to be stored in polythene lined gunny bags or polyethylene bags.

Chemical properties of SSP:

1) SSP has 2/5 Mono calcium phosphate and 3/5 Gypsum by weight

2) SSP manufactured in India consists of two grades viz., Garde I: 16% P2O5 % or (7% P) by weight of water soluble P2O5 and Grade II: 14% P2O5 or (6%P)by weight of water soluble P2O5 .

3) SSP also contains 21.0% calcium, 12% sulphur and traces of micronutrients .Obviously it contains more sulphur (12%) than phosphorus (6 to 7%).

4) SSP has a pH of about 3.0.