Phosphate fertility and fertilizer for your support

Soil

1. 1 CHAPTER: 5 PHOSPHATIC FERTILIZERS

2. The phosphorus (P) nutrient of all phosphatic fertilizers is expressed as P2O5. In soil, P is present as ( i ) Organic P and (ii) Inorganic P. The forms of inorganic P are H 2 PO - 4 ; HPO -2 4 ; and PO -3 4 ; Out of which, H 2 PO 4 and HPO 4 ions are available to plant. In soil, water in is changed to HPO -2 4 and PO -3 4 ions with increase in pH. -H + -H + H 2 PO - 4 HPO -2 4 PO -3 4 2

3. 3 Firstly, the P in soil is immobile or slightly mobile around one cm diameter and therefore, they should be applied in root zone. Secondly, the requirement of P is maximum in the initial stages. The crop takes up 2/3 of total P when the crop gains 1/3 of total dry matter and hence, the entire quantity should be applied at one time that is at the time of sowing as a basal dose. Thirdly, water soluble-P is changed to insoluble form as Fe and Al –PO 4 (Phosphate) under acidic and calcium phosphate in calcareous or high Ca content or in higher pH soils and hence, there is no danger for the loss due to leaching and volatilization. The applied P remain as in available form in less quantity while greater quantity is changed to insoluble form.

4. 4 Chemistry of P compounds: Phosphorus when burns gives P 2 O 5 and with water, it forms HPO 3 ( Metapohosphoric acid) and H 3 PO 4 (orthophosphoric acid) P 2 O 5 + H 2 O=2 HPO 3 ; HPO 3 + H 2 O = H 3 PO 4 . These H 3 PO 4 is important in agriculture as it forms three compounds (salts) by replacing one hydrogen every time. -H + -H + H 3 PO 4 H 2 PO - 4 HPO -2 4 PO -3 4

5. 5 When H 3 PO 4 combines with calcium, it forms three salts. They are i ) Ca(H 2 PO 4 = ) 2 Monocalcium phosphate ii) Ca 2 HPO - 4 Dicalcium phosophate iii) Ca 3 (PO 4 ) - 2 Tricalcium phosophate Classification of phosphatic fertilizers: The phosphatic fertilizers are classified into three classes depending on the form in which H 3 PO 4 combined with Ca .

6. 6 1) Water soluble P containing Super phosphate (SSP) (16 to18% P 2 O 5 ) Double Super phosphate (DSP) (32 to 36% P 2 O 5 ) Triple Super phosphate (TSP) (46 to 48% P 2 O 5 ) Mono ammonium phosphate (20% N and 20% P 2 O 5 ) Diammonium phosphate (18% N and 46% P 2 O 5 ) 3) Citrate and water insoluble-P containing Rock phosphate (20 to 40% P 2 O 5 ) Raw bone meal (20 to 25% P 2 O 5 and 3 to 4% N ) 2) Citric acid soluble P containing Basic slags (14 to 18% P 2 O 5 ) Dicalcium phosphate (34 to 39% P 2 O 5 ) Rhenania phosphate (23 to 26% P 2 O 5 ) Steamed bone meal (22% P 2 O 5 ) (Part of P 2 O 5 soluble in citric acid) Phosphatic fertilizers

7. 7 i ) Characteristics and conditions for the use of water soluble P containing fertilizer They contain water soluble-P as H2PO4 ion which can be absorbed quickly and available to plants when root system is not fully developed. Water soluble-P is rapidly transformed into water insoluble form in soil and hence there is no danger of loss due to leaching. These fertilizers should be used on slightly acidic, neutral to alkaline soils but not on acidic soils as the water soluble-P is changed to unavailable Fe and A1-PO4. These fertilizers are applied when a crop requires quick start and for short duration crops.

8. 8 ii) Characteristics and conditions for the use of citric acid (1%) soluble P containing fertilizers They contain citrate soluble-P and hence this P is less available than water soluble-P. They are suitable for moderately acid soils because it gets converted into water soluble form. They are basic in reaction and Ca content. There are less chances of getting fixed by Fe and Al. They are suitable for long term crops and where immediate and quick start to crops is not important.

9. 9 iii) Characteristics and conditions for the use of citrate and water insoluble P fertilizers They are suitable for strongly acidic soils They contain insoluble P and hence not available to crops The P is available when ploughed with green manuring crop or organic residues . They are used for long duration crops and in large quantity 500 to 1000 kg/ha They are used where immediate effects are not important

10. Manufacturing of Phosphatic Fertilizer

11. 11 Single super phosphate (SSP): SSP is manufactured by mixing equal amounts of rock phosphate and concentrated H 2 SO 4 (approximately 70%) and allowing to react for one minute in mechanical rotators. It is left for 12 hrs to harden and to cool down. It is then matured and after some weeks, it becomes ready for use. Due to free H 2 SO 4 present in it, it is responsible for destroying gunny bags and hence first fill in polythene bags and then in gunny bags. Ca 3 (PO 4 ) 2 + H 2 SO 4 + 5H 2 O = Ca(H 2 PO 4 ) 2 . H 2 O + 2CaSO 4 . 2H 2 O

12. 12 It contains two part by weight Ca (HPO 4 ) 2 monoalcium phosphate (16 to 18% water soluble-P) and three parts by weight gypsum. The formula of superphosphate is Ca(H 2 PO 4 ) 2 . H 2 O, CaSO 4 . 2H 2 O. Superphosphate supplies P, Ca and S due to gypsum, it improves physical conditions of soil when added to soil. In double superphosphate, there is no separate process but gypsum is removed and when the P 2 O 5 per cent of the content comes to 32 to 36, it is called double super phosphate. In triple super phosphate, phosphoric acid is used instead of H 2 SO 4 with calcium phosphate.

13. ii) Diammonium phosphate: (DAP) : DAP or monoammonium phosphate is prepared by reacting phosphoric acid (H 3 PO 4 ) with NH 3 . In this if one H + ion of H 3 PO 4 reacts with NH 3 if forms MAP (NH 4 H 2 PO 4 ) and NH3 reacts with two H+ ions of H 3 PO 4 forms DAP [(NH 4 ) 2 HPO 4 ]. H NH4 NH 3 + H PO 4 H PO 4 H H Mono ammonium phosphate (NH 4 H 2 PO 4 )

14. H NH4 2NH 3 + H PO 4 NH 4 PO 4 H H (DAP(NH 4 ) 2 HPO 4 )

15. 15 Behaviour of P fertilizes in soil : (SSP and ammonium phosphate) Super phosphate (Mono Calcium Phosphate): When superphsphate (normal or triple) is applied to soil, water vapour moves rapidly into each granules. The saturated solution of mono and dicalcium phosphates is formed and a residue of dicalcium phosphate remains at the site of original granules and saturated solution which moves out contain Ca and P and is extremely acidic (pH 1.8). This saturated acid solution reacts with different constituents of soil under different conditions. In acid soil, it dissolves Fe, Al and Mn and with them reacts with forming their insoluble Fe, Al and Mn- phoshpates . In calcareous soil, it reacts with Ca forming insoluble Ca 3 (PO 4 ) 2 . The reactions are as follows:

16. 16 In slightly acid or calcareous (alkaline) soils : Ca(H 2 PO 4 )= +Ca (HCO 3 ) 2 = 2 CaHPO 4 + 2H 2 CO 3 Ca(H 2 PO 4 ) 2 +2Ca (HCO 3 ) 2 = Ca 3 (PO 4 ) 2 + 4H 2 CO 3 Slowly the water soluble Ca (H 2 PO 4 ) 2 is changed to CaHPO 4 and then insoluble Ca 3 (PO 4 ) 2 . b) In markedly acid soils : 2Ca (HCO 3 ) 2 + Fe 2 O 3 = 2FePO 4 +2CaHPO 4 +3H 2 O

17. 17 The phenomenon is diagrammatically shown below :

18. 18 ii) Ammonium phosophate (MAP and DAP): Ammonium phosphate when applied in soil the water vapour moves rapidly inside the granules and form saturated solutions. The saturated solution of MAP has pH-4.0 and DAP 9.0. All salts of ammonium are soluble in water and hence the entire granules of ammonium phosphate is dissolved by soil water. No residue is left at the site of granule because of absence of Ca in fertilizer. After it is dissolved, it reacts with calcium particularly in a soil containing large amount of exch. Ca. One of the reaction products is CaHPO4. The other reactions are similar to that of superphosphate.