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Soil Science
b. Phosphatic Fertilizers :
The nutrient Phosphorus present in Phosphatic fertilizers is usually expressed in terms of Phosphoric
anhydride or simply as phosphorus pentaoxide (P 2
O 5
). The amount of Phosphorus available to the plants
depends on the extent to which the fertilizer supplies (HPO 4¯ ¯ or H PO 2 4¯ ions. Phosphorus moves
very slowly from the point of placement. It should thus be placed where it will be readily accessible
to the plant roots. Therefore, drilling of Phosphatic fertilizers has been considered to be superior to
surface application. The phosphatic fertilizers are divided into three groups according to the solubility
of Phosphoric acid as follows:
1. Fertilizers containing water soluble phosphoric acid or monocalcium Phosphate [Ca(H 2
PO 4
) 2
],
commonly called water soluble phosphatic fertilizers.
Name of fertilizer Chemical Composition Percentage of P 2
O 5
Acidity or Alkalinity
Single Superphosphate Ca(H 2
PO 4
) 2
, H 2
O 16.0-20.0 Neutral
Double Superphosphate 2Ca(H 2
PO 4
) 2
, H 2
O 30.0-35.0 Neutral
Triple Superphosphate 3Ca(H 2
PO 4
) 2
, H 2
O 45.0-50.0 Neutral or Acidic
Characteristics:
i. Phosphate is water soluble and quickly absorbed, as plants absorb Phosphorus in the form of
H 2
PO 4
ions.
ii.
iii.
Leaching loss of this group of fertilizers is less as its Phosphoric acid is rapidly transformed into
a water insoluble form in the soil.
This type of fertilizers should be applied in neutral to alkaline soils. In acid soil and free iron
and aluminium oxide containing soil, the water soluble Phosphoric acid gets converted into
un-available iron and aluminium Phosphate and calcium Phosphate in Calcareous soils and
this reaction is known “Phosphate fixation.” But in alkaline and Calcarous soils, water soluble
Phosphorus is converted into insoluble calcium Phosphate.
The organic matter prevents the Phosphate fixation and even releases phosphates that had already been
fixed.
2. Fertilizers containing citric acid soluble phosphoric acid or dicalcium Phosphate
[Ca 2
(H 2
PO 4
) 2
] commonly called the Citric acid soluble fertilizers.
Name of fertilizer Chemical Composition Percentage of P 2
O 5
Acidity or
Alkalinity
Basic Slag (India) (CaO 3
P 2
O 5
SiO 2
3.0-8.0 Alkalinity
Diacalcium Phosphate CaHPO 4
35.0-40.0 Acidic
Characteristics:
i. The fertilizer of this group is insoluble in water, but soluble in citric acid. For this, the nutrient of
this fertilizer does not become available to plants.
40 Reading Materials for Village Level Workers