Fertilizers undergo various chemical reactions in soil that determine their availability to plants. Nitrogenous fertilizers like ammonium sulfate and urea release ammonium ions through cation exchange or hydrolysis reactions. These ions can then be further transformed by soil microbes. Phosphate fertilizers like single superphosphate dissolve in soil water but can precipitate or react with soil minerals to form insoluble compounds depending on the soil pH. Potassium fertilizers like potassium chloride and potassium sulfate readily dissolve to release potassium ions for plant uptake. After application, the nutrients in fertilizers may be taken up by crops, react with the soil, leach below the root zone, or be lost through erosion, runoff or gas emission.

1. FERTILIZER REACTIONS IN SOIL

2. FERTILIZER
Materials having
 definite chemical composition
 higher analytical value
 capable of supplying plant nutrients
in available form

3. NITROGENOUS FERTILIZER
Ammonical form (Eg. Ammonium sulphate – 20% N)
Nitrate form
Eg: NaNO3– 16% N
Both Ammonium- Nitrate
Eg: CAN – 20% N
Amideform
Eg:Urea -46% N

4. Reaction of Ammonium sulphate in soil
1. In Upland condition:
Cation exchange - NH4
+ displaces some other cations.

5. 2. In calcareous soil:
When (NH4)2SO4 applied as
Broadcast – losses of Nitrogen is appreciable
Drilled - losses of nitrogen is not appreciable

6. REACTION OF UREA IN SOIL
Urea undergoes enzymatic hydrolysis to produce
ammonium carbamate.
NH3 converted to NH4
+
Further reaction is similar to
Ammoniacal form

7. •High concentration of urea – combined toxic effect of
NH3 and NO2
• Higher concentration of NH3
increases
•Due to low population of Nitrobacter
inhibited

9. REACTION OF SLOW RELEASE NITROGENOUS FERTILIZER

10. PHOSPHATIC FERTILIZERS
Water soluble
Eg: SSP – 16% P2O5
Citric acid soluble
Eg: Dicalcium
phosphate- 34 % P2O5
Both insoluble
Eg: Rock phosphate-
20-40% P2O5

11. REACTION OF SUPERPHOSPHATE IN SOIL
Superphosphate applied to moist soil
• Dissolved in soil moisture
• Within short time gets precipitated
• SSP not leached from soil by Rainfall

12. REACTION OF SSP IN ACID SOIL
Combines with Fe and Al – forms insoluble phosphate
- becomes unavailable

13. REACTION OF SSP IN NEUTRAL AND CALCAREOUS SOIL
Monocalcium phosphate converted to Tricalcium phosphate
Tricalcium phosphate converted to insoluble compounds like apatite
REACTION OF SSP IN ALKALI SOIL
SSP reacts with sodium and forms monosodium phosphate
Monosodium phosphate – highly soluble and become
available to plants

15. REACTION OF POTASSIC FERTILIZER IN SOIL
Both KCl and K2SO4
– soluble in water
-on application in soil they ionize to K+, Cl-, SO4
2-
- available to plants

16. REACTION OF POLYPHOSPHATE FERTILIZER IN SOIL

17. LOSS AND GAIN OF NUTRIENTS

18. Fate of
applied
fertilizer
Taken up by the crop
Reacts with soil
minerals and
organic matter
Can leach from the
root zone with water
lost to the atmosphere
as a gas
Can move from
the field through
soil erosion and
water runoff

19. •REFERENCE :
•W. L. Lindsay ,H. F. Stephenson. Nature of the Reactions of Monocalcium Phosphate
Monohydrate in Soils: The Solution That Reacts with the Soil. Soil Science Society of
America Journal . Vol. 23 No. 1, p. 12-18