17.2 Fertilisers
🎯 Learning outcomes
By the end of this chapter, Students should be able to:
- Describe Fertilisers
- Describe Chemical fertilisers
- Describe the types of chemical fertilisers
- Describe production of urea with flow-sheet diagram.
- Plants need water, carbon dioxide, sunlight and different nutrient elements like N, P, K, Ca, Mg, F, Zn, etc. for their growth.
- Naturally, plants get these nutrients from the soil.
- After cultivating the land for a long time, the soil becomes poor in these nutrients.
- So, these nutrients have to be supplied to the soil of the farms.
- In the traditional way, natural or organic fertilisers are supplied, which include dead and decaying vegetable or animal matter, wood and animal excreta.
- But the natural or organic fertilisers are not enough to fulfil the increasing demand for food production.
- Hence, a large amount of artificial or chemical fertilisers are being manufactured and used for the growth of crop production.
- Chemical fertilisers play a vital role in modern agriculture by enhancing soil fertility and addressing nutrient deficiencies quickly and efficiently.
- However, their overuse can lead to environmental issues like soil degradation, water pollution, and harm to beneficial organisms.
- Finding a balance between their usage and sustainable practices is crucial for long-term agricultural success.
Among the nutrients required by plants, N, P and K are essential. These play the following vital roles in plants.
Nitrogen
Rapid growth, increase yield, increase protein content and production of green colouring material chlorophyll (for photosynthesis)
Phosphorus
Stimulates early root formation and growth, maturity of crop, formation of seeds and resistance to frost, helps in energy transfer
Potassium
Development of healthy roots, vigour and resistance to diseases, takes part in carbohydrate formation, improves fruit quality and size, helps to remove excess nutrients, etc.
Hence, N, P and K are the major nutrients to be supplied as chemical fertilisers.
Based on chemical composition, the chemical fertilisers can be classified as:
1. Nitrogenous Fertilisers
These mainly supply nitrogen. Some examples are listed below:
- Urea, NH₂-CO-NH₂
- Ammonium nitrate, NH₄NO₃
- Ammonium sulfate, (NH₄)₂SO₄
- Calcium ammonium nitrate, CAN – CaCO₃.NH₄NO₃
2. Phosphatic Fertilisers
- Diammonium phosphate (DAP), (NH₄)₂HPO₄
- Monoammonium phosphate (MAP), NH₄H₂PO₄
- Superphosphate, CaH₄(PO₄)₂
- Double superphosphate, CaH₂(HPO₄)₂
- Triple superphosphate, Ca(HPO₄)₂
3. Potash Fertilisers
- Potassium chloride
- Potassium sulphate
- Potassium nitrate
Chemical fertilisers can also be classified into simple and mixed fertilisers.
- Simple fertilisers contain only one primary nutrient, e.g. urea, KCl, etc.
- Mixed fertilisers contain two or more primary nutrients. E.g., Ammonium phosphate, potassium nitrate, etc.
Urea
Urea is manufactured by reacting ammonia and carbon dioxide at high temperature (about 200 °C) and pressure (150 atm) to form carbamate, which decomposes into Urea and carbon dioxide. Ammonia is generally obtained from Haber’s process, and carbon dioxide is obtained by thermal decomposition of calcium carbonate.
Figure: flow sheet diagram for the production of Urea
Quick Summary of the Chapter
- Plants need N, P, K (and other elements) from soil; depleted soil requires supplementing with fertilisers.
- Chemical fertilisers boost yield fast but must be balanced against soil degradation and pollution risks.
- N → growth & chlorophyll · P → roots & seed formation · K → disease resistance & fruit quality.
- Fertilisers are classed as Nitrogenous, Phosphatic or Potash, and further as Simple (one nutrient) or Mixed (2+ nutrients).
- Urea (NH₂CONH₂) is made from NH₃ + CO₂ at 180–210 °C and 150 atm, via an unstable carbamate intermediate, with unreacted gases recirculated.
Test your understanding
Take a short quiz on fertilizers.