Remember the relationship between CRH and hygroscopicity:
\[ \text{Higher CRH} \implies \text{Lower Hygroscopicity (Better Storage Quality)} \]
Ammonium sulphate has one of the highest CRH values among nitrogenous fertilizers, making it highly resistant to moisture absorption.
Step 1: Understanding the Concept:
Hygroscopicity is the property of a substance to absorb moisture from the surrounding atmosphere.
For chemical fertilizers, high hygroscopicity is an undesirable trait because it leads to caking, lump formation, and difficulty in handling and mechanical application.
The hygroscopic behavior of a fertilizer is measured by its Critical Relative Humidity (CRH). Step 2: Detailed Explanation:
The Critical Relative Humidity (CRH) is the atmospheric relative humidity above which a fertilizer material will rapidly absorb moisture from the air.
A higher CRH value indicates that a fertilizer is less hygroscopic, as it requires a wetter atmosphere to absorb water.
Let us compare the CRH values (at \( 30^\circ\text{C} \)) of the fertilizers listed: Ammonium nitrate: This is an extremely hygroscopic fertilizer with a very low CRH of approximately \( 54\% \).
It absorbs moisture rapidly even in dry climates, making it difficult to store. Urea: Urea has moderate hygroscopicity, with a CRH of approximately \( 5\% \).
It can cake and clump in humid environments. Ammonium sulphate: This fertilizer has low hygroscopicity, with a high CRH of approximately \( 2\% \).
It remains free-flowing and does not readily absorb moisture from the air under normal atmospheric conditions.
Therefore, among the common nitrogenous fertilizers listed, ammonium sulphate has the lowest hygroscopicity. Step 3: Final Answer:
The fertilizer having the least hygroscopicity is Ammonium sulphate.
