Immediately after application (injection) of anhydrous \(\text{NH}_3\) into soil, it moves laterally as well as vertically (limited to \(< 5 \text{ cm}\)), and a localized zone known as \(\text{NH}_3\) retention zone is formed. A number of temporary yet dramatic changes occur in the \(\text{NH}_3\) retention zone. These are:
(A). A retention zone of both ammonia (\(\text{NH}_3\)) and ammonium (\(\text{NH}_4^+\)) having circular to oval shape (3-13 cm diameter) is formed.
(B). The concentration of both \(\text{NH}_3\) and \(\text{NH}_4^+\) increased in the range of 1000-3000 ppm.
(C). The pH increased up to 9.0-9.5 in the vicinity of the point of application.
(D). Concentration of \(\text{NO}_2^-\) (nitrite) increased to toxic levels (100-300 ppm).
Choose the correct answer from the options given below:
Show Hint
Anhydrous ammonia injection temporarily sterilizes a small zone of soil around the injection point because of the high pH (\(>9\)) and free ammonia toxicity. Normal microbial activity returns as the pH drops over time.
Step 1: Understanding the Concept:
Anhydrous ammonia (\(\text{NH}_3\)) is a highly concentrated gaseous nitrogen fertilizer (\(82\%\) N) injected under pressure into the soil. Its interaction with soil moisture causes localized chemical changes. Step 2: Detailed Explanation:
- Statement A is correct: Upon injection, ammonia gas expands and dissolves in soil water, forming a localized circular/oval retention zone of \(3-13 \text{ cm}\) in diameter.
- Statement B is correct: Due to high application rates, nitrogen concentrations in this central zone can reach \(1000-3000 \text{ ppm}\).
- Statement C is correct: Dissolving ammonia consumes hydrogen ions, which temporarily drives the local soil pH up to \(9.0-9.5\):
\[ \text{NH}_3 + \text{H}_2\text{O} \rightleftharpoons \text{NH}_4^+ + \text{OH}^- \]
- Statement D is correct: High pH and ammonium concentrations temporarily inhibit Nitrobacter (which converts \(\text{NO}_2^-\) to \(\text{NO}_3^-\)), while Nitrosomonas continues to function. This causes a temporary accumulation of nitrite (\(\text{NO}_2^-\)) to toxic levels (\(100-300 \text{ ppm}\)).
- All statements accurately describe the chemical dynamics of the retention zone. Step 3: Final Answer:
All statements (A), (B), (C), and (D) are correct.
