Step 1: Understanding the Concept:
The atmosphere is a mixture of various gases, water vapor, and aerosols that surround the Earth.
Dry air refers to the chemical composition of the atmosphere in the homosphere (the lower 80-100 km) after removing moisture.
Understanding the relative abundance of these gases is fundamental in agricultural meteorology as it dictates plant-environment interactions.
While Nitrogen and Oxygen are considered "permanent gases" because their concentrations are nearly constant, Carbon dioxide is a "variable gas" whose levels are gradually increasing due to anthropogenic activities.
Step 2: Detailed Explanation:
To arrange these gases in descending order (highest to lowest), we must examine their volumetric concentration in the lower atmosphere:
1. Nitrogen (C): It is the most dominant gas, occupying approximately \( 78.08% \) of the dry atmosphere by volume.
In agriculture, atmospheric nitrogen is the ultimate source for all nitrogenous fertilizers, though it must be "fixed" into reactive forms (like ammonia or nitrate) by lightning, industrial processes (Haber-Bosch), or biological agents (like Rhizobium) before plants can utilize it.
2. Oxygen (D): It is the second most abundant gas, making up about \( 20.95% \) (roughly \( 21% \)).
Oxygen is essential for the aerobic respiration of all living organisms, including the roots of crops and beneficial soil microbes. Adequate oxygen levels in the soil atmosphere are a prerequisite for healthy root development and nutrient uptake.
3. Argon (B): It is an inert, noble gas that constitutes about \( 0.93% \) of dry air.
Being chemically inactive, it does not directly participate in biological or agricultural processes, but it is the third most prevalent gas, significantly outnumbering carbon dioxide.
4. Carbon dioxide (A): Despite its critical importance for life, it exists in trace amounts, currently averaging around \( 0.04% \) (or 400-420 parts per million).
In the context of agriculture, \(CO_{2}\) is the primary substrate for photosynthesis. The "Carbon Dioxide Fertilization Effect" suggests that increasing levels can boost the growth of certain C3 crops, although it is also the major greenhouse gas responsible for global warming.
Comparing the percentages: \( 78.08% (C)>20.95% (D)>0.93% (B)>0.04% (A) \).
The correct descending sequence is therefore C, D, B, A.
Step 3: Final Answer:
By comparing the standard volumetric concentrations of Nitrogen (\( \approx 78% \)), Oxygen (\( \approx 21% \)), Argon (\( \approx 0.9% \)), and Carbon dioxide (\( \approx 0.04% \)), we determine the descending order is C, D, B, A.
Therefore, the correct option is (D).