Step 1: Understanding the Concept:
This question deals with a key design constraint for sprinkler irrigation systems. The application rate of the sprinkler (how fast it applies water to the soil surface) must be matched to the soil's infiltration rate (how fast the soil can absorb the water). If the application rate exceeds the infiltration rate, the excess water will not be absorbed, leading to ponding on the surface and runoff, which causes water loss and soil erosion.
Step 2: Detailed Explanation:
1. Infiltration Rate: This is the rate at which water enters the soil from the surface. Heavy clay soils have very fine particles and small pores, which results in a very low infiltration rate.
2. Sprinkler Application Rate: This is the rate at which the sprinklers deliver water to a specific area, usually measured in mm/hour. It is a design parameter of the sprinkler system, determined by the nozzle size, operating pressure, and sprinkler spacing.
3. The Matching Principle: For a sprinkler system to be effective and efficient, the following condition must be met:
\[ \text{Sprinkler Application Rate} \le \text{Soil Infiltration Rate} \]
4. Heavy clay soils have very low infiltration rates, often falling below 4-5 mm/hour.
5. Standard sprinklers, especially impact sprinklers used for field crops, often have application rates that are higher than this. It is difficult and expensive to design a sprinkler system with a very low application rate.
6. The question asks for the infiltration rate below which sprinkler systems are "not usually suitable." This implies a threshold where it becomes impractical to design a system that doesn't cause runoff.
7. A soil infiltration rate of less than 4 mm per hour is generally considered the lower limit for the practical use of most conventional sprinkler irrigation systems. Below this rate, runoff is very likely to occur, making the system inefficient and potentially harmful to the soil structure.
Step 3: Final Answer:
Sprinkler irrigation is generally not suitable for soils with a very low infiltration rate, typically less than 4 mm per hour. Therefore, option (A) is the correct answer.