We use chemical fertilizer . . . . . . . soften the soil. A. to B. for C. on D. at

The correct answer is: A. to

We use chemical fertilizer to soften the soil.

The other options are incorrect because:

• For is used to indicate the purpose of something. For example, “We use chemical fertilizer for growing crops.”
• On is used to indicate the location of something. For example, “We put the chemical fertilizer on the soil.”
• At is used to indicate a specific time or place. For example, “We put the chemical fertilizer at the edge of the field.”

In this case, we are using chemical fertilizer to soften the soil. Therefore, the correct preposition is “to.”