John is a farmer who wants to keep his soil as productive as possible. which of these actions would him achieve this goal?
excessive irrigation
crop rotation
soil exhaustion
leaching

Think about this logically. 

Excessive Irrigation would be flooding the land. This would wash away soil and nutrients for the plants. 

Crop Rotation is rotating crops to preserve the soil. 

Soil Exhaustion is when you exhaust the soil of its nutrients, making it basically barren until the nutrients replenish.

Leaching also takes away nutrients from the soil. 

Answerair masses develop near the equator are warm; tropical air mass