That is a form of what to keep his soil as productive as possible which of these actions would him achieve h
is goal

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. So by not doing things other than mentioned above will make his soil productive

Answer volcanic outgassing;