The answer is soil degradation. It is an artificial evolution related to the local climate and vegetation. It is caused by the replacement of primary plant communities (known as climax) by the secondary communities. This replacement changes the humus composition and quantity, and affects the formation of the soil. It is directly related to human activity. Soil degradation may also be defined as any change or disturbance in the soil perceived to be deleterious or undesirable. In this question, the more the farmer worked, the more he incidentally changed the formation and structure of the soil which resulted in the decrease in his crop yield.
