The soil becomes less fertile and crops cannot be grown on such a type of soil.
Plants are continuous eaters of nutrients and minerals,Thus they take away all the nutrients from the soil,leaving the land with less or no nutrients.
Thus,continuous plantation might add up to the problem,continuously planting will lead to continuous depletion of underground nutrients.
Thus,the soil does not get enough time to replenish itself with nutrients.
This how how the soil gets affected :)