This is due to several reasons:
(1) Nutrient uptake by plants
Plants take up several mineral ions such as nitrates(for general growth), phosphates (for root growth), magnesium (for chlorophyll).
In order for the plant to keep growing healthily, the soil has to be constantly replenished to supply them with the mineral ions required.
(2) Soil erosion/Percolation
Heavy precipitation, can result in the washing away of nutrients via percolation.
