Fertility of soil is measured by its ability to

The fertility of the soil is measured by its ability to support life.
Soil fertility refers to the ability of soil to sustain agricultural plant growth, i.e. to provide plant habitat and result in sustained and consistent yields of high quality.
It also refers to the soil's ability to supply plant/crop nutrients in the right quantities and qualities over a sustained period. The ability to supply essential plant nutrients and water in adequate amounts and proportions for plant growth and reproduction.