How will the amount of fertilizer used on a field affect the number of earthworms found there? What is the independent variable?

The amount of fertilizer used on a field WILL INCREASE the number of earthworms that is found in the field. The higher the amount of the fertilizer, the higher the number of earthworms.
There are many evidence which shows that most inorganic fertilllizers favor the buid up of large amount of earthworm.