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.

The changes to agriculture in the united states since 1900 is that increased mechanization has increased food production, thereby increasing the need for labor. there has been an increased need for higher speeds of food production due the fact that the population in the united states continues to rise.