optimal level of a new fertilizer

A farmer wishes to determine the optimal level of a new fertilizer on his soybean 
crop. Some factors that affect crop yield are fertilizer, precipitation, sunlight, method of tilling the soil, type of soil, plant and temperature. Many of these factors cannot be controlled or manipulated to increase crop yield, but the amount of fertilizer can be. An experiment is designed to study the effect of various amounts of fertilizer on crop yield. The soybean plants are randomly assigned to the different treatments.

Treatment A: 20 soybean plants receive no fertilizer

Treatment B: 20 soybean plants receive 2 teaspoons of fertilizer per 
gallon of water every 2 weeks

Treatment C: 20 soybean plants receive 4 teaspoons of fertilizer per 
gallon of water every 2 weeks

Diagram the experiment. Indicate the subjects of the experiment, the explanatory variable, and the response variable.