An agricultural field experiment was conducted by Bo D. Pettersson in the Nordic Research Circle for Biodynamic Farming in Järna, Sweden, which began in 1958 and lasted until 1990. The field experiment included eight different fertilizer treatments with a primary focus on aspects of soil fertility. Treatments were assigned to subplots with identical specifications. During the time, the yield increased in all treatments but the organic treatments resulted in higher soil fertility. Identify the

(a) response variable,

(b) explanatory variable,

(c) experimental units, and

(d) treatments and

(e) explain what it means to say "the organic treatments resulted in higher soil fertility".