A farmer wishes to test the effects of a new fertilizer on her potato yield. She has four equal-sized plots of land. One plot has sandy soil, one has rocky soil, one has clay-rich soil, and one has average soil. She divides each of the four plots into three equal-sized portions and randomly labels them A, B, and C. The four A portions of land are treated with her old fertilizer, the four B portions of land are treated with the new fertilizer, and the four C portions of land are treated with no fertilizer. At harvest time, the potato yield is recorded for each section of land. What is the treatment in this experiment?

A. the four types of soil

B. the soybean yield at harvest time

C. the soybean plants on the various plots of land

D. the tree types of fertilizer