Consider Figure, which illustrates the response of wheat plants (shoot weight) to increasing soil P, Zn, and bulk density,

(a) Explain, using specific data, how the graphs illustrate the principle of limiting factors (or "the law of the minimum").

(b) Describe the conditions in the experiment under which compaction (rather than zinc or phosphorus) is the most limiting factor.

(c) Explain in detail, using specific data, how the graphs also illustrate exceptions to or limitations of the "law of the minimum."