Figure 4. Soil available N (a), Olsen-P (b) and available K (c) gap variation by N (SNGN, soil available nutrient difference between NPK and PK plots, triangle up), P (SNGP, the difference between NPK and NK plots, triangle down), K (SNGK, the difference between NPK and NP plots, square) or NPK (SNGNPK, the difference between NPK and Control plots, circle) fertilizer omission from 1994 to 2006 during maize season at Yucheng, Shandong Province of China.
