Fig. 6. Trend changes of soil fertility in long-term fertilizers experiment in Pingliang, Gansu, China. Data showed here were continuous from 1979 through 1991 and 1996 through 1998; soil samples for 1992–1995 were not analyzed (Dashed lines), and total P, available P, and available K for 1996 were not determined except total N and soil organic matter.