| The temporal and spatial variability of soil N, P and K contents in the plough layer(020 cm)of farmlands in different agroecological regions of Ningxia was analyzed according to the data of soil nutrients tested from 1970 to 2000. The results showed that the contents of total N, P and K in the soils were relatively steady in the past 30 years. The contents of soil alkali-hydrolyzable N and available P were increased gradually. The content of readily available K was not obviously changed in the semiarid mountain region, but was decreased significantly in the irrigation region along Yellow River during 1985 to 2000. By comparison the contents of soil nutrients in the irrigation region were higher than that in the mountain region as a whole. There was the largest difference between the two regions in available P, and there were very significant differences in total N, alkali-hydrolyzable N and available P contents. There were also significant differences at 10%5% level in total K and readily available P contents, but no difference in total P content between the two regions. The variation coefficients of all soil nutrients (except total K) in mountain region were obviously higher than that in the irrigation region. The order of variation coefficients in the semiarid mountain region was: total N>available P>readily available K>alkali-hydrolyzable N>total P>total K, and in the irrigation region was: alkali-hydrolyzable N,available P>readily available K>total N>total P>total K. The variation coefficient of total K content was the lowest in both the mountain region and the irrigation region in Ningxia. |