Abstract: Geostatistics has proven to be useful for characterizing and mapping spatial variability of soil properties, however, most of the previous geostatistical studies were focused on data at small scale. With the development of GIS and GPS, geostatistcs and GIS are becoming indispensable in characterizing and summarizing spatial information in large regions to provide quantitative support to decision and policy making for soil, agricultural and natural resources management. In this paper, we applied geostatistics combined with GIS to analyze the spatio temporal variabilities of the available potassium, available phosphorus and organic matter in soil surface （0～20 cm） in Zunhua county （municipality） of Hebei province over 20 years from 1980 to 1999. Results showed the average content of the available potassium, available phosphorus and organic matter in soil surface in 1980 was 77 78 mg/kg, 19 46 mg/kg, 1 14%, respectively; and that in 1999 was 90 12 mg/kg, 25 7 mg/kg, 1 54%, respectively. Paired samples t test of soil nutrients and the results estimated by block kriging indicated the content of the available potassium, available phosphorus and organic matter in 1999 was significantly higher than that in 1980. The ratio of nugget to sill of the three soil nutrients varied from 32 3% to 60%, indicating the spatial correlation of the three tested soil nutrients at this large scale was moderately dependent. The ratio of nugget to sill of available potassium was 56 5% in 1980 and 41 1% in 1999, that of the available phosphorus was 45 5% in 1980 and increased to 54 1% in 1999. The ratio of organic matter changed considerably from 60% in 1980 to 32 3% in 1999. The range of the available potassium and available phosphorus between the year of 1980 and 1999 was almost the same, however, the range of organic matter was from 4 8 km in 1980 to 2 7 km in 1999. The improvement of the available potassium and available phosphorus in soil was mainly contributed to the more application of fertilizers, and the content of organic matter was affected by the changes of tillage and land use, which also altered the spatial distribution of soil nutrients to different degrees over the past 20 years.

Key words: Geostatistics, soil nutrients, spatio temporal variability, Kriging, GIS