Abstract: 【Objective】Soil available potassium (AK) is one of the main soil fertility indicators and has a close relationship with grain quality and yield. With the rapid adjustment of agricultural structure, improvement of mechanization level and recommendation of straw return technology in recent 10 years, it is urging to explore the effect of those factors on the change of soil AK. The objectives of this study were to analyze the spatio-temporal variability and its influencing factors of soil AK over the past 30 years in Quzhou county, Hebei Province, and to provide a guidance for soil fertility evaluation and soil management.【Method】By field investigation and soil sampling, the contents of AK in the plow layers in 1980, 2000 and 2010 were collected and measured. The traditional statistical method was used to analyze the datasets of soil AK in three periods. Normality of the datasets was assessed using the Kolmogorov-Smirnov test. Geostatistical analysis consisting of variogram calculation, cross-validation, kriging and mapping was performed using the geostatistical analyst extension of ArcGIS 10.0. Firstly, the unobserved value was estimated by the kriging method, then transferred the file format into raster format; finally the raster calculator was used to obtain the distribution map of the difference in AK between two time intervals. Overlap analysis was used to assess the effect of soil texture, soil type and land use type on soil AK content. Analysis of variance and t-test was also conducted using Least Significant Difference procedure.【Result】It showed that the average AK contents were 167.1, 90, and 87.7 mg•kg-1 in 1980, 2000 and 2010, respectively, showing a decreasing trend over the investigated period. The average AK content was decreased by 46.1% from 1980 to 2000, while it was only decreased by 2.6% from 2000 to 2010. AK levels were classified into five grades from high to low (＞200, 150-200, 100-150, 50-100 and ＜50 mg•kg-1), the percentages of land with the highest and second-grade AK were 5.2% and 82.9% in the study area in 1980, respectively, but the area of the highest and second-grade reduced to almost zero in 2000 and 2010. The third-grade land in 1980 accounted for 11.9%, while it was increased to 22.3% in 2000 and 31.2% in 2010. There were no fourth-grade land in 1980, but it was increased to 78.7% in 2000 and 68% in 2010. Semivariogram analysis results showed that the spatial correlation distances of AK were 8.1 km, 2.8 km and 9.8 km in 1980, 2000 and 2010, respectively, reflecting that the influence of human activities on AK content increased over time. The spatial correlation range decreased with time with a rapid decrease of 5.3 km in the first two decades (1980-2000), while it showed an increase trend in the recent decade (2000-2010), which due to the combined effect caused by anthropic activities and regional factors. The AK contents for different soil textures had the following order: clay＞medium loam＞light loam＞sandy loam＞sand. The AK content in fluvo-acquic soil was significantly higher than that in saline soil. It was found that the AK content in forest land was the lowest in 1980 and 2000, but it was the highest in 2010. 【Conclusion】The AK content showed a decreasing trend over the past 30 years. Soil type, soil texture, land use type and farming practices were the main influencing factors. Straw return and fertilizer application based on soil test technology should be recommended. Straw return practice and balanced fertilization technology played an important role in effectively increasing the AK content in this region and should be adopted as the long-term management practice.

Key words: plow layer , available potassium , geostatistics , spatio-temporal variability , influencing factors