Spatial variability of soil chemical properties in a paddy field

To obtain basic information for site-specific soil management to improve nutrient use efficiency by plants, spatial variability of soil properties was evaluated in a. 50 m x 100 m paddy field. Ninety-one surface soil samples were collected after harvest to investigate the spatial variability of their chemical properties: pH, EC, total C content, total N content, C/N ratio, contents of available P, exchangeable Ca, Mg, K, and Na. Fifty samples were also collected after transplanting to investigate that of nitrogen-related properties: total C content, total N content, C/N ratio, and contents of mineralizable N and inorganic N. Geostatistical analysis was carried out to examine within-field spatial variability using semivariograms and kriged maps as well as descriptive statistics. Descriptive statistics showed that the coefficient of variation for the EC, total C content, total N content, contents of available P, exchangeable K, Na, mineralizable N, and inorganic N exceeded 10%, suggesting a relatively high variability. Geostatistical analysis indicated a high spatial dependence for all the properties except for the pH and inorganic N content. The ranges of spatial dependence were about 20 m for EC, total C content, total N content, C/N ratio, contents of exchangeable Ca, Mg, Na, and mineralizable N, and about 40-50 m for the contents of available P and exchangeable K. Based on the results of spatial dependence, kriged maps were prepared for the properties to analyze their spatial distribution in the field. The results reflected the history of soil management of the field in addition to the characteristics of the inherent soil properties. In conclusion, rational sampling interval was evaluated at 20-50 m depending on the soil properties, and the need for site-specific soil management and possibility of precision agriculture were demonstrated even in an almost flat paddy field.