[Distribution and characters of soil dry layer in north Shaanxi Loess Plateau].

Based on a whole survey and detailed analysis in artificial Robinnia pseudoscacia forestland, distribution and characters of dry layer in soil were studied in north Shaanxi Loess Platea. The results showed that soil dry layer was distributed universally from south to north in Loess Plateau, which can be classified into four zones according to the dryness: 1) In southern gully zone represented by Ziwu Ridge and Huanglong Moutain in Yijun County, the soil dry layer was gentle, so if the appropriate measures were taken and water-saving facility were available or in water-abundant year, the shortage of soil water could be alleviated; 2) In northern gully zone represented by Fuxian and Huangling County, light soil dry layer occurs and was getting worse and we should pay close attention to this; 3) In the large hill and gully zone, serious dry layer appears, it could be subdivided into south, west and north plots. In south plot represented by Yanan and Yanchang County the dry layer was moderate; in west represented by Wuqi and Ansai County the dry layer was severe; in north represented by Suide and Mizhi County the dry layer was disastrous; 4) In the windy sand zone represented by Shenmu County, since the precipitation decreased drastically and seriously sandy, very grave soil desiccation appeared. Also, soil dryness showed apparent horizontal differences with rainfall and the apparent vertical gradient in small hill area.