| Dam land and terrace are basic farmland in the Loess hilly region and considered as life saving land. The quality on soil physical and chemical properties of dam land and terrace is the decisive factor of agricultural production and regional economic development. The potentiality to expand cultivated area is very limited in the studying area, and managing existing dam land and terrace is very important to solve grain self-sufficiency problem and consolidate the achievement of returning farmland into forest in this region. This paper takes on the dam land and terrace in Nianzhuang small watershed of Yan'an City as research object, through field investigation and lab test, basic characteristics and existing problems and fostering way on soil physical and chemic properties of dam land and terrace were studied. The goal was to solve grain self-sufficiency problem and regional economic development in this region. The main results were as follows: 1. Soil physical properties have differences in various degree. Coarse silt particle is the most in soil separates, coarse silt particle and fine clay particle account for more than 75% equally. Coarse degree of soil separates has significant difference. Microaggregate content of coarse silt particle and fine clay particle account for about 80% in dam land, sloping cultivated land accounting for more than 90%, terrace lying in middle. Microaggregates have good stability. Stable total aggregate content is low(fewer than 10%), quality is poor(content of larger size aggregate little), and it is very bad to resist soil erosion. Soil water capacity of dam land is very rich, and higher by 33.33% than terrace, and higher by 55.06% than CK. Soil bulk densities of dam land is higher than terrace, infiltration is bad. 2. Soil fertility properties have differences in various degree. Imbalance of soil nutrients is very serious in studying area. Organic matter, available N and available P are poor. Total N is scarce. Total P, total K and available K are rich. CEC changes in 6~8cmol/kg, difference is insignificant. The pH value changes from 8.08 to 9.74, CK>dam land> terrace. The content of CaCO3 is abundant, but difference is insignificant. Soil conductance rate of top layer in alkaline dam land is very great (46.51Ms/cm), it is about 3 times of terrace's, and influencing the output of crops seriously. The spatial difference of soil nutrients is not obvious in different position within dam land. However, it is obvious difference among different slope location terrace. As for dam land and terrace, soil fertility of large age farmland is better than small age farmland's. 3. Significant correlations are showed between soil physical properties and soil chemical properties. Fine clay particle with all physical indicators is significant positive correlation. Stable total aggregate with soil structure coefficient, with soil total porosity are significant positive correlation. Significant correlations are found among all chemical indicators except for total P. Organic matter, total N, available N, available P, available K, CEC, soil conductance rate are enriched obviously in fine particle, but pH value, CaCO3, total P, total K is not obvious. Significant correlations are showed among between total N, available N, available K, fine clay particle and total stable aggregate. Total P and available P with soil structure coefficient are significant negative correlation. Soil conductance rate with soil bulk density is significant positive correlation. Organic matter with all physical indicators is significant positive correlation. 4.Soil fertility qualities have differences in various degree. Soil fertility qualities of terrace belonged to grade 1 and 2 with the better. Soil fertility qualities of dam land have obvious difference, soil fertility qualities of middle age dam land are better than small age's and large age's. Soil fertility qualities of dam land tend towards declining wholly. Soil fertility qualities of sloping cultivated land is the poorest, belonging to grade 4.
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