Tai-lake basin locates at the lower YanQtse delta under a northern monsoon climate. It covers an area of about 30 thousand square kilometers. Paddy fields occupy about 90% of the total cultivated area.Historically, most of the land are used for rice-wheat rotation annually. At present, when conditions are favorable, large areas have been adapted for three crops a year i.e. rice-rice-upland crops (barley, rape or wheat). It is one of the high productive areas in this country with annual yield averaging 9-10.5 tons of grain per hectare. Since paddy soils are developped under long time of submersion with periodical wet and dry cultivation, the main pedogenic process of paddy soil is alternative of reduction and oxidation accompanied with the formation of ferro-humic coating and streaks and formation of Paitu. In consequence, the morphological feamrc of paddy soils as well as their physical, chemical and biological properties are diverged Erom the original upland soils. The presence of a plowed layer, plowpan, perco-submugic layu or illuvial gley sported layer usually charactcrizes the specific features of a paddy soil profile.