2Unger P W. Tillage and residue effects on wheat, sorghum, and sunflower grown in rotation [ J ]. Soil Sci Soc Am J, 1984,48 :885--891.
3Unger P W, Parker J J. Evaporation reduction from soil with wheat, sorghum, and cotton residues [ J ]. Soil Sci Soc Am J,1976,40 : 938--942.
4Extension Service of Iowa State University. Conservation tillage:Effects on soil erosion[ R]. Agronomy 721. Iowa State University. 1991.
5Casady W W, Massey R E. Costs end returns[A]. Conservation tillage Systems and Management (2nd edition)[C]. Ames, Iowa:MWPS245. Iowa State University,2000.
6Drury C F. Red clover, tillage influence on soil temperature, water content, and corn emergence [ J ]. A gron J, 1999,91 : 101--108.
7Schertz D. Conservation tillage: an analysis of acreage projections in the United States[J]. J Soil Water Conserv, 1986,33:256--258.
8Derpach R. Historical review of no-tillage cultivation of crops[A], Proceedings, The 1st JIRCAS Seminar on Soybean Research. No-tillage Cultivation and Future Research Needs [ C].Iguassu Falls, Brazil. 1998,13:1--18.
9Conservation Technology Information Center (CTIC). What is conservation tillage[EB/OL], http://www2.ctic. purdue.edu/ctic/CRM2004/1990 - 2004dada. pdf, 2006 - 05 - 21.
10Conservation Technology Information Center (CTIC). Conservation tillage and other tillage types in the United States-1990- 2004 [ EB/OL ]. http://www2. cric. purdue.edu/ctic/CRM2004/1990 - 2004data. pdf,2006 - 05 - 21.
