1. Dep. of Civil and Construction Eng., Town Eng. Bldg.Iowa State Univ.AmesIA50011
2. Center for Agric. and Rural Dev. Dep. of Economics, Iowa State Univ.AmesIA50011
3. USDA‐ARS, National Soil Tilth Lab., Deep Loess Research Stn.Council BluffsIA51503
4. The Texas Agric. Exp. Stn., Blackland Research CenterTempleTX76502