During the Soil Moisture Experiments in 2002 (SMEX02), the Polarimetric Scanning Radiometer (PSR) instrument was flown over the Walnut Creek Watershed in Iowa. This agricultural region, dominated by corn and soybeans, was selected so that microwave remote sensing algorithms could be tested under conditions of rapidly evolving vegetation conditions. Vegetation water contents varied widely between the two crops and also increased significantly during SMEX02. Values increased from 0.3–0.7 kg/m2 for soybeans and from 3-5 kg/m2 for the corn sites. PSR collected C-band (6.0 – 7.35 GHz) and X-band (10.7 GHz) brightness temperatures to support validation of the Advanced Microwave Scanning Radiometer for EOS (AMSR-E). The resulting data set enables us to examine the effects of soil and vegetation parameters on the sensitivities and accuracy of a microwave soil moisture retrieval model. In order to evaluate the utility of C- and X-band brightness temperatures for retrieving soil moisture, we used PSR brightness temperatures at each frequency to retrieve watershed-mean near-surface soil moisture. We have evaluated the sensor’s sensitivity to observed soil moisture as well as the sensitivities of the retrieved soil moisture to surface roughness, the vegetation B parameter and single scattering albedo. Implications for large-scale soil moisture retrieval from AMSR-E will be discussed.