Polarimetric synthetic aperture radar (PolSAR) images are widely used for agricultural fields monitoring and change detection applications due to their all-weather acquisition possibilities and inherent properties including phase and amplitude information. The techniques used for such temporal applications can be cast in two groups: polarimetric (incoherent) and polarimetric-interferometric (coherent), being represented in this letter by the Kullback-Leibler distance and the mutual information, respectively. The goal of this letter is to characterize these two kinds of different information sources in terms of ground measurement parameters of the agricultural fields and to figure out the relationship between temporal trends of the similarity measures versus temporal trends of the physical parameters without dealing with inverse problems. For this purpose, multitemporal fully polarimetric SAR images, which are acquired in the frame of the AgriSAR 2006 campaign with synchronous ground surface measurements over a whole vegetation period, are analyzed. The results have clearly demonstrated that the coherent measures have a strong relationship with wet biomass of crops. Although incoherent measures would be the preferred ones due to their simplicity in implementation, they showed to be very sensitive to changes in precipitation, causing misleading temporal interpretation at longer wavelength in some cases.