Soil moisture is a key state variable in environmental monitoring and in the water, energy and carbon cycles [1] - [3] . Farm management decisions, including the timing of planting, application of fertilizers, pesticides, herbicides, and irrigation scheduling, are influenced by soil moisture status [4] , [5] . Moreover, it is highly variable both in space and time and the estimation of this variable is challenging because the amount of moisture in the surficial soil layer is influenced by soil texture [6] , [7] . Passive microwave remote sensing is a well-accepted technique for estimating soil moisture due to the large contrast between the dielectric properties of liquid water (~80) and that of dry soil matter (~3.5) [8] , and reduced sensitivity to surface roughness and vegetation as compared to active microwave [9] . Current missions, including the Soil Moisture and Ocean Salinity (SMOS; [10] ) and Soil Moisture Active Passive (SMAP; [11] ) operating at L-band radiometer (~21 cm) are only able to detect shallow soil moisture (up to 5 cm in depth; [12] ). Compared with L-band, P-band (~ 40 cm) is even less sensitive to vegetation water content [13] and surface roughness [14] , and is able to penetrate deeper into the soil providing information about moisture over deeper depths (~10 cm; [12] ).