Soil moisture content on a horizontal scale of hectometers and at depths of decimeters can be inferred from measurements of low-energy cosmic-ray neutrons that are generated within soil, moderated mainly by hydrogen atoms, and diffused back to the atmosphere. These neutrons are sensitive to water content changes, but largely insensitive to variations in soil chemistry, and their intensity above the surface is inversely correlated with hydrogen content of the soil. The measurement with a portable neutron detector placed a few meters above the ground takes minutes to hours, permitting high-resolution, long-term monitoring of undisturbed soil moisture conditions. The large footprint makes the method suitable for weather and short-term climate forecast initialization and for calibration of satellite sensors, and the measurement depth makes the probe ideal for studies of plant/soil interaction and atmosphere/soil exchange.