| Neutron probe | Samples a relatively large area
One sensor for all sites and depths
Unaffected by salinity and air gaps around access tube
Robust and the most accurate SMS (0.5% error) | Soil/site specific calibration usually required\nContains radioactive material (safety hazard).\nLicensing and certified personnel is required. Even at 16-inch depth, radiation losses through soil surface have been detected\nManual reading and recording (≈3 minutes per access tube)\nNot good at shallow depths (less than 6 inches).\nReadings close to the soil surface are difficult and not accurate\nHeavy, cumbersome instrument\nThe sphere of influence may vary according to the following reasons: a) It increases as the soil dries because the hydrogen concentration reduces, so that the probability of collision is smaller and thereby fast neutrons can travel further from the source. b) It is smaller in fine texture soils because they can hold more water, thus the probability of collision is higher. c) If there are layers with large differences in water content due to changes in soil physical properties, the sphere of influence can have a distorted shape | 10,000 - 15,000 per sensor
25 - 30 for access tubes |