" Neutrons richochet strongly from large atoms.. however dramatically slowed down .. if ..collide with hydrogen atoms.. In most soils, the only source of hydrogen would be water, hence the only slowing down on fast neutrons would be due to water."

Because of the nature of its operation would not the sphere of influence change with the following

1. THE DRYING CYCLE OF SOIL - as the soil dries, and the concentration of hydrogens is reduced, the probability of fast neutrons travelling further from source before colliding would be higher. Therefore sphere of influence grows as soil dries out. And of course the reverse would happen as the soil profile refilled.

2. SANDY SOIL compared to CLAY SOIL - If clay soil can have higher soil moisture content then the probability of collision is higher and sphere of influence smaller than sand.

3. DUPLEX SOIL or DRY TOP SOIL/WET SUBSOIL - If there is a change of soil structure or soil moisture content with soil depth would not the sphere of influence be distorted, no longer in a possible circle but odd shaped. For example if the top 20 cm is dry sandy soil and 30 and 40 cm has perched water table due to clay barrier at 50 cm. Then if the sensor is placed at 20 cm, would not the probiblity of collision vary between the 10-20 cm and 30-40cm and so distort the sphere of influence. The fast neutrons going into 10-20 would travel some distance before collision, some even being lost to atomsphere and never returning while any travelling to 30 cm would reach the required number of collisions to slow then down sooner and closer to the source.

********************************************** SURFACE
            ****************                5 CM
           ******************                10 CM
          ********************                15 CM
         ********Source********                20 CM
          ********************                25 CM
              ************                 30 CM
                 ******                  35 CM
                   **                  40 CM

How reliable does this make a Neutron probe when close to soil surface ?
[see disadvantages below]
How does the onion shape of a wetting front under drip irrigation effect the sphere of influence ?
[ditto]
I realise calibration is required for specific soil type. If such distortion does occur, would not using one standard calibration lead to cases of under and over calculation of irrigation requirements, especially in shallow rooted crops (eg Lettuce, Strawberries, Potatoes etc) ?

Has any person conducted research investigating such issues ?

AND Is having a LARGE SPHERE OF INFLUENCE AND ADVANTAGE OR DISADVANTAGE ?