Dear Drlatief

We did such measurements on natural runoff plots using access tubes in

northern Iraq about three decades ago. An excellent reference on the subject is that of Hillel, 2004. The device costed us several thousands

US $ at the time. By inserting the device radioactive probe in the access

tube the device gives volumetric soil moisture and soil bulk density

at the specified soil depth. In our case (Hussein etc al., 2017) the depths

were between 5 and 120 cm.

Regards

Hillel D. (2004) Introduction to Environmental Soil Physics. Academic Press

Hussein e.g. al. (2017) Rainfed farming prospects....

J Aridland Agric. 3: 11-17

https://sanangelo.tamu.edu/extension/agronomy/agronomy-publications/grain-sorghum-production-in-west-central-texas/how-to-estimate-soil-moisture-by-feel/soil-moisture-measuring-devices/neutron-moisture-meters/

please see it

We did such measurements some twenty year ago with a home-made system as we had in the lab both an appropriate neutron source (Am-Be) and an appropriate slow-neutron counter (BF3). A few important points to consider:

a) the activity of the source is well above the limits allowed for free use, thus the user should obtain the authorisation, for the permanent storage of the source, its transport and its use in the field, from the radiation protection authority. Obtaining an authorisation for use in the field could be subject to very strong constraints because of the risk of exposing the population to radiation and also the risk of loss of the source in the subsoil. The cost of radiation protection should not be neglected.

b) the calibration of the neutron measurement in terms of moisture somewhat depends on the bulk density and composition of the soil;

c) the calibration also depend on the depth down to 1 meter depth.

The method proved to be reliable and useful for the control of seasonal variations of soil moisture, which was our main objective.

I have never actually done this, but let me give you some random thoughts about the problem.

First, look at the excellent publication

Thank you sir