ARS | Publication request: Non-invasive measurements of soil water content using a pulsed 14 MeV neutron generator

Submitted to: Soil & Tillage Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 31, 2011
Publication Date: January 26, 2012
Citation: Mitra, S., Wielopolski, L., Omonode, R., Novak, J.M., Frederick, J., Chan, A. 2012. Non-invasive measurements of soil water content using a pulsed 14 MeV neutron generator. Soil & Tillage Research. 120:130-136.

Interpretive Summary: In growing crops under irrigation, it is important to know when to apply water. One technique used to set an irrigation schedule is to measure soil moisture contents. The traditional method to measure soil moisture is to collect numerous field samples and after oven-drying, determine the water lost by the samples change in weight. This is a laborious method and provides a one-time data point. We developed a new ‘on-the-go’ method where soil moisture contents can be non-destructively measured using an electrically switchable neutron probe mounted to a cart. As the cart is pulled across the field, water is measured nondestructively by neutrons that interact with its hydrogen atoms. Results collected using the neutron source across a cotton field in South Carolina showed a high correlation with moisture contents determined using the more destructive technique. This technology offers the potential to deliver a spatially or temporally continuous estimate of soil moisture across a large field. Continuous estimates of soil moisture contents can be more useful for setting up a crop irrigation schedule because the results are obtained more quickly than the oven-drying method which allows for more rapid adjustments of crop water needs.

Technical Abstract: Most current techniques of setting crop irrigation schedules use invasive, labor-intensive soil-water content measurements. We developed a cart-mounted neutron probe capable of non-invasive measurements of volumetric soil moisture contents. The instrument emits neutrons which are captured by hydrogen atoms of the water molecules. The gamma-ray detectors mounted on a cart recorded the number of characteristic neutron induced prompt gamma-rays from hydrogen and estimated their quantity. The cart was towed across several established transects dissecting a South Carolina cotton field containing both well and poorly drained soils. Soil samples were also collected along the transects down to 30-cm depth and their moisture contents determined gravimetrically. Regression analyses between soil moisture contents estimated using these two methods for the 0 to 30-cm depth samples were significant (r squared = 0.89). Using prompt-gamma neutron activation analyses in this study was shown to provide a reliable continuous estimate of volumetric soil moisture contents.