Neutron Meter Research Articles

Soil Moisture Interface Effects upon Readings of Neutron Moisture Probes

AbstractMeasurements of soil moisture by the neutron scattering method are inaccurate when made in the presence of abrupt moisture changes. The effects of air‐soil interfaces and of wet‐dry soil interfaces were investigated using neutron probes of four different designs. Three of the probes were used to study the effects of wet soil layers within dry soil masses.Readings taken at shallow depths were found to have the greatest errors. These errors were always negative. Absolute error and depth within the soil to which this effect occurs were related to soil moisture content. Some means of correcting such errors should be used. Errors in determining moisture content of soil zones containing wet‐dry interfaces or wet layers were not as serious, and could be neglected in many practical applications. This was especially true when depth reading increments of 6 inches or less were used. These errors were usually negative. Neutron probes tend to underestimate soil moisture content at interfaces or in stratified profiles.Probe depth reading increments of 1, 3, and 6 inches all gave about the same accuracy of moisture determination. Twelve‐inch reading increments gave errors of as much as 6 times greater than those of the smaller increments.The influence of probe geometry upon neutron meter readings is also discussed. When compared on a basis of equivalent source strength, in uniform soils, the probe with end placement of the source was found to be considerably more efficient than the probes with center placement of the source.

Measurement of Field Capacity with a Neutron Meter

An experiment to determine the moisture vs time curve, and hence the field capacity, was carried out at 6-inch increments to a depth of 5 feet on 4 soils. Plots of soil were soaked with water and a soil moisture meter employing neutron scattering used to determine the moisture content of the soil profile at different times following water entry into the soil. The data were plotted as curves of soil moisture content on a volume basis vs time in hours following wetting. The general shape of the curves was about as expected for 2 silt loam soils and 1 sandy loam soil, but certain layers in these 3 soils deviated from the normal pattern. The deviations are explained on the basis of physical conditions of the soil existing some distance from the soil layer in question. A clay loam soil showed extreme variability, with little movement of appreciable quantities of water; a field capacity for it could not be defined. The neutron meter used was found to be an excellent device for this type of study; confounding of soil variation in sampling errors was, through its use, eliminated.