Measurement of Field Capacity with a Neutron Meter

Abstract

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.

Winter water regimes of clay soils

“…8 at matric potentials of -300 mbar, Salter & Williams, 1965) do not conform with values measured under field conditions (Webster & Beckett, 1972); and that the Soil Survey of England and Wales later adopted -50 mbar as a practical measure of matric potential at 'field capacity' (Reeve et al, 1973). However, clay soils have always been regarded as problematic, showing erratic changes in 8 during experimental determinations of field capacity using standard procedures of thorough plot irrigation and sampling once drainage has measurably ceased (Burrows & Kirkham, 1958). On the other hand, experience of the Windsor Series after slow seasonal wetting by rainfall indicated a behaviour that was far from erratic ( Fig.…”

“…In this respect, clay soils pose particular problems. Burrows & Kirkham (1958) found that the moisture release behaviour of soils approaching field capacity after irrigation is erratic where clay content is high. In fact, they could not identify a specific value for field capacity in the clays that they tested, in contrast with lighter soils.…”

Determination of deep percolation losses in loessial soils

“…This was near field capacity for Ida silt loam (Burrows and Kirkham, 1958). Since the initial moisture content was relatively high, the NE plot was flooded using only the 1,000-gallon tank.…”

“…Field measurement of field capacity was facilitated by development of the neutron probe. Burrows and Kirkham (1958) Field studies related to deep percolation Many papers have been published concerning moisture and tension changes in the field during and after water application, (Hilgeman, 1948;Cannell and Stolzy, 1962;Richards ^ al., 1956;Ogata and Richards, 1957;Wilcox, 1959;Wilcox, 1960;Nielsen et a^., 1964;and van Bavel et al, 1968a). Richards (1956) reported results from an experiment where bare soil was deeply irrigated with water containing a known concentra tion of chloride.…”

Determination of deep percolation losses in loessial soils

“…Field measurement of field capacity was facilitated by development of the neutron probe. Burrows and Kirkham (1958) employed this technique to estimate field capacities of four soils in Iowa. The shape of the moisture content versus time curves was as expected for two silt loam soils and one sandy loam but a clay loam soil showed extreme variability in resulting estimates because of slow water movement in the soil.…”