Calibration Equations for the Neutron Probe

These papers have reported calibration equations normalised to a 200 l water drum count (WD), using 50mm aluminium tubes.

Ham, G.J.

Water Consumption by Sugar Cane under Commercial Growing Conditions, Fifth Afro-Asian Regional Conference, Proceedings, 'Planning and Management of water for Agriculture in the Tropics', Townsville, August 1985

VSW = 0.7447 * CNT/WD + 0.0133 (sandy clay loam)

VSW = 0.651 * CNT/WD + 0.9848 (loamy sand)

Poulton, D.C., Dale, M.O., Nagarajah, S.

Field Calibration of a Neutron Moisture Meter for the estimation of Soil Water Content in Mallee Soil Types of varying Texture, Conference on Agriculture Engineering, Adelaide, August 1986

The paper has graphs showing the derived equation and also the result from each of the soil samples, showing clearly the wide variability in soil moisture in real soil profiles.

VSW = 0.737 * CNT/WD + 0.108 (all Mallee 0.0-0.2m)

VSW = 0.740 * CNT/WD + 0.125 (all Mallee 0.3-1.2m)

McKenzie D.C., Hucker K.W., Morthorpe L.J. and Baker P.J.

Field Calibration of a neutron-gamma probe in three agriculturally important soils of the Lower Macquarie Valley, Australian Journal of Experimental Agriculture, 1990, 30, 115-122.

The authors suggest that when various soil types are encountered using a single equation gave better precision, and that accuracy is better improved by averaging at least 3 tubes rather than using soil-specific equations.

VSW = 0.707 * CNT/WD - 0.160 (all soils)

Cull, P.O. (1979)

Irrigation Scheduling of cotton by computer, Ph.D. Thesis, University of New England, Armidale.
These are the 'standard' equations shipped with both PR41 and Prwin.

VSW = 0.64873 * CNT/WD - 0.0100 (20 cm depth)

VSW = 0.64873 * CNT/WD - 0.0100 (30 cm depth)

VSW = 0.66773 * CNT/WD + 0.0142 (40 cm depth)

VSW = 0.62702 * CNT/WD + 0.0333 (50 cm depth)

VSW = 0.60666 * CNT/WD + 0.0337 (60 cm depth)

VSW = 0.54966 * CNT/WD + 0.0418 (80 cm depth)

VSW = 0.56730 * CNT/WD + 0.0339 (100 cm depth)

VSW = 0.60259 * CNT/WD - 0.0201 (120 cm depth)

These 8 equations are essentially the same, and many users just use equation 1 (20 cm) at all soil horizons and types.

Davoren, Tony: HydroServices, Christchurch, New Zealand

Unpublished ??

The author recommends using a single equation, but noting that very high salt could affect readings.

VSW = 0.6244 * CNT/WD + 0.00644 (all soils)

Burgess, P.J., Reinhard, B.R. & Pasturel, P. (2006).

Compatible measurements of volumetric soil water content using a neutron probe and Diviner 2000 after field calibration. Soil Use and Management, December 2006, 22: 401-404.

VSW = 0.754 (± 0.076) * CNT/WD + 0.057 (± 0.039) (all depths in trial)

Also quote: Jarvis NJ & Leeds-Harrison PB 1987 . Some problems associated with the neutron probe in swelling/shrinking clay soil. Journal of Soil Science 38, 149-156....

VSW = 0.769 * CNT/WD + 0.038 (clay, 30cm)

and: Bell JP 1987. Neutron Probe Practice. Institute of Hydrology. Report 19....

VSW = 0.958 * CNT/WD – 0. 012 (Clay and peat)