Soil Sampling Lubricants

Soil sampling can be a difficult task under adverse soil conditions. High clay content soils, along with high soil moisture levels are two variables, which cause major problems. Under adverse conditions, friction against the probe wall, causes core compression and sometimes completely stop soil from flowing into the soil probe. These conditions can also impede the removal of the soil core from the soil probe. The results are unacceptable soil cores, which must be discarded, and additional soil cores of good quality need to be collected. Lubricants such as WD-40 and PAM vegetable oil have been used in the soil testing industry for many years. Recent research at the University of Wyoming took a comprehensive look at possible contamination of samples while using several lubricants.

Table 1: Nutrient Content of Several Lubricants – University of Wyoming 1995

Lubricant NO3-N PO4-P K Fe Mn Zn Cu
ppm ppm ppm ppm ppm ppm ppm
WD-40 3 34 4 0.8 0.1 0.8 0.2
PAM 9 34 1200 2.9 0.2 1.4 0.3
Silicone 37 2 3 0.9 0.1 0.3 0.4

Table 2: Effects of Lubricants on Soil Test Levels – University of Wyoming 1995

Lubricant NO3-N PO4-P K Fe Mn Zn Cu
ppm ppm ppm ppm ppm ppm ppm
Control 6.8 10.7 218 15.0 1.5 0.8 1.1
PAM 7.8 10.5 227 14.6 4.0 0.8 1.1
Silicone 6.0 10.3 219 14.2 1.5 0.8 1.0
WD-40 7.0 9.9 221 13.9 1.6 0.8 1.0
LSD 0.05 ns ns ns ns 0.3 ns ns
LSD 0.10 1.2 ns ns ns 0.2 ns ns

Mean of three probe diameters (5/8″, 11/4″, 11/2″)

Two clay loam sites and one silty clay loam site (montmorillonite clay type)

Conclusion: Based on the results of this study and the experience of our technical staff, using lubricants is both acceptable and encouraged. WD-40 may be the best choice when testing micronutrients. Under adverse soil conditions, using moderate amounts of lubricants will greatly enhance sample quality with a minimum of risk of contamination.