Effect of Soil Probe Lubricants On Analytical Results

analytical results, dishwashing liquid, Dove, lubricants, motor oil, Pam, silicone, soil probe, vegetable oil, WD-40

A study by the University of Wyoming helps to answer the question of how lubricants affect soil test results. The study included five commonly used lubricants, two (motor oil and dish washing liquid) wiped on and three (WD-40, vegetable oil spray, and silicone) sprayed on the soil probe. Chemical composition of the lubricants is shown in Table 1.

Table 1. Chemical Composition of Five Commonly Used Soil Probe Lubricants
		ppm
Lubricant	% OM	NO₃	P	K	Fe	Mn	Zn	Cu
WD-40	69	3	34	4	0.8	0.1	0.8	0.2
“PAM” Vegetable oil spray	>70	9	34	1200	2.9	0.2	1.4	0.3
“Dove” dish washing liquid	30	205	44	2	0.2	<0.1	<0.1	<0.1
Synthetic motor oil	48	11	33	5	0.8	0.1	460	0.6
Silicone spray	2	37	2	3	0.9	0.1	0.3	0.4

Data shown in Table 1 indicates that lubricants can pose potential contamination problems. Their effects on test results for samples in the Wyoming study are shown in Table 2, and may be summarized as follows:

Lubricants had no significant effect on soil test levels of organic matter, nitrate-N and K.
Effects of lubricants on soil P tests were significant. WD-40, PAM and silicone increased soil P from 14 ppm in the control to approximately 16 ppm.
Some lubricants significantly increased soil test values for micronutrients while others decreased these values. For example: WD-40 increased the test value for Fe, whereas reduced values for Fe, Mn, Zn, and Cu… possibly by complexing or precipitating these elements.

Table 2. Effects of Soil Probe Lubricants on Soil Test Results
		ppm
Lubricant	% OM	NO₃	P	K	Fe	Mn	Zn	Cu
Control (no lubricant)	1.7	1	14	249	11.4	1.5	0.8	1.7
WD-40	1.6	1	16	248	13.2	1.8	1.0	2.0
“PAM” Vegetable oil spray	1.7	2	16	263	13.5	3.8	1.1	2.3
“Dove” dish washing liquid	1.7	3	14	280	10.1	1.3	0.7	1.2
Synthetic motor oil	1.6	2	16	265	12.5	1.4	0.9	2.0
Silicone spray	1.6	1	16	246	9.9	1.3	0.6	1.0

Conclusions:

Lubricants significantly reduce the labor needed to sample soils with probes.
The greatest benefit with probe lubrication are with depth samples, and with wet, clayey soils.
Soil probe lubrication seems generally acceptable when sampling macronutrients.
In this study, benefits realized from lubricants greatly outweigh the negligible nitrate-N contamination resulting from the lubricants used.
Effects of probe lubricants on micronutrients could be considered significant since their concentration in the soil is low. However, benefits of using a probe lubricant outweigh the contamination risk if effects of the lubricant are taken into account when interpreting the soil test results.

Reference: Alan D. Blaylock, Lyle R. Bjornstead, and Joseph G Lauer, University of Wyoming. Taken from The Soil-Plant Council Analyst, The Soil and Plant Analysis Council, Inc. March 1996.