Soil Sampling as a Basis for Fertilizer Application (continued)

SF-990 (Revised) August 1998

Sampling Tools

Soil is variable not only over an area, but also with depth. A proper soil sample is taken from a uniform volume from the top of the sample depth to the bottom. Wedge shaped samples, or a handful of soil from the surface and one at depth, are not appropriate and will not give consistent results. The best sample is taken using a soil probe (Figure 3). There are hand-probes and automated probes available at nearly every price range. The probe should be designed to gather soil from the appropriate depth.

Figure 3.

Soil hand probe. Automated probe.

After a recent tillage operation, an automated probe sometimes has difficulty obtaining a consistent 0-6 inch sample. The consistency of a surface sample may be improved by sampling in wheel tracks, but it is sometimes difficult to find a wheel track when the probe is centered in the cab. Hand probes may be a better method to take a 0-6 inch sample, because the soil can be firmed with pressure from a footprint and a consistent sample can be taken. Automatic probes are very good at taking a 6-24 inch and 24-48 inch core, even following tillage. Automatic probes take a much more consistent surface sample when fields have not yet been tilled.

In many soils, a lubricant is needed to prevent soil plugging in a soil probe. Table 2 shows the effect of lubricants on soil analysis. For most soil nutrients, the use of lubricants, especially the most popular lubricants, should not affect soil test results. Exceptions would be certain micronutrients, iron (Fe), Zn, Mn and Cu. For these micronutrients, obtaining a 0-6 inch core without a lubricant is suggested, especially where deficiencies are suspected.

Table 2. Effects of soil probe lubricants on soil chemical analysis
(Blaylock et al., 1995. Wyoming).

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SF-990 (Revised) August 1998