Taking a representative composite sample is a cost effective way to obtain
reliable field nutrient status and fertilizer recommendations, but a minimum number of
cores should be collected to make a composite to account for field nutrient variability.
Fifteen to 20 cores are suggested by most laboratories to form a composite sample. This is
because field variability is adequately represented when the number of cores reaches about
18.
As illustrated in Fig. 2, the averages of composite samples for nitrate-N vary
tremendously if the number of cores for the composite is less than 15 in a field,
therefore, the soil test results you get from those samples are not reliable. Make sure to
get at least 15 cores from the field the composite sample represents by following a random
pattern.
Figure 1. Average NO3-N values for randomly selected samples from point grid
area to estimate the number of core samples needed to make a representative composite
sample.
Appropriate Sampling Depth
For areas where field crops are grown, collect samples to the same depth
that the field is plowed since this is the zone where lime and fertilizer have been
incorporated, and the majority of crop roots are located. Fertilizer recommendations from
OSU Extension are calculated based on a 6-inch furrow slice (0-6") for surface, and
18 inches (6-24") for subsurface soil samples.
Inaccurate interpretation and recommendations will result if samples are collected at
other depths. Therefore, make sure to push the soil probe to the right depth for surface
and subsurface samples. There is a mark at 6" on most soil sampling tubes. It may be
a good idea to mark it with a colored tape so that you can see how deep the probe goes. A
flange stop was designed by the authors with assistance from OSU Agricultural Engineers
recently. When the stop is mounted on a hand probe at the desired distance from the tip
(Figure 2), it controls sampling depth very well, especially in newly cultivated fields.
However, this kind of stop is not commercially available to our knowledge.
It should be easy to make something like this if sampling depth is really important to
you. In order to meaningfully compare results from year to year, it should keep sampling
depth consistent too.
For fields where no cultivation is practiced and most roots are near the
surface, such as turf, samples taken to a depth of 4 inches will best represent lime and
fertilizer needs. Therefore, 4-inch depth samples should be taken from lawns and golf
courses.
Figure 2. Soil sample probes mounted with flange stops to control
sampling depth.
