SoilFacts
Deep Soil Sampling for
Nutrient Management
The soil samples that determine lime and fertilizer needs of crops routinely
come from the top 4 to 8 inches of soil. The results of soil tests help to
optimize the purchase of fertilizer, maximize yield, and minimize
environmental impact. However, there are times when deep soil sampling is
appropriate or necessary.
Deep sampling (28 to 32 inches into the
ground) represents a way to monitor the
movement of nutrients below the topmost 8
inches of soil. The nutrient phosphorus (P) is
of particular interest because it builds up in
the soil if overapplied, and it may leach into
groundwater, seep into streams, or bind itself
to soil particles and be carried to water
bodies when soil is washed from fields. Thus,
monitoring, via deep soil sampling, may be
required as part of the new Phosphorus Loss
Assessment Tool (PLAT), which seeks to
preserve water quality.
Growers and consultants normally do not
take deep samples, so the collection of
samples requires that they exercise extreme
care. Soil near the surface usually has higher
nutrient levels than soil in the deeper strata;
because of this, the sampler must avoid
contamination of the deep sample by soil
from near the surface.
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June 30, 1914. North Carolina
State University and North
Carolina A&T State University
commit themselves to positive
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North Carolina State University,
North Carolina A&T State
University, U.S. Department of
Agriculture, and local
governments cooperating.
Sampling Tools
Tool selection depends on price, availability,
quality of sample desired, and expected
frequency of use (Table 1).
„
Hydraulic soil probes usually are
mounted on a tractor or truck. The soilcollecting tubes usually have no open face
and come in various diameters. If
contamination is a major concern, plastic
liners can be inserted into the tubes to
provide intact cores that can be crosssectioned for easy acquisition of
subsamples for specific depths. Although
hydraulic soil probes are easy to operate
and very effective in removing cores with
little contamination from above soil, they
also are very expensive and not readily
available to the public. Another potential
drawback is the difficulty of accessing
sampling sites with vehicles when field
conditions are wet.
Table 1. Evaluation of effectiveness of deep soil sampling tools based on field use.*
Tool
Hydraulic probe
Soil probe with extensions
Soil auger
Post-hole digger
Cost & Accessibility
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Ease of Sampling
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* The higher the number of checkmarks (a), the higher the rating of the tool.
Quality of Sample
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SoilFacts
„
Soil sample probes are hand-held,
single units 18 to 36 inches in
length. The lower portion of the
probe (12 inches) is an open-faced
soil tube with an approximate
diameter of ¾ inch. Some probes
have screw-on extensions for more
versatile sampling by depth.
Agronomists and growers
commonly use the shorter, singleunit probes to obtain the routine
shallow soil samples used for
fertility and nematode analyses.
Probes are usually pushed into the
ground by hand, using necessary
body weight. Quality samples can
be easily obtained when soils are
moist. When soils are dry and
there is significant resistance, the
probe may be hammered into the
ground. It may be difficult to take
samples in dry, single-grain sands
due to their unstable, shifting
nature. Soil sample probes are
available through mail order from
outdoor or forestry suppliers at a
modest cost, usually $50 to $100.
„
Soil augers with various handle
lengths and extensions for deep
sampling are also available
through mail-order suppliers, but
they are more expensive than soil
probes. They are generally easy to
use except where rocks are
abundant. The auger itself is about
12 inches long and comes in
various diameters. Its sides may be
open or completely closed (bucket
augers). Planing augers provide
clean excavation of soil from a
hole’s bottom.
Soil extracted with augers comes
up in cores due to the twisting and
cutting action of the auger, so care
is essential in verifying the exact
depth of sampling. Since several
borings are necessary for sampling
deeper depths, contamination may
occur from soil sloughing into the
hole upon inserting the auger or
from side-wall contamination
when removing an auger with
open sides. Sampling dry, sandy,
single-grained soils may be
difficult with an auger.
„
Post-hole diggers consist of two
metal blades bridged together with
wooden handles about 4 feet long.
They are inexpensive and
commonly available. Sampling is
usually easy except in dry soil
conditions or when soil is
compacted. There is some
potential for contamination due to
soil sloughing. Samples, however,
will not be in cores, so depths are
best determined by inserting a
measuring tape into the hole.
Suggested Deep Sampling
Technique for PLAT
A site with high P leaching potential,
as determined by PLAT, requires a
soil sample from a depth of 28 to 32
inches. The composite sample
submitted for analysis should be taken
from a mixture of at least five cores to
ensure that it is representative of the
site. To minimize contamination and
acquire a quality sample, follow these
steps:
1. Using a post-hole digger or auger
at least 2 inches in diameter,
excavate to 26 inches. Clean as
much soil as possible from the
bottom of the hole. Use a
measuring tape to measure the
depth at the center of the hole.
2. Use a standard soil probe at least
36 inches in length to collect a 6inch soil core from the center of
the hole. Measure the top 2 inches
and discard. Place the remaining
4-inch core in a clean plastic
bucket.
3. Collect at least four additional
cores using the same technique.
4. Thoroughly mix the cores into a
composite sample. Mixing is
extremely important for best
characterization of the site.
5. Take a surface sample to further
confirm elevated P levels as found
in the initial site characterization.
Cores should be taken from areas
coinciding with deep sampling.
6. Submit samples to a laboratory
that is certified by the N.C.
Department of Environment and
Natural Resources, Division of
Water Quality, Laboratory
Section. Request Mehlich-3
methodology and results as a
phosphorus index (P-I) as
routinely performed by the N.C.
Department of Agriculture and
Consumer Services (NCDA&CS)
Agronomic Division, Soil Testing
Laboratory.
For more information concerning
deep sampling, contact the NCDA&
CS Agronomic Division (919-7332655) or the county offices of any of
these agencies: Division of Soil and
Water Conservation, N.C. Cooperative Extension, or U.S. Department of
Agriculture Natural Resources
Conservation Service (NRCS). Direct
PLAT inquiries to NRCS at 919-8732100.
Reference: North Carolina
Phosphorus Loss Assessment, Model
Description, http://www.soil.ncsu.edu
/nmp
Prepared by
David H. Hardy, Soil Testing Section
Chief, Agronomic Division, North
Carolina Department of Agriculture and
Consumer Services
Deanna L. Osmond, Department
Extension Leader, Department of Soil
Science, North Carolina State University
Richard Reich, Director, Agronomic
Division, North Carolina Department
of Agriculture and Consumer Services
© 2003 North Carolina State University
Published by
NORTH CAROLINA COOPERATIVE EXTENSION SERVICE
10/03—JL
E04-43950
AGW-439-40