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 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.

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.
• 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, single piece 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 online 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 online 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). Planer 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.

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 6- inch 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 NC Department of Environmental Quality, Division of Water Infrastructure. Request Mehlich-3 methodology and results as a phosphorus index (P-I) as routinely performed by the NC Department of Agriculture & Consumer Services (NCDA&CS) Agronomic Division, Soil Testing Section.

For more information concerning deep sampling, contact the NCDA&CS Agronomic Division or the county offices of any of these agencies: NCDA&CS Division of Soil and Water Conservation, N.C. Cooperative Extension, or US Department of Agriculture Natural Resources Conservation Service (NRCS).

North Carolina Phosphorus Loss Assessment Tool

This publication is a revision of an earlier version. The authors would like to thank R. Reich for his earlier contributions.