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SOIL
SAMPLE COLLECTION GUIDE
 General
Tips
Soil samples must be collected properly and carefully to be
useful for evaluating the nutrient status of a crop. Analytical
results can only be as accurate as the sample. The most critical
objective of sample collection is ensuring that it is representative
of the area in question. If a field, orchard, or vineyard
is not uniform, separate samples from each distinct area are
recommended. Areas that differ in slope, color, drainage,
or texture should be sampled separately if they can be fertilized
separately. Avoid sampling fertilizer bands, fencerows, wet
spots, severely eroded areas, or any atypical sections unless
specifically evaluating these distinct locations.
Generally,
try to sample when soil is relatively dry. Avoid collecting
samples immediately after fertilizer application or after
watering or a heavy rainfall, as misleading or inaccurate
analyses may result. Best sample collection method depends
on the type of plant or crop, tillage practices, and fertilization
strategies. Recommendations for some situations are listed
on this page. Please contact our lab for sample collection
recommendations for other types of crops or strategies.
Sample Collection Tools
Use stainless steel or chrome-plated sampling tools and plastic
buckets to avoid contaminating samples with traces of microelements
from the tools. Avoid brass, bronze, or galvanized tools.
Tools should be clean and free of chemical, lime, or fertilizer
residue. A trace amount of residue can seriously contaminate
the sample and cause inaccurate results.
Soil samples may be taken with a soil tube, auger, narrow-bladed
trowel, or a spade. At each sampling site, scrape away surface
debris and remove a core sample to the appropriate depth.
If using a spade, dig a spadeful of soil to the appropriate
depth and discard it. Then take a 1-inch slice of soil from
top to bottom. Keep the entire slice or keep only a strip
from the middle of the slice, top to bottom.
Sampling Depth
In general, soil samples should be taken from the root zone
of the crop in question. For field crops, sample soil to the
depth that the field is plowed, usually 0 to 8 inches. This
zone is where lime and fertilizer have been incorporated.
Where no-till conservation-type tillage systems are used,
collect a second sample at a depth of 0 to 4 inches. Also
sample from 0 to 4 inches for pastures, turf, and forage fields.
Studies have shown that for most orchard crops, depths of
8 to 14 inches are best. For home gardens, sample soil from
0 to 6 inches. To sample subsoil, which is recommended every
2 to 3 years, sample 6 inches beyond the root zone of the
crop. Be careful to maintain a constant depth for all samples
collected for best results.
Number of Samples
The number of samples that should be collected depends on
your overall nutrient management strategy. Conventional, whole-field
management usually requires less sampling than a more systematic,
site-specific or grid sampling approach. Choose a system that
best fits your strategy.
Whole-Field Nutrient Management
Traditional whole-field management requires only enough samples
to determine the average nutrient concentrations for the field.
Collect 15 to 20 soil cores or aliquots taken at random locations
and combine them into one sample. Thoroughly mix and subsample
the composite sample if necessary. Generally, one sample should
represent a uniform area of not more than 20 acres. Place
the sample in a clearly-labeled Soil Sample Bag and submit
to the lab within 48 hours for best results.
Site-Specific Nutrient Management
Site-specific soil sampling is more labor-intensive than whole-field
sampling. The objective is to accurately describe the size
and shape of each soil condition or property to allow variable
rate application of nutrients and other inputs. A field is
mapped and divided into blocks or cells to form a grid. Using
GPS software, a random sampling location within each grid
cell is identified and sampled. Results are incorporated back
into the mapping software and used to manage nutrient applications.
Recommendations for number of samples per field depend upon
the nature of variability within the field. Generally, one
sample per acre or 200-foot cell widths are recommended to
begin. Many high-value crops are sampled using 100-foot grids.
Sample each grid cell as mapped. Place samples in a clearly-labeled
Soil Sample Bag and submit them to the lab within 48 hours
for best results.
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