Ohio State University FactSheet
Horticulture and Crop Science
2001 Fyffe Court, Columbus, OH 43210-1096
Guidelines for Choosing a Soil-Testing Laboratory
(HYG-1133-99) Maurice E. Watson Extension Specialist
Choosing a Soil-Testing Laboratory
Growers should consider several factors in choosing a soil-testing
laboratory. This Fact Sheet refers to laboratories that determine concentrations
of plant-available nutrients in the soil. Soil-testing laboratories are
generally not regulated by state or federal agencies. Consequently, it is
important for growers to investigate these laboratories by obtaining information
about their performance, operation, and service before sending soil samples for
analysis. A grower requires assurance that the test results will be of quality,
be credible, and meaningful. Specific guidelines are discussed here to aid the
grower in evaluating a soil-testing laboratory.
Factors to Consider
- Test Methods - The use of appropriate test methods is very
important in order to accurately determine the concentrations of
plant-available nutrients in the soil. Research at many land-grant
universities over many decades has resulted in soil- testing methods that
are specific for soils in particular regions of the United States. For
example, methods developed for testing the predominant soils in the Southern
region of the United States may not be applicable for soils in the North
Central region. The North Central Regional Research Committee (NCR-13) has
developed methods that work best on soils in the North Central region. A
publication of these methods is entitled Recommended Chemical Soil Test
Procedures for the North Central Region (1). Laboratories that test Ohio
soils should use these procedures. Therefore, potential clients need to
determine if these testing methods, recognized for Ohio soils, are being
used by the laboratory.
- Laboratory Proficiency - The proficiency of a laboratory refers to
its ability to produce accurate and precise test results. It is difficult
for a laboratory to independently assess this factor. Thus, regional
soil-testing research committees and other organizations established the
North American Proficiency Testing (NAPT) program in 1998. This program is
backed by the professional scientific organization, the Soil Science Society
of America. A main purpose of the NAPT is to provide
"double-blind" check samples to laboratories who wish to monitor
and improve the quality of their soil-testing data. NAPT not only provides
the check samples but also collects and statistically analyzes the data from
laboratories in the program. Participating laboratories receive a summary of
their performance for each soil-test method. Continued self-evaluation and
adjustment improves the integrity of the soil-test results. A prospective
client should ask the laboratory management if they are members of the NAPT
- Laboratory's NAPT Results - It is important that a representative
of the laboratory review with the potential client their NAPT quarterly test
results with those summarized for all NAPT participating laboratories.
Information for each test parameter of interest to the client should be
included. Growers should ask for this comparison in order to make a good
decision about a laboratory.
- Other Customers - The potential client should ask the laboratory to
provide the names and telephone numbers of 10 customers. This allows the
grower to evaluate the laboratory from the perspective of users like
- Units of Results - Ask a laboratory representative what units are
used for each test parameter. Some laboratories use lbs/a, ppm, or lbs/1,000
square feet. If results from different labs are compared, make sure the
units associated with the results are the same. For a valid comparison, a
simple conversion may be necessary. For example, to convert ppm to lbs/a,
multiply the ppm value by 2. Certain test parameters may have unfamiliar
units, such as meq/100 g for cation exchange capacity. Ask the laboratory
representative to explain the meaning of the units if they are unclear.
- Categories of Quantity - Some laboratories may place test results
into categories. Examples are the categories of low, medium, and high. There
may be additional categories or different categories than these. These
categories usually denote a range of test values. It is likely that the
categories given by one laboratory do not represent the same nutrient
concentrations for another laboratory. Ask the laboratory to define each
range that is used. In addition, find out if the categories are
crop-dependent or calibrated for specific soil conditions (e.g., soil
types). That is, results that may be regarded medium for one crop may be
considered low for another crop.
- Lime and Fertilizer Recommendations - Determine if the soil-testing
laboratory provides recommendations for the application of lime and
fertilizer for the crops of interest. The Tri-State Lime and Fertilizer
Recommendations provide guidelines for corn, soybeans, alfalfa, and wheat
that will be grown in Ohio soils (2). In addition, lime and fertilizer
recommendations for these crops and other agronomic crops are available
through The Ohio State University's Ohioline Internet service. The web
address to access Ohioline is: http://ohioline.ag.ohio-state.edu. The Ohio
Vegetable Production Guide lists fertilizer recommendations for vegetable
crops (3). The basis for these recommendations is the university research
that has been conducted for the soils and growing conditions of Ohio. Ask
the laboratory representative if these recommendations are used. Also ask
about the basis for lime and fertilizer recommendations that are used for
other crops. Are they calibrated for your specific soil types or growing
conditions? Ask if crop rotations and yield goals are considered. In
addition, ask if the timing of the application of lime and fertilizer is
included in the laboratory's recommendations.
- Turn-Around Time - Ask how long it takes the laboratory to do the
routine soil testing and return the results. In order for the results and
recommendations to be useful, the turn-around time must be as short as
possible. A good laboratory should be able to provide the results in two to
three working days for the routine soil tests of pH, lime requirement,
phosphorus, potassium, calcium, and magnesium. It is also very important to
make sure the laboratory does not sacrifice accuracy by short cutting the
methods to attain this turn-around time. It is a good idea to check the
turn-around time with those who have used the laboratory.
The Internet can be a useful system to obtain test results rapidly. Find out
if the laboratory can provide the results on the Internet. In addition,
determine if the recommendations for the application of lime and fertilizer
can also be obtained on the Internet. In some cases, the laboratory may be
able to accept the customer's sample identification information over the
Internet, rather than using the sample information form. Most laboratories
will also have an e-mail address that will allow direct and rapid
communication with the laboratory manager and/or laboratory professional.
- Visiting the Laboratory - It is important to visit the soil-testing
laboratory before submitting samples. A representative of the laboratory
should not hesitate to show a potential client the testing area. During the
visit, observe the orderliness and cleanliness of the work area. Ask how the
samples are handled. In addition, ask how the data is handled and ask about
quality control that is used.
- Reference Check Samples - Find out if the laboratory
routinely uses internal "blind" and "double-blind" check samples where
possible. A "blind" check sample is one that the laboratory
technician knows is a check sample and is aware of the range of acceptable
values for the parameters being tested. The technician uses this kind of
check sample to make sure the method and instrument are performing normally.
A "double-blind" check sample is one that the laboratory
technician does not know is an internal check sample. In this case, the
laboratory manager evaluates the data and determines if the test results
produced are in the acceptable range. If they are not, then corrective
action must be taken to solve the problem.
- Charting Quality Control - The testing laboratory should
continuously evaluate its quality by charting its check soil-sample results
over time. This allows for measurement and assessment of the variation over
time. Warning limits and action limits should then be established to assist
in the recognition of unacceptable results if a problem with the test should
arise. Ideally, quality-control charts need to be used for each test
parameter. A potential client should ask to review these charts with the
laboratory management prior to selecting a laboratory.
- Sample Information and Test Result Forms - Ask the laboratory for
examples of the information form and the final test result form. Study these
forms and ask for an explanation of anything that is unclear. Determine how
many samples can be represented on each form. Also, sampling instructions
are usually provided on the information form. Containers that hold the
sample are usually provided along with the information form. Ask to see an
example of the container.
- Test Kits - Most soil-testing laboratories supply test kits for
their customers. As a minimum, the test kits should contain the sample
information form and soil sample container. Some additional information may
be included with the test kit. Find out about the sample kits and how they
are obtained from the laboratory.
- Production Professionals - Find out if the laboratory has
professionals who are trained in agronomy, horticulture, or soil science to
work with the customer. Before deciding on a soil-testing laboratory, visit
the laboratory and meet with a professional to discuss concerns about
testing soil. When visiting a laboratory, ask to review the educational
credentials of the professionals. Find out about the training background.
- Laboratory Test Prices - Prices for soil testing often vary greatly
from one laboratory to the next. Ask about the prices. Determine if the
price for each test or test package is given in writing. Also find out if
discounts are given for large numbers of samples and whether prices are
- Other Testing Services - Determine what other services the
laboratory offers that are in conjunction with soil testing. Especially,
find out if the laboratory offers plant-tissue analysis. This tool can be
very useful along with soil testing to monitor the nutrient status of the
soil or to isolate problem fertility situations in the field. Find out if
the laboratory includes sample collection as an optional service.
Consultation with your local county Extension agent may also be worthwhile in
deciding which laboratory to use. In addition, if a crop consultant is used,
then the grower should discuss these factors with the consultant in regard to
the selection of a soil-testing laboratory. Additional time and effort in
selecting a quality soil-testing laboratory will pay off. Don't just assume the
laboratory gives quality test results. Find out for sure.
- Brown, J. R. (ed.). 1998. Recommended Chemical Soil Test Procedures for
the North Central Region. North Central Regional Research Publication No.
221. Missouri Agricultural Experiment Station SB 1001. Columbia, MO.
- Vitosh, M. L., J. W. Johnson, and D. B. Mengel (eds.). 1995. Tri-State
Fertilizer Recommendations for Corn, Soybeans, Wheat & Alfalfa.
Extension Bulletin E-2567. Michigan State University. East Lansing, MI.
- Precheur, R. J. (ed). 1999. 1999 Ohio Vegetable Production Guide.
Extension Bulletin 672. The Ohio State University. Columbus, OH.