soil testing laboratories are not the same. You will often find that
two laboratories will give totally different results and
recommendations on the same soil sample. How do you choose? The two
things that you must look for in choosing a laboratory are the
analytical methods used and the quality control system in place.
There are many
different methods used to determine the amount of available
nutrients and they do not always give the same description of the
soil. Some methods have been developed for use under a limited range
of soils. For example, the Troug method for analyzing phosphorus
works well on acidic soils, but gives false results on alkaline
soils. There are many regional laboratories that target
growers in a fixed area with one general soil type. They choose the
simplest, quickest analytical methods that will give reliable
results on that soil. If you send a sample from another area with a
different soil type, the results may be misleading.
laboratories, such as Agro Services International, do not target any
one soil type and must therefore use methods that are applicable
over a wide range of soils. These methods are more tedious than
those used by regional laboratories, but they must be used in a
laboratory that accepts samples from a wide geographic range or from
regions with variable soil types.
How do you
know if your laboratory uses suitable methods? Ask them if their
methods have been calibrated with the
soil types that you are growing on or if calibrations were done with
soils completely different from yours. You may be surprised to
discover that many laboratories cannot answer that question.
Even if your
laboratory uses suitable methods, you need to know if there is a
strict quality assurance program in place. Errors can occur and the
laboratory must have procedures that immediately detect if something
has gone wrong. For example, at Agro Services International, all of
our instruments are regularly checked and calibrated using certified
analytical standards. Also, in every batch of analyses, we
include standard samples, that is, samples with known nutrient
levels. If the results from these standards do not match the known
values, ALL results are thrown out and the entire process is checked
until the source of error is found and corrected.
at the end of the day, the purpose of soil analysis is to
predict the nutrient needs of crops growing on your soil. If you are
getting clear responses to a nutrient, but your laboratory says you
should not have problems with that nutrient, it is time to look for
a new laboratory.