Soil test analysis and correlation of results to field performance can affect your fertility program. Know what you are getting.
November 19, 2007 By Bruce Barker
Soil testing has a firm foundation in science, but like most things in life,
the application of the science can take on many shades of gray. It is like selecting
a metric wrench for a SAE nut; sometimes a person can get away with it, but
most often they cannot. To get the best fertilizer recommendations, growers
should understand how soil test laboratories analyze and calibrate the results.
Rigas Karamanos, manager of agronomy at Westco Fertilizer in Calgary, Alberta,
says there are two primary criteria for selecting a soil test method in a particular
region: the suitability of the extractant for the soils in a particular area,
and the calibration of the test results with yield data from field experiments
in a region. Both should be taken into consideration when looking at soil test
Extractants vary for many different soil properties
According to Karamanos, there are 11 different ways to determine soil pH, depending
on the extractant and soil-to-extractant ratio. Three commonly used methods
in western Canada are the soil paste method, and a soil:water method in either
a 1:1 or 1:2 ratio. The soil paste method is commonly used in the oilfield reclamation
industry. Comparing the results, if a pH of 7 was found with the soil paste
method, the 1:1 ratio would produce a result of 7.3 and the 1:2 ratio would
produce a pH reading of 7.6.
"It is important to accurately know your pH for assessing soil herbicide
residues," says Karamanos. "Use the lab's analysis as a guideline
to follow." Soil organic matter estimation also uses various methodologies.
The two main approaches are chemical oxidation of soil organic carbon and ignition
to destroy organic carbon, although any inorganic carbon will also be destroyed.
Karamanos says there can be as much as 0.8 percent difference on soils that
test less than four percent organic matter. Organic matter estimates are used
for guiding herbicide applications with some soil residual herbicides, but also
for estimating nitrogen (N) mineralization in the Black soil zone. Again, use
the results as a guide and not as firm baselines.
Nitrogen is a nutrient that does not normally differ in extraction rates. Soil
test nitrogen is normally determined as soil nitrate-N (NO3-O),
which is water soluble. As a result, nitrate-N results are generally the same
from soil test laboratory to soil test laboratory, although how the laboratories
make their recommendations for fertilizer N can differ.
Phosphorus is the nutrient with the most diverse methodology. Bicarbonate (Olsen)
was the most commonly used, but many laboratories moved away from it because
it was difficult to use. Now, western Canadian laboratories use a variety of
methods; Kelowna, Weak Bray (P1) and Strong Bray (P2). Karamanos says that Bray
tests extract more P from the soil than the old Olsen test. However, the results
with the Bray tests are variable, depending on the amount of calcium carbonate
in the soil. On soils with less than two percent calcium carbonate, the Bray
tests extracted more than 200 percent more P, but with more than 10 percent
calcium carbonate, the extraction rate was 18 percent of the Olsen test.
The Mehlich-1 test is used in the US, and when pH is higher than 7.5, the extract
rates are very high compared to the other test.
Methodologies for K and S typically yield similar results.
Adrian Johnston, with the International Plant Nutrition Institute (formerly
PPI/PPIC) at Saskatoon, Saskatchewan, says that extraction and chemical analysis
are generally well monitored and regulated. Most North American laboratories
participate in proficiency testing programs to evaluate their methodology and
equipment. Errors or deviations from standard values must be addressed promptly.
However, that is not to say that every methodology is applicable to western
Calibrating results with field experiments
Apart from being a scientist with a background on soil analysis, how is a farmer
to be certain that his laboratory is using the most appropriate methodology?
The answer lies in what the laboratory does with the numbers.
"There is a common misconception that a nutrient extracted equals availability.
That's not necessarily true," explains Karamanos. "The soil test analysis
provides an inventory of the nutrient in the soil, but it is not available unless
it is accessible to the plant roots in a form that it can be absorbed."
As a result, soil test laboratories must calibrate the nutrient extracted against
crop yield responses. Laboratories conduct and gather results from hundreds
of field experiments across many different soil types to develop crop yield
responses for a specific extraction method. So when a certain methodology produces
a data value, the fertilizer recommendation has been calibrated using that data
value. The process is costly and time consuming.
Johnston says almost all soil test laboratories have some research on file
that is used to correlate soil sample results with the data, which provides
some sort of calibration that allows the development of fertilizer recommendations.
"However, remember that a lab based in the southern US or Ontario will
likely use data from that region, and it is often not relevant to a western
Canadian farm," cautions Johnston.
In the absence of field data, some laboratories use crop removal data to calibrate
soil test results and fertilizer recommendations. While this avoids the need
to have a database for every area samples come from, it does not allow for regional
specific growing conditions. This is especially true in areas where leaching
during the growing season results in the loss of mobile nutrients like nitrate
and sulphate. This approach is of little relevance to the semi-arid prairie
where residual soil test levels in the fall are utilized to derive soil test
Karamanos splits out which soil tests have been calibrated with field experiments
for western Canadian soils. He cautions growers to avoid laboratories that base
fertility recommendations on soil test methods that have not been calibrated
for western Canadian conditions.
|Table 2. Soil test methods that have
been calibrated for western Canada.
|Soil test method calibrated
|Soil test method 'not' calibrated
|Water (bicarbonate; Kelowna modifications).
Mineralization indices like: hot KCL, amino sugars,
|Olsen (bicarbonate) Kelowna modifications Miller Axhley.
|Bray (weak and strong) mehlich extractants.
NH4OAC (ammonium acetate) Olsen
|Based on percent K saturation, K/Ca.
|Hot water extractable.
CI electrode. Chromatography. AgNO3.
Water mercury (II) thiocyanate.
|Exchange membranes and resins.
Base cation saturation ratios mislead fertilizer recommendations
Proposed in the 1940s and 1950s in the US, the Base Cation Saturation Ratio
(BCSR) is used to describe the ideal proportions of the major exchangeable cation
nutrients: calcium (Ca), magnesium (Mg) and potassium (K). BCSR is being used
by a number of soil test laboratories for fertilizer recommendations for these
three nutrients: it does not apply to N, P, S and micros. The original work
in 1945 proposed a percentage of 65 percent Ca, 10 percent Mg, five percent
K and hydrogen at 20 percent.
However, Karamanos says there are numerous studies in the literature in western
Canada that show no relationship between yield increases and the K saturation,
mainly due to high levels of K on western Canadian soils. He says this approach
often recommends K application even though the supply of K is more than adequate
with levels of more than 500lb/ac in the top six inches of soil.
"It is surprising that the cation exchange capacity is being used for
fertilizer recommendations in western Canada. Our soils are well supplied with
these nutrients and where deficiencies do occur, particularly with potassium,
traditional soil testing techniques provide satisfactory predictability,"
explains Karamanos. "Saturation ratio is meaningless in western Canada,
and in any case, it has not been calibrated with western Canadian soils and
environmental conditions – no correlation exists."
Karamanos says when growers consider selecting a soil test laboratory, or consider
the laboratory recommended by their agronomist, they should ask questions about
soil test methodologies and how those results are calibrated to their local
conditions. Armed with this knowledge, they can start to build a soil fertility
program based on performance in their own field, with the assurance that the
program is science-based and field tested. -30-
Part I of this story: Understanding
soil test philosophies, appeared in the February 2007 issue of Top Crop Manager,