Interpreting Soil Test Results – The Extraction Method
Different soil labs might provide different results for the same soil sample. This can be very confusing for growers and agronomists, who need to design proper fertilization program. One of the major reason for this inconsistency is that different laboratories use different extraction methods (testing methods).
So why do labs use different methods? Which methods are the most suitable?
In order to understand that, we must first understand what stands behind the term “extraction method”.
Extraction method – The term “Extraction Method” refers to the combination of the analytical procedure and the chemical (extractant), which are used to determine the level of a certain element in soil. It might sound surprising, but for most extraction methods, the value obtained for a specific element DOES NOT represent its total amount in the soil, nor its actual available amount.
In most cases, extracting the total amount of nutrients doesn’t even have any practical meaning. This is because a large fraction of the nutrient in the soil is not readily available to the plant.
Each method can extract a different amount of the nutrient from a given sample, as shown in figure 1 below. Method B extracted a higher amount of the nutrient than method A. Moreover, the amount of nutrient extracted by any extraction method does not necessarily represent the actual amount of nutrient that is available to the crop.
Fig. 1 Using different extraction methods on the same soil sample may extract different amounts of a given element from the soil.
Therefore, the test result must not be taken as an absolute number and the results are almost always empiric. A rating of ”Low / Adequate / High / Excessive” is used to classify the results. The rating describes the likelihood of the crop to respond to addition of fertilizer.
For this purpose, “crop response” means an increase in yield and/or in quality.
For example, the rating “Excessive” means that “It is very unlikely that the crop will respond to addition of fertilizer”. The rating “Low” means that “Most probably the crop will respond to addition of fertilizer”. A proper extraction method provides a good correlation between the amount of nutrient that it extracts to the uptake rate of the nutrient by the crop.
It is extremely important to use the right extraction method. Since the crop response to fertilizers may be different under different soil conditions and geographies, developing an extraction method requires an extensive research and many field trials.
In figure 2, two extractants were used on the same soil sample in order to extract phosphorus from it. The phosphorus uptake by a certain crop was measured for each phosphorus level that was extracted. From the chart, it is quite obvious that Extractant A gives a better correlation than extractant B. Therefore, extraction method A is the preferred extraction method for phosphorus under the given conditions.
Fig. 2 Calibration of extraction methods
A particular extraction method may be suitable for one nutrient but unsuitable for another.
Therefore, various extraction methods may be used to extract different nutrients from the same soil sample. In other words, in most cases, more than one extraction method is used on the same soil sample. Remember: In order to be able to interpret the soil test results, the laboratory must indicate which extraction method was used for each element.
Unfortunately, many soil labs do not indicate which extraction method they used. If neither the extraction methods nor interpretation categories are provided in your soil test report, you must contact the soil lab and ask them to provide this information. Else, the test results cannot be interpreted.