Soil Test Interpretation – An Introduction
At Earthwise we are often asked by our growers to “take a look at” and to interpret soil tests for them. And generally, we are glad to do it. Soil tests can be an important tool in understanding the physical and chemical makeup of a soil; they can guide a grower’s decisions about applying inputs to the soil and crop in general. And a comprehensive soil test can often help us to make better recommendations for their crops. But a misinterpreted (and/or misunderstood) soil test can lead to bad decisions and cost a grower quality and yield. In the following series of essays we will look at:
1) Why take a soil test?
2) What kind of soil test should be taken? How many?
3) Why is it best practice to pick a good lab and use the same lab year after year?
4) What is meant by pH, Soil Texture, CEC, OM, PPM, ME, etc.?
5) How and why should both the amount and quality of organic matter in the soil be evaluated?
6) Why test only for available minerals and not total minerals in the soil?
7) How do petiole and tissue testing relate to soil testing?
8) Should I take a soil test?
Why take a soil test
Soils used for growing crops, ornamentals, and pastures vary widely in their physical characteristics (grain size, porosity, permeability, electrical attraction), in their chemical profile (mineral makeup, mineral availability), and in their carbon profile (organic matter quantity and type). A soil test can give important information about all 3 of these characteristics, but not every lab evaluates soil the same way or in the same detail. In fact, there is quite a bit of disagreement between labs (and agronomists) about the type of test that should be performed on each nutrient in order to give growers the most useful information about their soils.
When submitting a soil test the grower generally wants to know whether and what nutrients need to be added to the soil to give their crop the best practical chance of producing a target yield and quality. The nutrient requirements of a given crop at a given yield goal are generally known. The soil test is expected to show the amount of nutrients available in the soil to the plant roots during the next growing season. In theory, then, it becomes a simple subtraction problem to determine how much fertilizer to add to the soil:
Amount of Nutrients needed to produce the desired yield
minus
Amount of Nutrients available in the soil
equals
Amount of Nutrients added to the soil in the form of fertilizer
Of course fertilizing plants is not quite that straightforward for lots of reasons, but we will stick to the problems with this simple subtraction formula as it relates to setting up and interpreting the soil test. Here are a couple of examples why soil test results and fertility recommendations should be taken with a grain of salt.
Example 1
As mentioned previously, there is some disagreement between labs about how to test for the amount of nutrients that will be available to the plant during the growing season in a particular soil. Determining Phosphorus (P) availability is a particularly tricky and controversial measurement. Examples of possible tests for phosphate availability are: the Mehlich I, Mehlich II, Mehlich III, Olsen, Morgan, Strong Bray, Weak Bray. Sometimes 2 different techniques are reported in order to get a better perspective of phosphorous availability.
Example 2
It is generally agreed that the availability of some nutrients should be tested with different techniques in alkaline soils as opposed to acid soils. But there are very acid soils, somewhat acid soils, somewhat alkaline soils, etc. And the results of even the ‘most appropriate’ testing techniques must be calibrated to known local conditions to be truly useful. By calibration it is meant that records of the application of inputs of various fertilizers are compared to yields reported by farms in the local area.
So you can see that interpreting a soil test and deciding what to do with the results is not exactly straightforward! Still, it can be very useful if you are aware of the limitations of the information it provides. Without a soil test you are depending only on your own knowledge of your growing soil or growing medium. And even the most experienced grower can be led astray by symptoms of plant struggles that don’t always fit into neat categories. That possibility points to the importance of a soil test and why it can be useful and important when interpreted correctly.
Next up
What sort of soil test should I be taking?