Why Are Soil Tests Not Gospel Truth?
It is well known among Growers people that Jim Halbeisen is not a big fan of soil tests. He and Carla do nearly all of the tests that come into the office, except when I can come down to the office and do a few. However, his dislike for them is not just from doing so many. Soil tests are held up by the agricultural establishment as the best way to find out what your soil, and therefore your crop, is lacking. Recommendations for massive amounts of fertilizer are based solely off them, so it should follow that they are an accurate indication of actual soil nutrient levels. Most of the evidence to support a strong test result connection is anecdotal, as Dr. Tiedjens demonstrates in his book More Food From Soil Science. Adding potassium because a soil test claims it is low may boost yield for a season, but the nature of potassium is to bring water into the plant. Therefore, the yield boost could come from either correcting a deficiency, or from this short-term green-up. His book contains countless examples like this.
I wish to caution readers before I delve further into why Growers has the view of the soil tests that we do. We do not think soil tests are completely useless in your program, but rather that they inherently do not present an accurate picture of your soil and therefore should be used only as a starting point and reference. We believe strip tests to be much more valuable. There are several reasons we take this position, including soil tests' flawed methodology, their disconnect from a real field environment, and the natural variability of all land.
In any scientific experiment, the methodology must be accounted for and its impacts, if any, on the outcome need to be quantified. If not, you end up with confounding variables, meaning there are observed results but we don't know what to attribute them to. The case above with potassium is a good example of this. I believe this principle can account for a lot of the inaccuracies of soil tests.
A saline solution is used to soak the soil and strip nutrients from the bases. It is this solution that is actually tested. Different laboratories use different concentrations of saline solution, stripping more or less nutrients compared to other labs. Further, according to mass action, the initial response in any chemical reaction is exaggerated, meaning that the saline solution will initially strip more nutrients than any natural process, artificially increasing those tested. Before the soil is even soaked, it is dried and ground, also artificially increasing the nutrients tested. Once the saline solution has soaked through the soil, it is mixed with various chemicals depending on which nutrient is being tested for. It will color the solution, which is compared against known results to determine the levels of the nutrient. Depending on which chemicals are used, different amounts of nutrients will be accounted for. Additionally, nutrients can come in many forms, such as organic or inorganic, that are not all equally available to plants. Soil tests may include unavailable types in their final product.
Second, the conditions present in a lab are very different from those in each field. The limitations I explained in the prior paragraph are also valid here. The changes a sample must go through as it is processed do not happen in a natural environment, at least in any time frame we can imagine. However, perhaps the greatest reason that we do not wholly trust soil tests is that they can only account for the physical and chemical properties in the soil, not the biological. Given our modern knowledge of biological soil life and its huge impact on crop health, we are only shooting ourselves in the foot by ignoring it in our fertilization decisions.
Lastly, we would have to sample every square foot of a field (more than 6-8 inches deep) in order to get a precise idea of what is present in the soil. Furthermore, the time variable can affect certain soil properties, such as pH and temperature. Soil tests provide a glimpse in both time and space of a field, and cannot capture the variability present in all fields. A certain nutrient may be present in high concentration where the sample is taken, but may be low in many other parts of the field. Likewise, pH can be different across a field and also change throughout the day. Recommendations for entire fields are made from a few tests (at most), and do not reflect the field's true condition.
As I stated above, we do not wish to completely abandon soil testing, as it is useful in some regards. Our position is taken from a quote by Dr. Tiedjens, who said, "It is often easier to do a soil test than to explain to a farmer why he doesn't need one."
This is an excerpt from the Early Fall Growers Solution (2019) written by Zach Smith.
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