There are two ways to measure the value of a soil test. One way is to look at the cost of lost production from inefficient fertilizer use, and another is to look at potential cost savings if over-fertilization is stopped. I would like to illustrate these situations to you in this article. The data I am using are a compilation of producer samples tested by the Noble Research Institute. It represents about 10,000 samples taken over five years. All illustrations assume bermudagrass is the crop/forage being grown.
First let's look at phosphorus (P). P is a macronutrient, which means it is needed in relatively large quantities by growing plants. Most soils in the Noble Research Institute service area are inherently low in P. Our data show that only 33 percent of the soil tested has sufficient P (Fig. Phosphorus (P) Levels). Let's assume you are in the slightly deficient range or below. That is 48 percent of you. If so, you are 80 percent sufficient in P or less and need at least 40 pounds per acre P2O5. According to Oklahoma State University, if you do not add the needed P, you limit your yield by at least 20 percent. That means without adding the needed P, you waste at least 20 percent of your nitrogen (N) fertilizer dollar. With today's N cost, that is not an attractive option.
Next let's look at the inverse of this example with potassium (K). K is also a macronutrient and second only to N in the amount needed by a growing plant. However, most soils (66 percent) that we test are sufficient in K (Fig. Potassium (K) Levels). Let's assume you are in that 66 percent and that you annually apply 300 pounds per acre of 17-17-17 fertilizer. That is 51 pounds per acre of K2O you are paying for and applying that you don't need. At $0.15 per pound of K2O, you waste $7.65 per acre.
Please don't use this information to fertilize by the odds of what you need. Yes, most fields are low in P and sufficient in K. However, the only way to know is to soil sample and test. Hopefully this article will encourage you to soil sample if you haven't done so. Who knows what it could be worth to you?