Soil Testing Interpretation is Broken
By Doug Soldat
The old saying goes “don’t guess, soil test”. The thought behind this axiom is that soil testing can prevent unnecessary applications of fertilizer that would’ve been applied if not for the soil test. This is probably true in some cases; but EIFG’s recent nutrient use survey shows that managers who soil test apply more fertilizer than those that don’t. The implication is that soil test results are telling people to fertilizer when the grass doesn’t need it. Sounds surprising, but it is consistent with what our research has shown over the years. For example, we found that the critical level for a bentgrass green on a sand root zone was 7 ppm Mehlich 3. Below that the grass turned purple, and at or above 7 ppm, the grass was great. But we didn’t change the University of Wisconsin recommendation to 7 ppm, we made it 25 ppm (MLSN is 21 ppm). That’s because our critical level (7 ppm) was specific to the grass species, rooting characteristics, and soil type. A different green might have a critical level of 5 ppm or 15 ppm. So to be safe, we built in a big safety buffer. Same thing for the potassium research we are doing. UW Soil Testing Lab says to keep Mehlich 3 potassium above 50 ppm. But my research green has healthy bentgrass when Mehlich 3 K is 20 ppm. In fact, we haven’t seen a sign of a deficiency yet, so we don’t really know where the lower level is. Rutgers University research on annual bluegrass suggests that annual bluegrass was more resistant to anthracnose disease when Mehlich 3 levels were above 43 ppm, anthracnose isn’t a problem on bentgrass so we haven’t had the same issue. The point here is that soil/grass systems have different optimum levels. So, overestimating soil nutrient requirements is a feature of even the most data-driven recommendations. There is no good way around this. The best recommendations are going to have some degree of over-estimation, which means you will continue to over-apply fertilizer when you follow them.
We have a solution for this at GreenKeeper. First, we are offering low cost Mehlich 3 soil tests, allowing you to test more areas and/or test more regularly. The soil test results are automatically piped in to the app and displayed in a graph for tracking changes over time. GreenKeeper will show you MLSN and other University based soil testing guidelines, but we won’t recommend that you apply fertilizer simply to chase a number. Take calcium for example, we don’t have any interpretations for calcium. If your soil pH is above 5.5, then you won’t have to worry about calcium deficiency. If you are below 5.5, you should apply lime, which will raise pH and simultaneously add calcium. You can keep track of calcium levels, but we don’t want you to over-react to them. For the majority of managers, the standard soil test package will be all that is ever needed. Only the salt affected properties will want to go with the salt package, and while we offer a complete soil test package with micronutrient values and sulfur, we don’t think measuring micronutrients in soils is necessary in most cases.
|Standard Soil Test ($20)||Standard + Salts ($30)||Complete Soil Test ($40)|
|Organic matter||All tests in standard package||All tests in standard + salts package|
|pH and buffer pH||Mehlich 3 Na||Mehlich 3 Fe, Mn, Zn, Cu, S|
|Mehlich 3 P, K, Ca, Mg||Exch. sodium percentage|
GreenKeeper Premium users get a further discount on all testing services.
We want you to think about applying nutrients when you see changes in soil nutrient levels, or changes in performance. Here is an example. A couple of years ago I worked with a golf course that had a phosphorus deficiency show up on a tee a few weeks before a PGA Tour event. The grass was slowly turning purple, it was not localized dry spot. They applied some phosphorus fertilizer and the grass turned green in time for the tourney. Nutrient deficiencies are super easy to correct. I think people worry too much about them for how rare and agronomically inconsequential they are. Now that PGA Tour course has their own Mehlich 3 critical point that they can use as a guide for future soil test interpretations. They don’t have to chase numbers from a soil test interpretation sheet anymore, they know the level where their grass turns purple and they can apply P to stay just above that number.
In summary, use existing interpretations as a guide. Do not apply nutrients when above MLSN values, with rare exceptions. If your values are below MLSN guidelines, you can either apply nutrients to attempt to achieve MLSN values, or if your turf is healthy, maintain the levels existing in your soil. A GreenKeeper program builder feature is currently under development and will give nutrient application recommendations to maintain your soil nutrient levels that happen to fall below MLSN thresholds but support healthy turf. In the meantime, feel free to reach out to me at firstname.lastname@example.org with all your soil testing questions.