More about that Bah! Humbug! thing.
In reference to this discussion on Pitchcare, Mario Ernst sent me this question:
There is a passage that has confused me a bit:
"Grass with a good population of mycorhhizae in the root system may need 50% less fertiliser than non mycorrhizal grass. Unfortunately high iron concentrations sometimes reduce beneficial fungal levels but generally mycorrhizal colonisation rate is one of the most important factors in determining the amount of fertiliser required to maintain healthy plants.
The analysis (soil test) only measures the available extractable/exchangeable levels of nutrient. It does not measure the total levels. Locked up nutrients become available due to solubilisation by organic acids and by microbial activity, mycorrhizae produce enzymes which make a lot of nutrients available, especially phosphate and many bacteria and fungi also solubilise locked up nutrients."
The person that wrote this seems to argue that soil test (extractant not known here) results in general do not give a clear picture since they don't consider the influence/activities of acids and organisms between consecutive soil tests. The logic behind it also seems to be that there may be a higher amount of nutrients in solution during the summer as opposed to early spring when many people send out their soil samples...or perhaps I am reading too much into it. In one of your posts you wrote
Not all nutrients in the soil are available for plant uptake. But they don't have to be. We just need to know if there are enough, or not enough...
...nutrients available in solution and on the exchange sites, that is, correct?
Could you help me clarify whether the points (mycorrhizae in particular...e.g. should the impact of the fungi be taken into account somehow for example by calculating with an adjusted rootzone depth) above do affect how one should look at soil test results or is the answer once again Bah! Humbug!...?
My response in short -- this is making it more complicated than it needs to be. Here's what I wrote immediately, before I had seen the entire thread on Pitchcare:
The person that wrote that is making the common error of not understanding how one uses and interprets a soil test.
Because the soil test level is calibrated to turfgrass response, we can say that turf growing in soil with 8 ppm P by the Mehlich 3 test is very likely to respond to P application, while turf growing in soil with 50 ppm P by the Mehlich 3 test is almost certainly supplied with enough P.
The soil test data are not just random numbers unrelated to turf performance. The soil test guidelines, if they have been developed in a standard way, are related to probable turf response. So the soil test value is compared to the guideline and one can then make a decision about application of that nutrient.
That means that ALL of the information about the availability of a nutrient, in terms of is there enough of it available or not, is embedded in the soil test result.
If one interprets soil tests in another way, what is the point of testing?
After I found the thread and read it, I have a few more thoughts.
First, I wouldn't be so harsh in my wording about "the person that wrote that." After I read the thread, I thought it was a reasonable post, with a reasonable interpretation of the soil test, and a reasonable recommendation of how to fertilize the rugby pitches in question. Although I would add a little manganese in this case too.
Second, the mycorrhizae are not really related to interpreting soil tests. We can expect mycorrhizae in turf soils, and if there are more mycorrizae, the effective root system will be larger, so the quantity of an element such as P required on the soil test would be lower in a soil with more mycorrhizae than in a soil with fewer. But this makes it unnecessarily complicated. P in the soil will change through the season. The root system size and architecture also changes through the season. The plant demand for P changes through the season. The amount of mycorrhizal infection of the roots changes through the season too. All these things are in flux. And largely irrelevant.
What is relevant is that the value of an element (P in the example I am using) is enough or is not enough to produce good turf. It is not about locked up, exchangeable, or seasonal variables, or mycorrhizal counts, or anything like that. One just gets the soil test number, compares it to a guideline, and decides if the soil levels of P can meet the plant requirement, or if fertilizer P is required.
If one cannot trust the guideline, or believe the number, then the soil test was a waste of time and money. Soil testing is useful as a decision-making tool to determine if an element should be applied as fertilizer. If one does a test, and then cannot make a decision, what was the point?
I like to use the MLSN guidelines because they are developed from more than 3,000 soil samples, with turf of multiple species, sampled throughout the year, and in all the turf was performing well. That means that at the time the sample was taken, that amount of an element, as measured by the Mehlich 3 extractant, was sufficient to produce good turf. And we can then make calculations of how much the grass may use going forward, to make sure that the fertilizer recommendations keep that element from dropping below the MLSN guideline.