Soil potassium, from significantly unavailable to highly available
Burning grass

I misunderstood the question about pH

Allan Dewald asked about upward revision of the MLSN guidelines at high pH:

I misunderstood that question at first. I thought he meant that if one has good grass at pH 9, then would an upward revision of suggestions about pH be warranted? But as I started composing my answer and read again his question, I'm now taking it to mean that if one has a root zone with pH 9, is an upward revision of the minimum levels for elements such as K, Mg, P, and Ca necessary?

My answer, to both of those questions, is no. Here's why.

First, calcareous soil that doesn't have an alkalinity problem will have a maximum pH of about 8.3. When the pH is more than 8.5, then it is not calcium carbonate causing that -- there is sodium or some other source of alkalinity. A pH of 9 can be problematic, and one doesn't want to grow grass at that pH. Therefore, one will likely be doing things like acidifying irrigation water, or leaching, or applying elemental sulfur, and doing more leaching, in order to get the pH closer to 8.3. In that case, I don't think it matters what the soil nutrient levels are. There is no need for guidelines for those type of soils. My recommendation is to make treatments to deal with the alkalinity and just supply the grass with the amount of each element that it can use. Don't worry about soil nutrient levels.

Second, the current MLSN guidelines were developed from an analysis of soils with pH from 5.5 to 8.5. That's the pH range at which most good turf is found. It is probably more common to have good turf at less than 5.5 than it is to have good turf at pH above 8.5. In the Global Soil Survey through 31 August 2015, the samples have a pH from 4.6 to 8.2 with a median of 6.4.

pH of Global Soil Survey samples

Because our analyses of the MLSN data were restricted to soils with pH from 5.5 to 8.5, and because the data collected from the Global Soil Survey so far have a range from 4.6 to 8.2, we can make some suggestions about nutrient levels in soils within those pH ranges. But for pH 9, the data we have don't give us any information about what the nutrient levels should be for good performing turf at that pH.

That's why I would not worry so much about soil nutrient levels at extreme pH levels, and I would (especially if the pH was high) try to get it closer to 8.3, and I would not use the MLSN guidelines, but rather would just supply the grass with the amount of nutrients it uses. This is essentially the Precision Fertilisation method of STERF.


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