How Much Potassium Does Grass Require?
Turfgrass Mystery at the sod farm

The Unambiguity of Nutrient Availability Indices

I've been asked a question about nutrient elements being "locked up" in the soil and if I could explain that. This question is in some way related to these posts about soil testing and nutrient requirements:

IndexIn those posts, I have not used the terms "locked up" or "tied up" to refer to nutrients in the soil that are unavailable for plant uptake. Those terms should not be used by professional turfgrass managers because they are ambiguous and do not give the mechanism by which the nutrients are supposedly unavailable.

If we would say that some of the phosphorus (P) applied as fertilizer becomes "locked up" in the soil, what does that really mean? Does it mean the P will be forever unavailable for plant uptake? Does it mean the P is temporarily unavailable? Does it mean that all the soil P is at risk of being unavailable? Should we use a liquid fertilizer instead, to avoid this ambiguous "lock up" in the soil? And what about calcium (Ca) in a high pH soil, is it "locked up" in the form of calcium carbonate? The questions go on and on, but cannot be answered clearly because of vague terminology. It is often the case that this ambiguous terminology goes together with dubious chemical assumptions.

We avoid these problems by using nutrient availability indices, of which the MLSN guidelines are an example. A nutrient availability index does not try to say that a certain pool of measured nutrient are all of that element available. It is simply a number that, if it has been carefully calibrated with turfgrass response or turfgrass performance, can be used to determine the probability of a plant response to additional applications of that nutrient. 

Pace_mlsnIn the soil, there are various forms (pools) of the nutrient elements. With potassium (K) for example, there will be some in soil solution, some on cation exchange sites, some in what is termed non-exchangeable forms, some in the soil particles (structural K), and some, perhaps, in undissolved or unreleased fertilizer granules, and a small amount in soil organic matter. But an availability index for K does not need or attempt to assess all of this. In the case of the Mehlich 3 extraction for K, we would be extracting from the soil almost all of the soluble and exchangeable K and a small portion of the non-exchangeable K and none of the structural K. But that index is useful, because after many experiments and calibration and study, we are confident that a value of 37 ppm K is sufficient for excellent turfgrass performance. 

If the index is less than 37 ppm, we would expect a positive response to added K. If the index is above 37 ppm, we do not expect a response to added K. Availability indices such as the MLSN guidelines are unambiguous and already incorporate the "locked up" nutrients. 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. The MLSN guidelines provide an unambiguous answer.

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