N:K ratios on warm-season grass
Monthly Turfgrass Roundup: February 2015

Surprises, conservatism, and what one can learn from soil testing: part 1

Last week at the Siam CC Waterside Course, I talked about N:K ratios on a seashore paspalum tee. I showed some demonstration plots, on which had been applied two applications of these products, with the application rate being 5 g N/m2 at each application, for the products that contained N:

  1. nothing (control, no N or K applied)
  2. urea (46-0-0, a 1:0 N:K ratio)
  3. 19-0-19 (1:1 N:K2O ratio)
  4. 15-0-26 (~ 1:2 N:K2O ratio)
The seashore paspalum test plots at the Field Day last week with collaborating scientists from the Thailand Institute of Scientific and Technological Research

We collected soil samples after the first application of fertilizer treatments, and before the second, and sent them to a lab for analysis. Unfortunately, the results came back to me the day after the Field Day, so I wasn't able to discuss them there. But the results were illustrative of a some points about soil testing I would like to make anew here, and others that I'd like to reiterate. I'll start with three in this post.

1. One really should do soil tests on a regular basis. I suggest once a year will be sufficient frequency for the average facility. If tests are not done, there is no way to know the conditions in which the grass is growing. On the tee where we had these demonstration plots, the soil pH ranged from 4.9 to 5.5; I suggest maintaining soil pH at 5.5 or above in most cases. And in these demonstration plots, the soil calcium (Mehlich 3 extractant) ranged from 155 to 246 ppm, way below the MLSN guideline of 331 ppm. And the magnesium in the soil (Mehlich 3 extractant) was even lower, from 25 to 41 ppm -- all demonstration plots were way below the MLSN guideline of 47 ppm.

I demonstrate how we can trim turf in pots to a simulated fairway height on this seashore paspalum growing in soils with low pH and Ca and Mg well below the MLSN guideline

2. The MLSN guidelines are conservative. I've mentioned that before, and this is a good example of that conservatism. If you were at the Field Day last week, you saw that the seashore paspalum on that driving range tee looked pretty good. If you weren't there, you can see from the photos (provided courtesy of Dr. Rujira Deewatthanawong) above -- it looked green, actively growing, divots filling in quickly, a dense sward -- the turf was performing well and did not appear deficient in any nutrients. Yet the soil calcium and magnesium were both below the MLSN guideline. One meaning of conservative is "(of an estimate) purposely low for the sake of caution." In the case of nutrient guidelines and the resultant fertilizer recommendations, we want to avoid a deficiency, so we could say also that an estimate is purposely high for the sake of caution. That is, a conservative estimate is one that errs on the side of caution.

Soil tests are used to determine how much fertilizer should be applied, and the MLSN guidelines are set in such a way that for the sake of caution the amount of fertilizer recommended will be more than the grass actually requires. One of the concerns that has been raised is that perhaps the MLSN guidelines are too low. Read more about those concerns here:

 This driving range tee, at which the grass performance was fine, but soil Ca and Mg were way below the guidelines, is an excellent example of this conservatism. One can look at the turf -- Ca and Mg aren't really required as fertilizer here, because the grass performance is fine, and the soil is supplying enough of those elements to produce high quality turf. But using the MLSN guidelines to calculate the fertilizer requirement, I would recommend application of 25 g Ca m-2 (5 lbs Ca/1000 ft2) to supply enough Ca to reach the MLSN guideline. And using the MLSN guidelines to calculate a Mg requirement, it would be 5 g Mg m-2 (1 lb Mg/1000 ft2). That's what I mean by conservatism. Even though the MLSN guidelines are way lower than conventional guidelines, it is the conventional guidelines that are set almost impossibly high. The MLSN guidelines are much lower, but they are still conservative, in that for the sake of caution they will recommend application of elements that are unlikely to cause an improvement in turf performance.

3. Supply of nitrogen (N) really controls nutrient demand. This is embedded into the fertilizer requirement calculations that one makes when using the MLSN guidelines. And the fertilizer treatments and amount of K applied and remaining in the soil in these demonstration plots illustrates that demand.

In the control plot, where no N or K was applied, the soil K (Mehlich 3 extractant) was 62 ppm. In the plot to which urea was applied, the soil K was 41 ppm. Why was it lower? Well, because the added N stimulated growth and increased the grass demand for K. And in the plot receiving the 19-0-19 fertilizer, the soil K was 48 ppm. How can it be, that a plot receiving a 19-0-19 fertilizer would have a lower soil K than the control plot? It's because adding N increases the uptake (root uptake = soil depletion) of K. It was only the 15-0-26 plot that got soil K elevated above the control plot, to 71 ppm. Of course, we didn't see much of a difference between the plots, on the surface -- all the turf looked good, and looked pretty much the same. But in the soil, there were differences, and they were related both to how much K had been applied to the plots, and also, whether N had been applied. Those data are plotted below, and the color of the points indicates whether N was applied to that demonstration plot, or not.



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