"Even at extremely high concentrations, the bicarbonate did not seal off the soil"
"I started using the MLSN guidelines and was amazed with the huge impact this made"

Of drought resistance, N:K ratios, experiments, and inferences

The evidence continues to accumulate. Adding more K in relation to N doesn't seem to do much. Rowland et al. wrote about their experiment that looked at drought resistance of warm-season putting green grasses grown in sand, treated with different rates of potassium.

They studied drought resistance of four cultivars:

  • bermudagrass, Tifeagle and Tifdwarf
  • seashore paspalum, SeaDwarf
  • zoysiagrass, PristineFlora

Here are some of the results from their research:

Increasing K in relation to N failed to increase drought resistance for the cultivars studied.

Applying K at ratios above 1N:1K did not increase drought tolerance and may have actually hindered it, as wilting increased (P < 0.10) on two rating dates when 1N:4K was compared to 1N:1K. This was likely due to saturated K levels within the leaf tissue and an increase in soil solution salts.

Our results indicate that increasing N/K ratios above 1N:1K is not beneficial, and in fact may impart a negative effect on drought resistance.

In a previous discussion of these results, Jon Wall had this suggestion:

That's a good idea. I have a few comments on those specific rates.

1. If I were doing a follow-up on this, I would try to minimize the number of treatments. The more treatments there are, the more time, space, and expense involved in doing the experiment. Bermudagrass leaves usually have from 3 to 4% nitrogen (N) and about 2% potassium (K). I conservatively estimate the ratio in the leaves of this species at 3:2, which would be 1:0.66. 

If the soil has a reasonable amount of K, then K added as fertilizer won't have an effect, because the grass will be able to obtain all the K it requires, even if the N:K ratio is 1:0. One could do the experiment in a rootzone with low K, and I would try to do it with just 2 or 3 treatments – 1:0.33 and 1:1 for sure, and maybe 1:0.66. By measuring what happens at the 1:0.33 and 1:1 treatment levels – bracketing the levels of K that the grass can use –  and what the magnitude of the effect is going from one level to the next, one could infer what would happen at intermediate levels of K supply.

2. On the topic of inferences, there is lots of data showing that supply of K in the amount the grass can use is the rate of K that optimizes a number of plant performance characteristics. These cited results are for bermudagrass in Florida.

Cisar et al. found a ratio of 10:1.25 sufficient even with high sodium application.

Snyder and Cisar saw reduced visual quality when the N:K ratio dropped below 2:1.

Snyder and Cisar reported this: "Severe K deficiencies were observed in the absence of K fertilization. However, increasing K fertilization beyond a K/N fertilization ratio of 0.5 to 1 had virtually no effect on turfgrass appearance, growth, on resistance to bermudagrass decline, or on root weight."

What inferences can one make from these experiments and from Rowland et al.? Adding more K than the grass can use doesn't do anything good. Adding less K than the grass can use, if the soil is also low in K, will cause problems.

3. Because the results are so consistent, I think the optimum N:K ratio is pretty predictable. It is in the range of 2:1 to 3:2, assuming the soil K is relatively low, say at less than 50 ppm by Mehlich 3. When the soil K is higher, the amount of K added can be reduced, because the soil will be able to supply some of the K used by the grass.

4. This would be a great test to do on a nursery area. Using the procedures described in PACE Turf's guide to testing products and practices, one could apply different N:K ratios. That is straightforward. And then one could withhold irrigation and wait for the grass to start wilting, and look at the grass treated with different N:K ratios to determine if there was any N:K effect on when the grass starts to wilt. That, basically, is drought resistance. 


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