Increasing potassium “may impart a negative effect on drought resistance”

Does adding more potassium improve drought resistance? This article by Rowland et al. takes an in-depth look at that important question.

They looked at bermudagrass (Tifeagle and Tifdwarf), seashore paspalum (SeaDwarf), and zoysia (PristineFlora) grown in a USGA sand rootzone.

Unirrigated manilagrass (*Zoysia matrella*) in the middle of Thailand's dry season.
Unirrigated manilagrass (Zoysia matrella) in the middle of Thailand’s dry season.

The amount of potassium applied to the grass ranged from 4.9 to 19.6 g m-2 30 d-1. Nitrogen was applied at 4.9 g m-2 30 d-1, so that created N:K application ratios of 1:1, 1:2, 1:3, and 1:4. Irrigation was applied at 25, 50, and 100% of daily ETo. Then they measured wilting, soil moisture, root length, turf quality, leaf chlorophyll, normalized difference vegetative index (NDVI), and thatch depth.

One might have expected potassium application, or increasing the amount of potassium applied in relation to nitrogen, to have some positive effect on at least some of these parameters. You might be surprised then, at what actually happened. Rowland et al. report that:

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.

How about the soil volumetric water content (VWC)?

There were no differences in VWC among N/K ratios.

And that is what was observed in every parameter measured. Whether it was turfgrass quality, NDVI, root length, or thatch depth, adding more potassium didn’t improve anything. They conclude:

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.

These results show that when the grass is supplied with enough potassium, adding more does not provide any benefit.