Test for response

Phosphorus and potassium are problem nutrients for superintendents trying to find the correct application rates. Dr. Beth Guertal, alumni professor at Auburn University’s Department of Agronomy and Soils, talks about the difficulty in getting it right.

Why is it important for superintendents to be aware of their phosphorus and potassium rates?

Well, particularly on the phosphorus issue – golf courses are not yet constrained as much as others – but we’re certainly up to 11 or 12 states now that have some legislation as how to much and when phosphorus can be applied. We’ve got some legislative controls as to when and how some people can apply phosphorus fertilizer. Of course, the other big area is certainly financial and plant response. There’s no need to be applying nutrients if we’re not seeing a response to them.

For potassium, it’s an interesting issue. I’ve had a superintendent who sat all the way through my talk and at the end said, “Yeah, but I still put it out for stress.” But all the data clearly shows we don’t see a response. Potassium, to some degree, is kind of almost a no-harm, no-foul input. We don’t talk about the environmental negatives of the overapplication of potassium. There’s no leaching like there is with nitrate. There’s no runoff and subsequent pollution like there is with phosphorus. So it’s sort of frustrating, but a lot of guys will use some extra K when they’re thinking they’re going into a stress situation like drought or traffic, even when the data doesn’t necessarily support that.

What has the data shown?

Certainly, Micah Woods’s work shows that we see little response to added potassium. Down in Florida, the work that John Cisar did on bermudagrass showed there’s really little benefit to going above a 1:1 ratio of N to K. In general, if your soil test is at the sufficiency level for soil-test K we do not see any benefit to adding additional K. In fact, the work that Micah Woods and Larry Stowell are currently doing with soil tests indicates that we can probably operate below that soil test recommendation. There’s probably a dozen research papers out there that clearly show overapplication of K doesn’t come back in anything: No better ball roll, no better turf quality, no better clipping yield, no better shoot density. Typically, we often will see more K in the plant (as measured by a tissue test), but that extra K does not provide any agronomic benefit. This is not a new finding – this idea of ‘luxury consumption’ was first mentioned in 1921. In the past couple years, Bruce Clarke at Rutgers is starting to see some effects with disease suppression. Bruce has been looking at potassium and anthracnose, and there was some work where there was less anthracnose with the potassium put out. It had to be N and K together. He did more with greater rates of potassium, so 1:1 or 2:1 ratio had significantly less disease than the lowest potassium rate, which was 4:1. So he saw some reduction in anthracnose.

I’ve seen some response for phosphorus. The response for phosphorus is typically less than the soil test recommendations at which they’re made. A lot of our soil test recommendations we use in turfgrass are based on years and years of crop production research. But P and K fertilization should be based on soil tests, so for instance, if a super takes a soil test in Alabama and it comes back that your soil test for phosphorus is 60 pounds per acre, I know from years of doing this they’re not going to see a response to adding extra phosphorus to the grass. Even though the soil test may say, “Go out and put down 45 pounds more of phosphorus,” I can get that soil test as low as 20 pounds before I see a response. Then it comes back to, “What is that response?” Well, I see more phosphorus in the plant. Maybe I see more rooting, more shoot density.

A lot of us have data that do show response to phosphorus, but most of us think we could back off our soil test recommendations and be just fine. Here’s the next question superintendents ask: “Well, if I’m seeding, I think I’m seeing a response.” If your soil test is in the ‘Medium’ or ‘Low’ level, you may see a response to adding P at seeding, but if your soil test is high, you don’t need to add any more. The bank is fine, you’re good to go. In general, phosphorus is not prone to leaching, and it will accumulate in the soil if long term applications are made. That’s why overapplication is bad – the P will collect, and the soil becomes over-saturated with P, and that soluble P can move with water and sediment to streams and lakes. It’s really important – and I tell superintendents this all the time – intact swards of turfgrass are our absolute best defense against sediment movement and resultant pollution. Intact stands of turf are a great sponge for sediment and nutrient removal. The easiest thing they can do is go around their water bodies and leave a 10 to 15-foot strip that doesn’t get fertilized. That’s their absolute best filter. It works, we know it, and we have the research to show that.


What should superintendents do about this?

Most of the good guys know and are already doing the right things. They’re asking really good questions, like, “Should I even put this out?” My answer is that I don’t know. They want to know specific rates. Rates are just really hard – it varies with the grass species, the level of turf management, the soil type and the soil test extractant.
I tell them to get their soil test and base their application off that, and then they can usually go half of what that recommends, and I say that cautiously. That’s the problem, it’s all based on soil chemistry and it’s a really challenging area.