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. 

"Even at extremely high concentrations, the bicarbonate did not seal off the soil"

When it comes to fearmongering in turfgrass, I'm not sure there is a bigger one than bicarbonate (HCO3-). One often hears of supposed problems associated with bicarbonate. I'm still waiting to see a soil or turfgrass problem associated with this ubiquitous anion. It is ubiquitous because atmospheric carbon dioxide is dissolved in water, and carbonate minerals dissolve in soil water, and thus bicarbonate will always be present. 

Brian Whitlark wrote about an interesting project by Obear and Soldat to see if bicarbonate applied at high rates could actually "seal off" a soil. Even when high rates of bicarbonate were applied, the water movement in the soil was not affected.

There are plenty of things one needs to worry about in the irrigation water quality and soil moisture and soil chemistry management of turfgrass. Bicarbonate isn't one of them.

What one needs to know is the salinity (the amount of salt) in the irrigation water, and in the soil, and in the water one also needs to know the sodium adsorption ratio (SAR). If the SAR of the irrigation water is high, then one will also want to know the exchangeable sodium percentage (ESP) of the soil.

Unless one is growing turf in a sand rootzone. In a sand rootzone, the SAR and the ESP don't matter, so one only needs to worry about the salinity. 

The importance of irrigation water testing

Brad Burgess wrote:

I would appreciate your thoughts and comments re this water test. I just read your PACE Turf Article and thought I would run this by you. It could be a nice study for you re salt tolerance in Zoysia …

Never seen water this bad before and tested it after the fact. 

Others have said this water is not even suitable for Paspalum … 

Look forward to your comments. Also attached some photos … at 90 days after planting.

This water had electrolytic conductivity (EC) of 1.4, pH of 9, calcium at 4.7 ppm, magnesium 3.7 ppm, and sodium 314 ppm.

I responded:

Thanks for the photos and the water test.

Photo of turf at the site being grown-in with this irrigation water.

The grass looks great. And that is a pretty poor water. It would be an interesting site to do some tests.

My thoughts on the water -- the 2 most important things to look at are total amount of salt (EC) and SAR [sodium adsorption ratio]. 

EC is what one looks at to see the effect salt in the water is going to have on the grass, how that may accumulate in the soil, and how much extra water will be required to keep the soil salts at a level the grass can tolerate.

For the salt content, it isn't too bad. The leaching requirement [for more about leaching requirement, and how to make these calculations, see this handout] for zoysia using that water, if I use a soil EC tolerance level of 8 ds/m, is 0.037, so the amount of extra water required is minimal, ET / (1 - 0.037). But if the soil structure would deteriorate, then one couldn't leach to maintain the soil EC at 8, and then the salt would damage the grass. As a comparison, the irrigation water at [golf course name redacted] has had 4 times as much salt as this water, and Tifeagle can still be maintained to a high level, as long as one leaches properly.

SAR is what one looks at to see how the sodium may cause a problem with soil structure.

I think [this lab’s water test is flawed] because it does not provide the SAR directly, forcing the customers to calculate it themselves, while emphasizing [less relevant data]. 

For this particular water, the SAR is about 26, which is especially bad for soil structure, especially because the water doesn't have a high salt content. One expects the regular use of this water to cause problems with soil structure (unless it is a sand rootzone) over time, exhibited by slowing of water infiltration. This problem can be addressed by regular applications of gypsum. The amount of gypsum to apply is based on the amount of sodium added in the water, or based on the ESP of the soil. Gypsum can be applied at pretty high rates, like 200 to 400 g/m2. I make a rough calculation that for every liter of water added, one should apply 1.5 g gypsum/m2 to prevent soil structure problems. So if 150 mm of water were added in a month, that would be a 225 g/m2/month gypsum requirement.

To summarize, I'd be concerned about soil structure with this water, would apply 1.5 g gypsum/m2 for each L of water that was applied, with that being done to prevent soil structural problems (disregard that advice on a sand rootzone), and I would make sure that slightly more water was applied than ET, to prevent salinity problems.

I’d like to emphasize three things.

1. It is really important to test the irrigation water. Because Brad had this water tested, he can identify and prevent potential problems. What is in the water is invisible. Many sites have water that is perfectly fine, and a test will confirm that. For locations with high salinity or high SAR, that problem is invisible in the water, until there are visible problems on the turf, and by then it is way too late.

Seashore paspalum has died at this site where salt has accumulated in the soil. This problem can be prevented by knowing what is in the water and then carefully managing the salinity through leaching.

2. Make sure the water is being tested for the right things. One needs an irrigation water suitability test. A comprehensive guide for this is Harivandi’s Interpreting Turfgrass Irrigation Water Test Results. In that, he writes:

When irrigation is applied to the soil, the best indicator of sodium effect is a water’s Sodium Adsorption Ratio (SAR), a value which should be provided in all laboratory water analyses. 

3. If for some reason SAR is not reported, one can calculate it from this equation:

\[SAR = \frac{Na}{\sqrt{\frac{Ca + Mg}{2}}}\]


SAR is sodium adsorption ratio

Na is the sodium concentration of the water in milliequivalents per liter

Ca is the calcium concentration of the water in milliequivalents per liter

Mg is the magnesium concentration of the water in milliequivalents per liter

Water use and growth

Turfgrass water use is a real thing. The temperature-based growth potential (GP) is not real in the same sense. GP is a value between 0 and 1 that gives an indication of the potential for the grass to grow, based on how close the actual temperature is to the optimum temperatures for photosynthesis and shoot growth for that species. 

There are many practical uses of GP. I think GP is more useful than water use, or evapotranspiration (ET) as an estimate of water use, in the planning and prediction of turf nutrient use. 

But the water use is important. Nutrients go into the roots with water, so when lots of water is flowing through the plant, there will be more nutrients being used too. In winter, for example, dormant turf won't be using water, and thus there won't be any nutrient use. This has important implications for late autumn N fertilizer and how much of it is utilized by the grass.

As grass grows more, it uses more water. Or, as more water is used, the grass grows more. I'm not sure which is cause, and which is effect, but there is a definite connection between water use and growth. An experiment with grass in Thailand is illustrative on this point.

Bermudagrass, seashore paspalum, and manilagrass were grown in a plastic house in Thailand for 48 days. The water use and the clipping yield of the turf were measured.

This was a fertilizer experiment, with the same amount of N but different amounts of K applied as treatments. Looking at the water use and clipping yield, one can see that with more water use, there was more growth. Or with more clipping yield (growth), the grass used more water. This has obvious implications for how many nutrients are used and required by the grass.


Playing with numbers, evapotranspiration edition

I've used equation 52 in Crop Evapotranspiration: Guidelines for computing crop water requirements (FAO Irrigation and Drainage Paper 56) to calculate the predicted ETo for San Digeo, California, and for Madison, Wisconsin.

Rplot01I've also obtained the average ETo for San Diego (Torrey Pines, station # 173) from CIMIS and the average ETo for Madison from UW Extension Ag Weather

RplotPlotting the calculated ETo from equation 52 (predicted ETo) against the ETo average, one sees a pretty close relationship between the two values.

Rplot04But there is not such a close relationship between the temperature-based growth potential of PACE Turf and the ETo.

Rplot03I've suggested that the growth potential can be used to estimate turfgrass nitrogen use, for any grass, anywhere. Doug Soldat has pointed out to me that ET, or the consumptive water use, can be used as an estimate of how much nutrient uptake there may be. Nutrients (including nitrogen) go into the roots with water, and Bill Kreuser has written about this in Rethinking fall fertilization

The ET is one way to look at nutrient uptake and nutrient demand. And it is important to remember that nutrients do go into the roots with water, so there is going to be a huge, dominating role of consumptive water use (ET) in how much of each nutrient gets used by the grass.

Even so, I think the GP is more useful as a predictor of nutrient requirement extended across grass species and locations. These calculations don't show the whole story, but they are a first step at looking into this. And, it is interesting to see just how well equation 52, which requires only temperature and latitude data, predicts the average ET.

We have had our water tested and would like a little interpretation

I received an e-mail asking for some help with interpreting an irrigation water test. Since many people may have similar questions, I'll paraphrase the questions here, together with my response.

  1. Is salt the sodium, chloride, and salinity together? Actually, salt on a water test is the total salinity, that is, all the dissolved salts, so it will be sodium and chloride and potassium and nitrate and magnesium and sulfate and calcium and ammonium and so on. And for any irrigation water test, I suggest consulting Dr. Harivandi's Interpreting Turfgrass Irrigation Water Test Results. In fact, this is on my list of Five Articles Every Greenkeeper Should Read. Another great reference is the Irrigation Water Guidelines document from PACE Turf.
  2. What is the difference between SAR and adjusted SAR on a test? The SAR is the number to look at. I disregard adjusted SAR. The adjustment attempts to predict future sodicity problems by considering what chemical reactions may occur in the soil. But it also overestimates the hazard. For a bit more about this, see What's in the water from the University of Nebraska and this abstract from Obear et al..
  3. On our test it shows alkalinity expressed as bicarbonate is 89 mg/L. Is this a problem? No, that is a normal amount of alkalinity. I should add, this is not something that one even needs to check. I spoke about this in a presentation entitled Soil and Water Management: three problems, three solutions. The handout, here, explains how to check the two things that do need to be checked: salinity and sodium hazard.
  4. Do you use ppm or mg/L? These are the same thing. One mg per L is also one part per million.
  5. What does TDS mean? TDS stands for total dissolved solids. It is a measure of the amount of salt in the water. If one would evaporate all the water from one liter, the remaining mass of material is the total dissolved solids, or TDS.

It is important to understand the impact salt in the water can have on the grass, and how that salt should be managed. If it is not managed, the results can be disastrous. 

Salt from the irrigation water has accumulated in the soil, killing seashore paspalum turf on this golf course fairway near Bangkok.

Of course, in many cases there is no problem with the irrigation water. It is still good to know what is in the water, and to be able to interpret the results, because when the turf is good, one doesn't want to damage it in any way.

The manilagrass and creeping bentgrass at this course near Tokyo are irrigated with water low in salinity and with a low sodium hazard.

And in many cases, there may be a shortage of water for irrigation. In that case, one also needs to know exactly what is in the water, and how it may affect the grass and soil. That is the only way to ensure that this limited resource is used most efficiently.

When a limited amount of water is available for irrigation, it is especially important to know what is in the water and how it may influence the grass and soil.

I'll recommend again, print a copy of Interpreting Turfgrass Irrigation Water Test Results and keep it within easy reach. And the Irrigation Water Guidelines from PACE Turf is another good reference that is useful in understanding test results and identifying (or more likely, eliminating) possible problems.

Three of my favorite projects from Beth Guertal's Research Group

Fertilization of bentgrass with commercial foliar products - guertal ats 2010.pdf (page 1 of 10)
I was excited to read the press release from AGIF and GCSAA last week announcing that Beth Guertal will be teaching in September at events in the Philippines and Vietnam. I've always enjoyed studying the research she does, and these seminars are a great opportunity for learning from one of the world's experts on turfgrass management. 

I'm not sure that she will be talking about these particular experiments, but these are three of my favorites (of the many) from her research group.

  1. Fertilization of bentgrass with commercial foliar products: Greenhouse evaluations. "In most cases foliar application of urea was as effective (for N uptake and dry matter yield) as applying any of the commercial materials."
  2. Potassium Movement and Uptake as Affected by Potassium Source and Placement. "Over the 2 year study potassium application had no beneficial effects on turfgrass performance, and acceptable performance was achieved across a wide gradient of K content in soil and leaf tissue. Regardless of soil test K level, no deficiency symptoms were observed."
  3. Fan and syringe application for cooling bentgrass greens. "In general, reductions in the maximum observed temperature occurred in the following order: fan plus syringing, fan only, syringe only, and, no fan/no syringe."

Tropical carpetgrass part 2: ugly duckling or swan?

When I wrote about tropical carpetgrass being an unappreciated grass, the conversations (1 & 2 & 3) that ensued showed a mixed response. Some people really like tropical carptegrass (Axonopus compressus), and others have no use for it.

Tropical carpetgrass on a golf course fairway in Singapore

It was rightly pointed out that in a a subtropical environment, tropical carpetgrass will not be ideal throughout the year, especially when it is cool. And like other grasses, there will be issues with drought tolerance, and traffic damage, and so on. Where this species is really well-adapted is in tropical climates that receive more than 1,000 mm annual precipitation; one wouldn't want this grass where precipitation is less than 800 mm. It is not a perfect grass, even in the tropics, but no grass is.

Tropical carpetgrass through the green at Chumpon, Thailand

So why do I persist in writing about tropical carpetgrass? Because for many tropical sites, this is the grass that should be used, and it has some very attractive characteristics that are especially relevant to the way I think about turfgrass management. When possible, I think we should manage turf with a minimum amount of inputs. Tropical carpetgrass, more than other species, can be maintained as a multipurpose turfgrass with the fewest inputs.

Last year I was interviewed by Matt Adams on the Fairways of Life radio show. You can listen to the interview here.

07.09.13 INT ARCHIVE Fairways Of LIfe Micah Woods_3592772

I was expecting Matt to talk with me about tournament preparation and golf and grasses around the world. Instead, he started by asking about maintenance inputs and grass selection:

Today more and more, there is pressure upon every golf facility in terms of how they maintain the golf course – the general line that we're hearing is that golf courses need to embrace more of the brown because water is at such a premium anywhere and everywhere around the world.

Then he asked, how can the type of grass chosen help us out in terms of maintainenance cost and availability of water? Rather than talking about using more resources and spending more money, the focus at the global level is to use less resources in turfgrass maintenance. 

This was also a prominent theme of Don Mahaffey's recent conversations at Golf Club Atlas (first interview, second interview). These are really good discussions about what golf course maintenance should be about, and I highly recommend taking the time to watch both of them in their entirety. In the second video, Don said something that is very applicable to turf management: 

We cut our maintenance expenses greatly, because we just focused on what's good for golf, and interestingly, no one complained.

Tropical carpetgrass fairway at Phuket, Thailand

With tropical carpetgrass, in a tropical climate with annual precipitation of at least 800 mm, and preferably with 1,000 mm or more, this grass requires only mowing. It can be maintained without fertilizer or pesticides, and irrigation is only required if one wants to make the grass green, or if the turf is to be heavily trafficked with golf carts.

There will be many golf courses or turf managers that prefer to grow and manage a grass that has different characteristics. But let's not forget about the many good characteristics of the multipurpose tropical carpetgrass. As the recent research from Trinidad and Nigeria demonstrates, this species has a number of advantages compared to other turf species in a tropical environment.

Tropical carpetgrass lawn in Ayutthaya, Thailand