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October 2015

How soil K changes over time

These data show what happens to potassium (K) in the soil when different rates of K fertilizer are applied. Over two years, I made 25 applications of K to a plot of L-93 creeping bentgrass in Ithaca, New York. In 2002, I made 13 applications, and in 2003, I made 12 applications. K was applied at 6 different rates, and N was supplied in equal amounts to each plot.

This chart shows the starting soil test K, before any of the 25 treatments had been applied, and also the final soil test, two years after the first one, and after all those 25 fertilizer applications had been made. I'm showing data here from the 0.01 M SrCl2 soil test (that is "hundredth molar strontium chloride") because that test has high accuracy and sensitivity in sand rootzones. These data are proportional to Mehlich 3 data, but lower in this sand by about 50 ppm. So 30 ppm by 0.01 M SrCl2 would be about 80 ppm by Mehlich 3.


Before any of the treatments were applied, the soil test K was about 29 ppm. When no K was applied, the soil test K went down. When more K than N was applied, the soil test K went up.

This next chart shows those same data, with the 25 application dates when K was applied marked in green.

K2I'd like to point out that on the final date of sampling shown here -- 30 May 2004 -- it was 7 months after the final K fertilizer application of 2003. And you'll notice that there is a big difference in soil test K, with less than 20 ppm in the plots to which no K fertilizer was applied, and more than 50 ppm in the plots to which 4.6 grams of K were applied for every gram of N applied.

What about precipitation? Shouldn't heavy precipitation cause the K to leach? That's not the way it works. From the first soil test date of 4 June 2002, when the soil test K was 29 ppm, to the last date, there were 20 days during which the precipitation was greater than 25 mm. This chart adds on those dates, marked as blue asterisks. The asterisks are jittered up and down, to avoid overplotting.

K3Each of those 20 days had more than 25 mm of precipitation, for a grand total of 719 mm (28.3 inches) just on those 20 heavy precipitation days. There were 4 such days between the last K application and the soil testing on 30 May 2004. But the amount of K in the soil looks like it was controlled by the quantity of K fertilizer applied, not by the amount of precipitation.

I wrote about this in "I'd be applying potassium all the time" parts 1, 2, and 3. Adding K based on rainfall is a sure way to apply way more potassium than the grass can use or the soil can hold. For that matter, so is adding more K than N.

What is even more important than all the soil test numbers is the performance of the grass. And all the K added in this experiment, all 25 applications of K at different rates over 2 years, didn't cause any improvement in turf performance. Here, in the flagged rectangle, are those L-93 plots to which the K treatments were applied. This photo was taken on 19 August 2003.

K trial north

At the soil test levels of K in this experiment, there was enough K to meet all the grass requirements, across the range of adding no K for every 1 gram of N (a 1:0 ratio of N:K) all the way to the highest rate of 4.6 grams of K for every 1 gram of N (a 1:4.6 ratio of N:K). All the more reason not to worry about replenishing soil K after a rain.

What one should do is look at the soil test K, make sure it will stay above the MLSN guideline for K, and then don't worry about K.

A chart of PPFD at two locations this year from January 1 through last Friday

The photosynthetically active radiation (PAR) changes through the day and through the year. The PAR is measured instantaneously for a duration of 1 second as the photosynthetic photon flux density (PPFD), and by adding up the PPFD for all the seconds in the day, one gets the daily total of PAR, which is called the daily light integral (DLI).

These charts show the average PPFD on an hour by hour basis. With a look at a chart like this, one can see:

  • how length of the day affects PAR, by looking at what time in the morning and what time in the evening the PPFD goes to 0.
  • how time of the day affects PAR, by looking at the change in PPFD hour by hour through the day.
  • how day of the year, and consequently sun angle, affects the PAR, by looking at the maximum values of PPFD at midday and seeing how they change through the year.
  • how clouds reduce the PAR, by comparing PPFD on sunny hours or days to PPFD on hours or days that don't have full sun. For more about sun and clouds and time of year, see these descriptive slides with data from 4 days in Tokyo this year: a sunny summer day, a very cloudy summer day, a sunny autumn day, and a partly sunny autumn day.

This chart shows, for every hour of this year through last Friday, the average PPFD for that hour at Tokyo (red) and at Watkinsville (blue). Each panel of the chart is a single day, and the DLI in units of mol m-2 d-1 is written on each panel, in red for Tokyo and in blue for Watkinsville.

image from

There have been 296 days this year, through October 23. On one of these days, February 10, there were erroneous data at Watkinsville, so I don't have a DLI. That leaves 295 days with a DLI for both Tokyo and Watkinsville. These locations have similar temperatures, and similar latitudes. How do they compare for photosynthetically active radiation? There have been 115 days with a higher DLI at Tokyo than at Watkinsville, and 180 days with a higher DLI at Watkinsville than at Tokyo.

I've made a couple other similar charts. This one shows the average PPFD at Tokyo hour by hour this year through October 12. Because the chart shows data for only one location, I've used color to indicate the month.

image from

And the next one is the same location and dates as the above, with the addition of the DLI written on each panel.

image from

The Watkinsville data are from the US Climate Reference Network and the Tokyo data are from the Japan Meteorological Agency.

You'll want to listen to these when you have time

I really enjoyed these two podcasts by Frank Rossi on Turfnet Radio, with guests Doug Soldat and then Bill Kreuser.

Check Out Golf Podcasts at Blog Talk Radio with TurfNet RADIO on BlogTalkRadio

They talk a lot about turfgrass fertilizer, which is something that I'm quite interested in, and I learned a lot by listening to these two podcasts. I think you will too.

Check Out Golf Podcasts at Blog Talk Radio with TurfNet RADIO on BlogTalkRadio

Why light is more important for ultradwarf than for bent: my presentations at the Japan Turf Show

I'm giving two presentations at the Japan Turf Show in Tokyo this week. In the first one, I explain why light, by which I mean photosynthetically active radiation (PAR), is more important for ultradwarf bermudagrass than it is for creeping bentgrass. I use data from Tokyo and from Watkinsville, Georgia, to demonstrate this and to point out the difference in PAR between Japan and the region of the USA with similar temperatures.

The slides for this presentation about light are available in English and in Japanese.

In a second presentation, I talk about management of ultradwarf bermudagrass greens, explaining how this species performs compared to creeping bentgrass in Japan, and how it should be managed.

The slides for this presentation about ultradwarf management are available in English and in Japanese.

Bentgrass in hot and not so hot places

Creeping bentgrass is a cool-season grass. When temperatures are hot, it doesn't perform well. I was asked if bentgrass in southern China was comparable to bentgrass in Spain. I don't think that is the right comparison. It would be more appropriate to compare southern China to Florida.

I downloaded the 2014 daily temperatures for the international airports at the cities shown in this chart, then plotted the cumulative sum of the mean temperature for the year.

SumTemperatureGuangzhou and Orlando had the same cumulative sum of temperature. Bentgrass wouldn't be a good choice in Orlando, and I don't think it is a good choice in Guangzhou either.

A better way to look at bentgrass suitability is to look at the low temperatures. If the low temperatures are too high, for too many days, bentgrass will be really difficult to manage, eventually becoming too much of a problem and one would be better off with a warm-season grass.

For 2014, here's the number of days with a low temperature greater than or equal to 22°C. I'd look at anything more than 60 days in a year above that level as being difficult for bent.

BarChartLowsThis way of evaluating the temperature fits pretty well how one expects bentgrass to perform in these locations. Perfect in Kunming, the "Spring City." Pretty good in Spain. A challenge in Shanghai summers, with some warm-season greens there also, but possible with good management. Not used in Orlando. And I wouldn't want to try it in Guangzhou.

For more about temperatures and bentgrass, see:

"I'd be applying potassium all the time": Part 3

One doesn't need to apply supplementary potassium (K) after a rain, as I wrote in part 1 of this series, because such applications will invariably lead to application of way more K than the grass can use. In part 2, I showed a calculator that makes an estimate of how much K is reasonable to apply as fertilizer, based on how much K the grass will use.

Looking at this with soil test data, these four charts show what happens to K in the soil over time.

In 2002, I applied nitrogen (N) and K every two weeks to L-93 creeping bentgrass maintained as a putting green in Ithaca, New York. I collected soil samples every eight weeks. This summarizes what happened during the summer of 2002.

At the start of the experiment, before applying any N or K, the Mehlich 3 K was 86 ppm, and the water extractable K was 8.3 ppm. I've added a horizontal line at each of those levels, to indicate what the starting level of soil K was in this experiment.

Plot1Then the treatments started, N and K every 14 days. When no K was applied, what happened? The soil K went down. That is to be expected, because the grass uses K, so when the grass is growing one expects the soil K to go down if no K fertilizer is added.

Plot2What happened when a moderate amount of K was added? Over these 16 weeks in the summer of 2002, I applied 12 g N m-2, and the K rate supplying 13 g K m-2 in that time is close to a 1:1 ratio. From June to July, the soil K went up at that rate, because that is slightly more K added as fertilizer than the grass can use. Then from July to September, the soil K in plots supplied with the 1:1 ratio went right back to where they started the summer. The reason for the decrease is discussed below.

Plot3What happens when the K applied is way more than the grass can use? The highest rate in the experiment supplied 50 g K m-2 over this time period, roughly a 1:4 ratio of N to K. And the soil test levels went up, because when one supplies a lot more K than the grass can use, that's what happens.

Plot4Why was the soil K higher in late July than in September? That is because the irrigation of this area was increased in August, and the rain + irrigation from the end of July to the time the samples were collected in September was double the evapotranspiration (ET).  From the start of the experiment until the samples were collected in late July, the rain + irrigation was just slightly higher than the ET.

The grass performance was good in all the plots, and equally good no matter if no K was applied, if a moderate rate of K was applied, or if the highest rate of K was applied.

There were six rain events with > 25 mm (> 1 inch) of rain during these 16 weeks. Adding K after rain would have accomplished nothing, other than supplying even more K than the grass would use, and supplying K that would mostly be leached out sometime in the future. By supplying the amount of K the grass uses, one can maintain a pretty stable level of soil K. Of course if the soil K is well above the MLSN guideline, then no K is needed at all, because the grass can get all the K it needs from the soil.

For more details about the experiment, see this paper from Soil Science.

"I'd be applying potassium all the time": Part 2

In Part 1, I explained that adding potassium (K) after every precipitation event of 25 mm (1 inch) or more at Minneapolis or Fukuoka would supply from 2 to 13 times more K than the grass could use. Since there is no benefit to adding more K than the grass can use, it doesn't seem that such post-precipitation applications are necessary.

How can one determine how much K the grass will use? This calculator does, predicting how much K is required as fertilizer, all while making sure plenty of K remains in the soil as a buffer even beyond the K that is used by the grass.

And now, this calculator is available in a Japanese version too.


"I'd be applying potassium all the time": Part 1

When I saw this photo captioned "a little potassium replenishment being applied," I asked "replenishment from what loss?" As I found out, the potassium (K) was being applied after a heavy rain event.

I don't think it is necessary to apply K after a heavy rainfall. Potassium fertilization can be made a lot simpler by ensuring the grass is supplied with enough K to meet all the grass requirements, and to ensure the soil stays above the MLSN guideline. The approach I advocate will ensure the grass has more than enough K, and will avoid unnecessary K applications.

I started writing a blog post about this, and mentioned to a superintendent what I was writing about -- adding K every time it rains more than 25 mm (1 inch). "If I did that, I'd be applying potassium all the time," he said.

Rain doesn't change the grass requirement for potassium (K). I'll start by making a generous estimate of how much K the grass may use in a year. I'm going to compare two grasses at two locations -- creeping bentgrass in Minneapolis, Minnesota; and korai (Zoysia matrella) in Fukuoka, Japan.

Based on the average temperatures in Minneapolis, I estimate that a creeping bentgrass putting green, at the growth rates usual for this era, may use about 13 g N m-2 y-1 (2.6 lbs N/1000 ft2/year). The K use of bentgrass is half the nitrogen use, so if the grass uses that much N, the K use (requirement) would be about 6.5 g K m-2 y-1 (1.3 lbs K/1000 ft2/year). I don't think this is necessary, but for the purpose of these calculations I'll go ahead and round up the expected K use to 10 g and 2 pounds, just to make sure the grass gets supplied with a little more K than it will use. For every 1 gram of N I apply (or every 1 pound of N), I can apply 0.8 grams (or 0.8 pounds) of K. I would want to make sure the soil is above the MLSN guideline for K -- 37 ppm -- and otherwise I can disregard the soil, because that fertilizer ratio will be supplying more K than the grass can use.

Making these calculations for korai in Fukuoka, the N estimate is 11 g N m-2 y-1 (2.2 lbs N/1000 ft2/year). Korai uses N and K in a 3:2 ratio, so the estimate of the K requirement for korai in that climate is 7.3 g K m-2 y-1 (1.5 lbs K/1000 ft2/year). I'll round this up to the same annual overestimate as at Minneapolis -- 10 g or 2 pounds.

At both locations, I'm estimating that turf maintained as a putting green will use slightly less than 10 g K m-2 y-1 (2 lbs K/1000 ft2/year), so applying that much K will be supplying more than the grass will use. Whether it rains 25 mm (1 inch) or not, that rain is not changing how much K the grass will use. And I'm planning to supply that K whether it rains or not.

So what happens if I add in "potassium replenishment" after every day with precipitation greater than 25 mm at these two locations? I looked up the rainfall for Minneapolis (at the MSP airport station) and for Fukuoka for the past 5 years, and I counted up the number of days with precipitation greater than 25 mm. The counts for 2015 are through October 4.

5yearPrecipAveraged over the past five years, that is 6 days a year at Minneapolis and 19 days a year in Fukuoka. I'm not sure what the "replenishment" rate was. Let's calculate for two possible rates -- 2.5 g K m-2 and 5 g K m-2 (0.5 and 1 lb K/1000 ft2).

In an average year at Minneapolis, adding a half pound of K after each 1 inch rain event would add an additional 3 pounds of K per 1000 ft2; at the 1 pound rate it would be 6 pounds of K. And in a place with rain like Fukuoka, it would be an additional 48 g or 96 g K m-2 (9.5 or 19 lbs K/1000 ft2).

When comparing these amounts of K to the amount the grass will use, it is apparent that the quantity added as a replenishment is, depending on rainfall, from 2 to 13 times more than the grass can use. One wouldn't think of overapplying N to such a degree, or overapplying water to that extent. Since there is no evidence that overapplying K provides any benefit to the turf, I would not worry about adding K after heavy rain.

The eloquent Edwin Roald on Bogey Nights

This is a conversation I really enjoyed, and I think you will too. Edwin has some great ideas about matching the time window of a round of golf to the way we live today. And the implications of this are many -- time, cost, resources, quality, and land use are all naturally influenced by what he has to say.


This fascinating conversation is less than 15 minutes. Have a listen here: Edwin Roald on Bogey Nights, or visit for more information.

Another note of interest is this. Jason McKenzie, one of the hosts of the Bogey Nights show, worked in golf course maintenance during high school before going on to play golf at Mississippi State University. There is a lot of talent and knowledge on this radio show, and I'm glad I had a chance to listen.

The data prove me wrong

I was listening to Frank Rossi talking with Bert McCarty on the TurfNet Radio Network. It was an interesting conversation, about lots of things including ultradwarf bermuda, pigments, and heat stress on bentgrass.

One thing struck me when they were talking about ultradwarf bermudagrass, and Rossi mentioned that "light levels change, maybe temperatures change, but certainly light levels is a driving force" in the slowdown in growth at the end of summer. McCarty confirmed that "light starts it off" as the "grass starts to slow up" even though it is still hot.

I wondered if that was right. It seemed to me that temperatures would go down to slow growth before light would. Well, I looked up some data, and they were absolutely right.

Here's the average monthly temperature for the last 10 years at McClellanville, South Carolina. Temperatures for 2015 are included to October 4. Looks like July and August are the hottest months, no surprise, and I see a drop going to September.

TemperatureThen I looked up the global solar radiation, converted to photosynthetically active radiation expressed as a daily light integral, and calculated the monthly averages. Again, 2015 data are just up to October 4. The DLI peaks in June and then looks like a steady decrease to December.

DliOk, I had the temperature and the light, but how to see which one is dropping off at a different time? To do that, I looked at the change from month to month. I calculated something called the log percent (L%), which is $100(log_{e}(y/x))$. In this case I let $y$ be the value for the month, and $x$ be the value for the previous month. This calculation gives a symmetric, additive, and normed measure of the relative change in light, and of the relative change in temperature.

I didn't compare December to January, so there is no L% for January. That's alright, because I was most interested in what happened at the end of summer, specifically in August and September. The values on this chart for February represent the L% change from January, March shows the change from February, and so on.

LogPercentIt is really clear. Dr. McCarty was exactly right.

In July the temperature is still increasing from the previous month, and light is decreasing from its peak in June.

Then in August there is a big drop in light, and on average no change in temperature. And even in September, the relative drop in light is higher than is the relative drop in temperature. It is not until October and November that the temperature decrease is more than the light decrease.

It's good to know this!