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November 2014

Estimating the total root length in an average lawn

I read a seemingly unbelievable “fact”, that the average lawn has enough roots to equal 15 round trips between the sun and earth.

Healthy turfgrass will have a well-developed and fibrous root system, but 15 round trips, really? I’m taking that “fact” to mean that the length of all the roots in an average lawn will equal 30 times the average distance between the sun and the earth.

This is something that is pretty simple to estimate.

The average distance between the sun and the earth is 1 astronomical unit, which is about 150,000,000 km. And the average lawn is one-fith of an acre.

# 1 astronomical unit expressed in km
earth_sun_distance_km <- 149597870700 / 1000
# 15 round trips
earth_sun_15_round_trip_km <- earth_sun_distance_km * 15 * 2
# one acre in square meters
one_acre_m2 <- 4047
# average lawn size, 1/5 acre
average_lawn <- one_acre_m2 * 0.2

Now how many roots may there be in an average_lawn? I’ve used root length density (RLD) data from Schlossberg and Karnok (2001) and from Jordan et al. (2003) to get an estimate of the length of roots in a given soil volume for creeping bentgrass. Based on those data, I’m using in these calculations a RLD of 24 cm of roots per 1 cm3 of soil.

I’ll assume that in this average_lawn, the roots go to a depth of 40 cm, and have a RLD of 24 cm cm-3. The total root length in the average_lawn will then be the RLD times the soil volume.

# RLD of 24 cm/cm3
root_length_density <- 24
# volume of lawn rootzone in cm3, to depth of 40 cm
average_lawn_soil_volume_cm3 <- average_lawn * 100 * 100 * 40
# avg lawn root length in cm
total_root_length_cm <- root_length_density * average_lawn_soil_volume_cm3
# avg lawn root length in km
total_root_length_km <- total_root_length_cm * 10^-5

The distance of 15 round trips between the sun and the earth is 4.487936110^{9} km. And in the average_lawn described above, the total root length is 7.7702410^{4} km. That’s not close at all. The distance of 15 round trips between the sun and earth is 5.775801210^{4} times more than the total root length of the average_lawn.

The roots do make an impressive length however.

# circumference of the earth
earth_circumference_km <- 40075
# distance from the earth to the moon
earth_moon_distance_km <- 384400

The roots in an average_lawn with RLD of 24 and root depth of 40 cm have a total length 1.9389245 times the circumference of the earth and 0.2021394 of the distance from the earth to the moon. Pretty impressive, but not even close to a single trip to the sun, let alone 15 round trips.

Where are the Global Soil Survey Samples from?

Online Turf and Turfgrass Research Super Journal - PACE TurfYou've heard about the Global Soil Survey, and maybe you've joined this project. If so, thank you very much! You are contributing some incredibly useful data that is being used to improve turfgrass fertilization around the world. And you are also getting soil test data which you can use for your site.

Have you wondered where these samples are coming from? We anonymize the data, of course, so one can't tell which data are from which site. The list of participants so far is here, and it is a broad range, from high-budget private to 9-hole public courses, from major championship venues to military facilities, and pretty much every type of course in-between. 

To join this exciting citizen science project, purchase your Global Soil Survey kit here. The data from the soil at your site, the soil in which the turfgrass is performing the best, will be used to improve and refine soil nutrient guidelines for turfgrass all around the world. Plus you get soil test data!

For more about this project and related work, see:

Questions from a correspondent

Regarding potassium: from the info you have sent the minimum ppm for K should be 35. Is this a happy medium for most turf species?

I would not describe it as a happy medium. I would say that the current MLSN guideline for K of 37 ppm is the number that we want to keep the soil above, for all turf species. One should make fertilizer applications that will ensure the soil K does not drop below 37 ppm. And that is a number that works well for all turf species. Keep in mind that when there is 37 ppm K in the soil, that is about 5 g K/m2 in the top 10 cm. That amount, the MLSN guideline level, is like a buffer of K, one that will never be accessed by the roots, because the fertilizer applications will be made to ensure that 37 ppm never needs to be used. Of course, if the soil K is high, then the grass may be able to obtain all the K it can use from the soil, and no K will be required as fertilizer. You can check this, and see how the calculations work, with this Shiny App K calculator.

Would you agree that more K than is necessary may actually cause disease not to mention wasted money, leaching etc.?

Adding 2 or 3 or 4 more times K than the grass can use won’t cause disease, but such rates of K do have some association with increased susceptibility to certain diseases. If the grass doesn’t have enough K, there can also be increased susceptibility to disease. The important thing is to make sure the grass is supplied with just the amount of K it can use, and that can be done by making sure the soil is kept above the MLSN guideline. I would be less concerned about disease, and more concerned about wasted money when applying K that the grass can’t use and the soil won’t hold.

Do you ever see much response in colour, density, leaf turgidity etc from K apps because I have ever rarely seen a response from it in the field?

It would be rare to see a response to K addition in the field. If one keeps the soil above 37 ppm, I don’t think a response will be seen.

Another five articles every greenkeeper should read

ReadingThese five articles are ones that I refer to and recommend repeatedly. This list is a continuation of the first and second sets of articles in this series.

Testing products and practices: a guide for golf course superintendents, by PACE Turf, describes a method "to determine which products and management practices work best on your own golf course."

Just what the grass requires, by Micah Woods, Larry Stowell, and Wendy Gelernter, explains how to use the MLSN guidelines to ensure the grass is supplied with all of each macronutrient and secondary nutrient that it can use, all while keeping an additional amount (the MLSN guideline amount) as a reserve in the soil.

Manipulating creeping bentgrass nutrition, by Wayne Kussow, describes what happens when different elements are applied. And it contains this choice quote: "How many more times do I have to say that applying nutrients to turfgrass growing on soil already well supplied with the nutrients is a waste of time and money?"

Evidence, regulation, and consequences of nitrogen-driven nutrient demand by turfgrass, by Wayne Kussow, Doug Soldat, Bill Kreuser, and Steven Houlihan, is a little bit technical, but very informative in describing experiments with creeping bentgrass and with kentucky bluegrass. It shows how increasing rates of N increase yield, and also how that increases the demand for other nutrients. In the conclusions, they share these three characteristics of plant growth-driven nutrient demand, after first showing how nitrogen controls growth:

1. Nutrient uptake and tissue content are more closely related to growth rates than external nutrient supply

2. Nutrient uptake at a given level of external supply varies substantially in response to variable nutrient demand.

3. Tissue nutrient content tends to remain constant once external nutrient supplies allow plants to satisy their demand.

What's the ideal fertilizer ratio for turfgrass? by Bill Kreuser, gives an overview of turfgrass nutrient uptake and demand, explains how plant use is related to soil test levels, and suggests which fertilizer N-P-K ratios may be best for particular turfgrass situations.

Previously, I described Five articles every greenkeeper should read and Five more articles every greenkeeper should read.

A paper packed with data about N, P, and K

I've always liked this paper. It is about N, P, and K applied or withheld to kentucky bluegrass (Poa pratensis) grown in a loam soil and in a USGA sand rootzone. I was reviewing it recently in advance of an upcoming presentation about turfgrass nutrient use. As turfgrass papers go, this one is especially full of useful data. And it is open access, so you can download and read the full paper.


Here are a few highlights from this paper.

  • At the start of the experiment, the loam had P and K of 68 and 63 ppm, respectively, by the Mehlich 3 extractant. The sand had P and K of 58 and 39 ppm. 
  • "Added P had no significant effect on clipping yield and underground turf biomass in both sites."
  • "Despite the low initial soil K levels ... , clipping yield and underground turf biomass showed no significant response to K addition in both sites."
  • "Potassium showed no significant main effect on shoot density or foliage colour."
  • I suggest having a look at Figure 2 in the paper, which shows how N rate has a large effect on turf shoot density and turf color, but adding more P and K has negligible effects.

Note that the K in the control plot in the sand would have been below the MLSN guideline for K very early in the study, and yet the addition of K fertilizer had no effect, except to decrease the color of the turf at the highest N application rate. This is another indication that the MLSN guideline for K is not an absolute minimum, but it is a level that if one stays above, there is a high level of assurance that turf quality can be optimized, with no risk of K deficiency.

Turfgrass Mystery: the curious case of the Oregon Obtrusion

Engelke and Woods on zoysiagrass test plots in Oregon.

I was in Oregon last month, and I had the great opportunity to spend a few hours with Dr. Milt Engelke, studying and discussing grass in general, and Zoysia grasses specifically.

You might not expect it, but most cultivars of manilagrass (Zoysia matrella), and all cultivars of japanese lawngrass (Zoysia japonica), can survive in Oregon's Willamette Valley. In fact, the grasses I saw on my visit, in mid-October, looked really good.

The place, and the season, will give some hints about the correct solution to this mystery.

The location is Oregon's Willamette Valley.

The season is autumn, in this case, mid-October.

Manilagrass and japanese lawngrass growing in Oregon's Willamette Valley.

The grass species are primarily Zoysia japonica, Zoysia matrella, and some interspecific hybrids.

And this is the mystery.

The curious case of the Oregon Obtrusion. Can you identify the cause of this patch?

Dr. Engelke and I were looking at variety after variety, and then we came to a plot of Zoysia japonica with this patch on it. Keeping in mind the species, season, and location, can you solve this mystery? What is the cause of the patch on the zoysia?

Update: a closer look at this mysterious blemish.

I thought this might be mistaken for the beginning stages of large patch, considering it was the onset of cooler autumn temperatures and the species is Zoysia japonica. However, the uniformity of the patch and the clear circular shape were a strong indication that this was manmade.

Here are a series of answers.

This is what that sprinkler looks like. The case makes it easy to drag the pipe across the field without damaging the impact sprinkler.


Everyone knows zoysia grows slower than bermuda, except when ...

... it doesn't. In fact, there is one variety of manilagrass (Zoysia matrella) that consistently grows faster than bermudagrass in Southeast Asia. I've had occasion to study and measure this grass, and there is no doubt that it grows faster than bermuda here.

Manilagrass fairway on the Player Course at Orchard Golf and CC in the Philippines.

Some recent measurements have provided a bit more information. The grass I refer to is nuannoi, the manilagrass variety that is grown on nurseries in Thailand, that I've seen growing as far east as the Philippines, south to Singapore and Bali, and west to Dhaka. 

Many golf courses in Thailand were planted to bermudagrass when they were constructed in the 1990s and through a natural conversion process, the nuannoi has taken over. Courses such as Windmill, Green Valley, Thana City, and Phoenix Gold have all had this happen.

Banyan GC in Hua Hin, Thailand was planted to this grass at the time of construction.

I've measured how fast this grass spreads, both when it is invading bermudagrass, and when it is establishing on bare ground without competition from other grasses. Here are four separate measurements.

1. At the ATC research facility

Different species and varieties of warm-season grass at the ATC research facility in 2008.

More than 50 varieties of grass from various species were grown at the ATC research facility near Bangkok from 2006 until 2009. One of the interesting observations was just how quickly nuannoi manilagrass grew into Tifway 419 bermudagrass. Remember, people keep telling me that bermuda grows faster, and zoysia grows slower, but as you can see here, the invasion was only happening in one direction. Which grass is growing faster?

The yellow pen marks the farthest extent of manilagrass invasion into a plot of bermudagrass.

From the plot border, to the farthest point of invasion, was 2 meters at this point, and the grass had been planted for 2 years. This is a rate of 1 meter per year, or 8 cm per month.

2. In Samui

The Santiburi Samui CC opened at the end of 2003. It was planted to seashore paspalum, except for the Tifeagle greens.

The 2nd shot at Santiburi Samui in January 2004, when the fairways were still seashore paspalum.

Most of the seashore paspalum on the fairways and roughs died during a drought in 2005. Where the paspalum died, bermudagrass took its place. I first noticed a few small patch of nuannoi on fairways at Santiburi Samui in January 2007. Since then, the nuannoi has continued to expand into the bermuda, with the largest patches now having an area of about 140 m2.

Large patches of nuannoi on the Santiburi Samui fairways in May 2014.

Assuming that the size was one small plant in January 2007, and the patch of manilagrass is now 140 m2 in size, gives an expansion rate of 7 cm per month. 

Based on the measurements at the ATC research facility and at Samui, the rate of expansion into established bermudagrass was 7 and 8 cm per month, respectively. This expansion rate is useful for doing planned fairway conversions, both to know how long it will take for the nuannoi to take over, and to calculate an appropriate spacing for planting.

3. In a greenhouse experiment

I've shared some results from a greenhouse experiment conducted last year in Thailand, in which nuannoi manilagrass, Tifway 419 bermudagrass, and Salam seashore paspalum were established from stolons in a sand rootzone and the growth and the nutrient uptake were measured. After the grass was already grown in, during the duration of the fertilizer and clipping yield experiment, the nuannoi manilagrass had 52% more clipping yield than did the Tifway 419 bermudagrass.

But before that, it grew in faster from stolons also.

42 days
Nuannoi manilagrass 42 days after planting by placing 5 stolons in the pot.
Nuannoi manilagrass 48 days after planting by 5 stolons in each pot.

The manilagrass had grown in after 42 days (6 weeks). The planting rate of the stolons was 144 g/m2

4. At a sod farm

I've explained the process of manilagrass production on sod farms near Bangkok in this video, where the grass goes from planting to harvest in about 6 weeks.

Of testing methods, Mehlich 3 as an extractant, and “bespoke testing”

Mario Ernst wrote with this interesting question:

I have been in contact with a lab in the UK for the past few days discussing the suitability and possible limitations of Mehlich 3 with regard to MLSN.

Below is an email I received today and I am very interested in your thoughts on this. If you have time you could post your view on this on your blog as I think this could be of interest to anybody thinking about using the MLSN guidelines.

Here it is: 

[from the laboratory]

Hello Mario,

Yes, you are absolutely right in your thinking that Mehlich III (M3) has gained popularity recently due to its ability to provide information on a great number of nutrients with a single extraction. Therefore, its costs are low and turnaround rapid.

However, it does have, in my opinion, some drawbacks.
Firstly, routinely only 2ml of soil is used. A very small amount, i am sure you'll agree. This 2ml, therefore, has to representative of all present nutrition. Be it a major, secondary or trace nutrient.

Secondly, M3 will over extract cation content compared to traditional exchangeable cation techniques. Especially calcium and magnesium on calcareous soils. This explains the lower guidelines in the MLSN system. It's not wholely due to a change of agronomic thinking but a reflection of M3's more aggressive nature.

Thirdly, as M3 is a weakly acidic extraction, it can quickly be neutralised by calcareous soils resulting in its ability to extract trace nutrients plateauing. I routinely see very low metals contents on calcareous soils tested by M3.

Having said all that, M3 most certainly has its place in modern agronomy and we do perform this test upon specific request. But, in addition to my observations above, as with any system, it is only as good as the guideline system employed for result interpretation and the trials work conducted to produce those guidelines.

This is why our preferred option, and that of all our major customers, remains to perform bespoke testing as per the details in our methods statement. 

Those are interesting points, and ones that I do have some comments on.

a. Mehlich 3 is the extractant with the most widespread use in the United States. For the development of the MLSN guidelines, we didn’t choose Mehlich 3 because we think it is the best extractant. Rather, we chose it because it is used at soil testing laboratories across the United States, including at some of the labs with the highest volumes of turf samples, and it is as much of a “standard” universal extractant as there is today. With Mehlich 3, we have access to a lot of data, and the guidelines produced for Mehlich 3 extractions will have the broadest application at the greatest number of turfgrass sites.

Because Mehlich 3 is so widely used, there are equations available to estimate what results would be on other common tests, and vice versa. I will share some of that research on this blog in the future.

I would prefer that all turfgrass soils were tested with 0.01 M SrCl2, which I think has a number of advantages over Mehlich 3 as a universal extractant for turfgrass soils. But since that is not realistic at this point, I do a lot of work with Mehlich 3 data.

b. Yes, the standard procedure for Mehlich 3 and for many other laboratory is to use a small volume or mass of soil, usually 2.5 cm3 soil or sometimes 2 g of soil. I don’t see that this is a problem. Soil samples submitted to a laboratory are dried and ground to pass a sieve. The sample to be analyzed is drawn from the prepared soil, and that sample is representative of the larger sample. I’ve not had a problem with this. For example, I applied K to a research putting green and then measured the amount of K in the soil. The amount measured by soil tests, even though the test analyzes just a small amount of soil, is representative of the amount actually in the soil.

c. Mehlich 3 is never a suitable extractant for calcium in calcareous soils, because the acidic extractant dissolves calcium carbonate. And depending on the presence or not of magnesium carbonate in the soil, Mehlich 3 may or may not be suitable. But this doesn’t really matter, because calcium won’t ever be required as fertilizer in a calcareous soil. One doesn’t need to test for it. So the erroneous calcium data can be thrown away. And the same goes for magnesium. If there is magnesium carbonate present, and the extractant dissolves some of it and gives an erroneous result for magnesium, it doesn’t matter because magnesium was not required as fertilizer in that situation.

d. As for why the MLSN guidelines are lower than conventional guidelines, it isn’t because of Mehlich 3 over-extracting cations compared to other extraction methods. In fact, the MLSN guidelines calculated from Mehlich 3 data are higher than what one would get if the same soils were analyzed with extractants that removed fewer cations than Mehlich 3.

e. The Mehlich 3 extractant starts at a pH of 2.5 and after mixing with soil the pH will change, and unless one measures it, one won’t know what the pH of the extraction was. That is one of the reasons I prefer 0.01 M SrCl2 — because the extractant adjusts to soil pH and we know that the extraction was done at the same pH as the bulk soil pH.

f. As far as micronutrients with Mehlich 3 in calcareous soils, I have a few thoughts on this. First, we have deliberately avoided micronutrients in the MLSN guidelines, because we have not studied this in detail. We may study this in the future and propose some MLSN guidelines for micronutrients.

There may be cases when Mehlich 3 is unsuitable for micronutrients. But micronutrient testing of soils is fraught with problems anyway. And turfgrass micronutrient deficiencies are so rare as to be almost unseen. In the case of moderate to high pH soils, one would generally apply micronutrients anyway, to make sure there is availability, and would not consult soil test results. In fact, I rarely look at micronutrient results, and I’m not aware of good guideline levels for turfgrass no matter what extraction method is used. There has not been much research on this topic. So I am not defending the micronutrient data from Mehlich 3, but I am saying that I don’t think they are especially useful as decision-making criteria for turfgrass managers making fertilizer decisions, nor are micronutrient data from any other method.

g. Regarding “it is only as good as the guideline system employed for result interpretation and the trials work conducted to produce those guidelines.” I agree completely, and I think that is the most important thing to consider when doing testing. Just saying that, however, doesn’t convince me that the lab in the UK has a better guideline system for result interpretation than the MLSN guidelines. As I’ve explained, conventional guidelines are high, broken, and a main reason for development of the MLSN guidelines was to have a set of guidelines that is representative of the soils in which turfgrass is grown today.

h. Bespoke testing with a number of different tests depending on the element or the soil would have some advantages. But again, it comes down to the guideline system used. With the MLSN guidelines, we are working to be open about what data we are working with and how we generate the guidelines. For those who prefer to use other guidelines, I would like to see why those levels are what they are, and just how extensive and convincing is the trials work done to justify those guidelines.

From what I’ve learned about conventional guidelines, and this comes right from the textbook, they were in many cases set artificially high, based not on agronomic research, but because it was considered that cost of fertilizer was not important in turfgrass management.

"Think of the soil as a nutrient bank"

Bill Kreuser has written an interesting article about the ideal fertilizer ratio for turfgrass. You will want to read the entire article, which discusses turfgrass nutrient demand, nutrient uptake, soil nutrients, and suggested nutrient ratios for various turf situations. Here are a few highlights:

We first need to recognize that nutrient uptake is controlled by plant nutrient demand and not fertilizer applications.

Fertilizing with P and K is a waste of resources when the soil test reports indicate nutrient levels are already adequate.

Think of the soil as a nutrient bank. When fertilization exceeds plant nutrient demand and other mechanisms of nutrient loss (leaching, denitrification, fixation) then soil test nutrient levels increase. Likewise, soil test nutrient levels will decline when plant uptake and nutrient loss exceed fertilization.

Uptake of all soil nutrients is dependent on turfgrass growth rate ... Since turfgrass is chronically N deficient, N fertilization promotes leaf growth and increases demand for other nutrients.

I encourage you to compare your Mehlich-3 soil test results with the Minimum Levels for Sustainable Nutrition (MLSN) guidelines generated by PACE Turf and the Asian Turfgrass Center. Briefly, if soil test P is much greater than 21 ppm and K is much greater than 37, then there is little reason to apply anything other than straight N.

Try to use the diversity of fertilizer ratios to your advantage.