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September 2014

Why is the grass so good, but the soil test results so bad?

I've been asked this question more times than I can remember. It usually goes something like this. 

"The grass at this golf/park/lawn/sports facility is performing really well. But we had the soil tested, and it turns out we are quite low in P/K/Ca/Mg. How can that be? And do we really need to add that element?"

The answer is, maybe it is not really low, and you may not need to add it. The reason is simple. Conventional soil testing guidelines for turfgrass, which ideally would be developed from calibration studies across multiple soil types that measure turfgrass response to different application levels of an element, have not really been developed that way. 

This comic identifies the problem and explains a solution. 

Here is a bit more information on this important topic.

Conventional soil nutrient guidelines for turfgrass appear to be, in some cases, based on analyzing a large number of samples from home lawns, which tend to be on relatively high nutrient content soil, and then choosing a value above the median of those results, and identifying that as a target level. The problems with that approach, in trying to optimize turf performance, nutrient use efficiency, or manage turf in sand rootzones, or sand topdressed rootzones, are obvious.

When I speak of conventional guidelines, I am generally referring to those given in Clarifying Soil Testing: Part 3 by Carrow et al.

In the definitive textbook on this subject, Turfgrass Soil Fertility and Chemical Problems, by Carrow et al., they explained the situation with turf and soil test interpretation:

More research is needed on soil test calibration for turfgrasses. Such research should include wide ranges of soil types, and should be conducted using various turfgrass species and cultivars.

In some cases, turfgrasses have been placed in a "high" P and K requirement category, while pasture grasses were in a "low" category. This decision was based on economics, not agronomics. The cost of fertilization was not considered of primary importance for turf. Considering current fertilizer prices, scarcity of some fertilizer materials, and the environmental concerns associated with fertilization and soil nutrients, it seems quite appropriate to utilize fertilizer recommendations based on sound research.

The MLSN and Global Soil Survey projects are an attempt to do just that. For new soil nutrient guidelines based on the latest research and a careful evalulation of turf and soil at thousands of locations, see the MLSN guidelines for turfgrass. 

And recognizing that it is impossible to do calibration studies for fertilizer on all turfgrass species and soil types, we address this problem in a different way, using the Global Soil Survey (join here) to identify, refine, and verify the quantity of nutrients required to produce high quality turf.

With more awareness of this problem, and the simple solution, I'm optimistic that I'll be asked that question about good turf and "low" soil nutrient levels a lot less. If you think you have that problem, please have a look at the MLSN guidelines and see if your good turf would still be classified as low. For a guide to calculating fertilizer requirements using the MLSN guidelines, see the GCM article, Just What the Grass Requires.

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.

Conventional nutrient guidelines: explaining what I mean by "broken"

With all the recent discussion of the newly updated MLSN guidelines and the first year report of the Global Soil Survey, I can imagine that some are wondering, what is all the fuss about? Why is someone trying to push and explain new guidelines?

The reason is, and this is no secret among turfgrass scientists – there isn't a lot of good justification to support what we call conventional guidelines for soil nutrients in turfgrass. And by that, I mean the conventional guidelines are broken. 

I'll explain it by taking a quick look at data from the first year of the Global Soil Survey. But first, you might like to have a look at these for some background.

  • In this 2013 GCSAA webcast on putting green nutrient use and requirements, I explained that conventional guidelines are broken because they "tend to recommend fertilizer additions when we do not see any benefit to the turfgrass."
  • In this analysis of soil test potassium, and a comparison of the soil test levels to the actual amount of potassium fertilizer required to increase the soil to the guideline recommendation, it became apparent that "conventional soil test guidelines are sometimes just impossible to reach."
  • The logic of this approach is summed up by Wayne Kussow, who wrote that "applying nutrients to turfgrass growing on soil already well supplied with the nutrients is a waste of time and money."
  • Last year Doug Soldat wrote about common soil testing mistakes and how to avoid them. He discussed costly calibration studies that can improve fertilizer recommendations based on soil test interpretation. What happens with these experiments? Let's hear it from Doug: "It is also worth noting that every time a researcher conducts one of these studies, they tend to find that the levels required are lower than what we previously thought – meaning that 'low potassium' you got on your last soil test report might be optimum down the road."

I hope that makes it clear. Conventional guidelines can be impossible to reach, turf performs at a high level in soils with nutrient levels less than the conventional guidelines, and new research continues to show that conventional guidelines are too high. That's why ATC, together with PACE Turf, developed the MLSN guidelines and are now conducting the Global Soil Survey.

For the conventional guidelines, I'm using the middle of the medium sufficiency range as given by Carrow et al. in this article from the Clarifying Soil Testing series in GCM. That is, if the range given, for example for calcium, was 500 to 750 ppm, then I take the middle of that range at 625 and say that will be our "conventional" guideline.

I took the data from the first year of the Global Soil Survey, from turfgrass sites from around the world, all in which the turf was performing well. Clearly, these soils contain enough nutrients to produce high quality turf. The soils were not deficient. And how do the soil nutrient levels in the surveyed soils (n = 84) compare to the "conventional" guideline?

In these dot plots, each soil sample from the Global Soil Survey is a single black dot, aligned with whatever the soil test level was for that sample. And the red line marks the "conventional" guideline for that element.



For potassium, 82% of the samples in the survey so far have been below the conventional guideline of 113 ppm.

PWith phosphorus, it is not so bad. Only 32% of the samples are below the conventional guideline of 40 ppm.

CaFor calcium, a full 46% of the tested samples are below the conventional guideline of 625 ppm. 

You can see where this leads. When the guidelines are so high, then fertilizer will tend to be recommended. And yet the turf is performing well at levels both above and below the guideline level.

Mg55% of the samples had magnesium below the conventional guideline of 100 ppm.

And for sulfur, 88% of the soils in the Global Soil Survey database at this time are below the conventional guideline of 27.5 ppm. 

After looking at those results, we can reach one of two conclusions. First, we could say that the conventional guidelines are correct, and the soils are dangerously low in those nutrients, and we must add those nutrients. Remember, however, that these soil samples were specially selected from the locations where turf was performing well. So it seems to me that the second conclusion is the one more likely to be true. And that second possible conclusion is, the conventional guidelines are set at a level higher than is necessary to produce good turf.

This disparity between the actual nutrient levels in soils on turfgrass sites around the world, compared to the conventional guidelines, is the primary reason for our development of the new MLSN guidelines.

Technical details of the Global Soil Survey report, including the entire year 1 dataset

The Global Soil Survey is a citizen science project. We are working with turfgrass managers from around the world to collect and analyze soil samples from good performing turf. Through an analysis of those soils, we are able to develop new and improved soil nutrient guidelines for turfgrass that are useful for the entire industry.

The ethos of this project is openness. We share the data that have been collected, and we share the code used to generate the guidelines.

Not everyone will be interested in all the details. But if you want to do your own project with these data, or compare our data to your data, or check for errors in our code, or just see what we've done to make such a pretty report, or find a way to improve on what we have done so far, all the files are in this GitHub repository. With these files, and the right software on your computer, you will be able to reproduce (and check) our analyses and the entire report. Or you can just have a look at the data in your web browser. Or do something we haven't imagined with this. It's open.


The first year Global Soil Survey report is now available

When I spoke with Kyle Brown on the Superintendent Radio Network, I mentioned that I had been working on an analysis of the first year data for the Global Soil Survey. That report is now completed and is available for download from the PACE Turf website.


The report includes a summary of the first year results, a map of the sample locations, and a description of just how the data are used in the calculation of a nutrient guideline.

You'll need to read the report to get the full description and see the charts that go step-by-step through the process. Briefly, we take a the soil test results for a particular element. That is a series of numbers. Then we identify a probability distribution that reasonably approximates the test results for that element.

In the Global Soil Survey, all samples are selected from turf that is performing well at the time of sample collection. Because of that, we can infer that the soil was NOT deficient in that element at the time of sampling. So whatever the nutrient level was in the soil, it was enough to produce good turf. All of the samples in the dataset, then, are sufficient in the element, based on the performance of the turf at the time of sample collection.

We combine these two things, the probability distribution that fits the data, and the good-performing turf at the time of sample collection, to identify a level in the soil at which we want to be sure to keep the soil at or above to avoid the risk of deficiency. That is the 0.1 level, the level at which 10% of the samples in the model will fall below. Even though turf can perform well at those levels below the 0.1 level that we select for the guideline, we consider this a way to be conservative and to keep a safety buffer of nutrients above the level at which a real deficiency would occur. Even with this conservative buffer, the guidelines developed using this approach, which involves analysis of good-performing turf from sites all over the world, produces soil nutrient guidelines that are considerably lower than conventional guidelines.

Updated MLSN: new turfgrass soil nutrient guidelines

The MLSN guidelines have been updated for the first time since their release in 2012.


Compared to the previous version of the guidelines, the new MLSN guidelines for potassium and phosphorus are higher, and the new guidelines for calcium, magnesium, and sulfur are lower. 

Why did the guidelines change? Because the design of this project includes regular updating of the guidelines as new data are added. These guidelines are not static. They are designed to be updated to provide the most accurate assessment of soil nutrient levels, based on the way turfgrass is managed today. The current version of the MLSN guidelines includes data from soil test results in the PACE Turf, ATC, and Global Soil Survey datasets.

For more detail, see:

Turfgrass ecology, part 2: abandoned turf in Thailand

In the southern Tohoku region of Japan, ceasing maintenance of creeping bentgrass leads to grass death and almost complete invasion by weeds. Manilagrass (Zoysia matrella) stays alive for at least 18 months with no maintenance, and has minimal weed invasion. Japanese lawngrass (Zoysia japonica) also stays alive, but has more weed invasion than seen on manilagrass.

What happens with manilagrass and other grasses like seashore paspalum (Paspalum vaginatum) and hybrid bermudagrass (Cynodon dactylon x C. transvaalensis) when maintenance is stopped in the tropical conditions of Thailand? And do observations of what happens when there is no maintenance have some implication on what the maintenance requirements may be for those grasses? 

The Asian Turfgrass Center research facility north of Bangkok in 2008. The grass was maintained at this facility from 2006 until April 2009.

At the TT Tour in January 2008, we studied various grasses at the research facility. The grass immediately surrounding the paved area is manilagrass, and adjacent to the sala with the red tile roof is centipedegrass (Eremochloa ophiuroides).

Manilagrass at the edge of the potted grass nursery at the ATC research facility in January 2008.

In April 2009, irrigation was stopped at the research facility, and mowing was stopped from October 2009. So what happened to this manilagrass by November 2010, 19 months after the last irrigation, and 13 months after the last mowing? This is in Thailand, where the temperature is always warm, and the grass (and weeds) have the potential to grow all 12 months of the year. 

The next photo shows the same area as the previous photo, but at a different angle; the paved area is now at left (and covered by weeds); the manilagrass that people are standing on in the above photo is in the center of the photo below.

Manilagrass remains alive and free of weeds after 19 months with no irrigation and 13 months of no mowing. The centipidedegrass beside the sala has been overrun by weeds in the same time.

 How about bermudagrass and seashore paspalum?

Mowing was continued, but irrigation had been withheld for 5 months. Seashore paspalum is in the foreground, and bermudagrass is in the background.

The rainy season in this part of Thailand goes from late May until the end of October. The photo above was taken in mid-September 2009. Irrigation was stopped in April. Even though the grass was growing through the rainy season, the seashore paspalum in the foreground has almost all died without supplemental irrigation in only 5 months. The bermudagrass in the background remains alive, as does the surrounding manilagrass.

This next photo is taken from the same plot of mostly dead seashore paspalum, but turned to a different angle to show manilagrass in the background. The manilagrass, of course, remains alive with the natural rainfall and no supplemental irrigation.

After 5 months without supplemental irrigation, the seashore paspalum in the foreground is almost all dead. The manilagrass in the background remains alive.

Those photos showed the seashore paspalum after 5 months without irrigation. What happens after 19 months of no irrigation and 13 months with no mowing? In that case, the seashore paspalum has all died.

The plot of seashore paspalum 19 months after maintenance was stopped. It has now all disappeared and some weeds and other grasses are invading.

Seeing what happens when maintenance is withheld gives some indication of how much maintenance (irrigation, fertilizer, pesticides, mowing) are required when a species actually is maintained. Under the conditions of central Thailand, one can make some general observations based on this comparison of unmaintained grasses.

  • manilagrass seems to require only mowing to persist as a turfgrass and is the most resistant to weed invasion
  • seashore paspalum dies without supplemental irrigation
  • bermudagrass does not die without supplemental irrigation but will eventually be invaded by weeds if not maintained intensively

These observations of manilagrass are very similar to what was seen in the photos from Japan. Also, these observations of dying seashore paspalum are similar to what was seen in a controlled experiment in southern China. Xie et al. found that seashore paspalum turf under low maintenance was naturally replaced by manilagrass within 2 to 3 years.

Turfgrass ecology, part 1: abandoned turf in Japan

These photos from an abandoned golf course in the southern part of the Tohoku region of Japan are fascinating. They show clearly how three different species perform when they are not maintained for 18 months in that climate. From a consideration of the grass performance when abandoned, one can get a good idea of the maintenance requirements for the grass when it is being actively maintained.

These photos are provided courtesy of Mr. Norifumi Yawata, who kindly shared them with me along with some details about this site.

Formerly a creeping bentgrass green, now covered in weeds, but the korai around the green has very few weeds by comparison.

This site, formerly a golf course, has not been maintained for 18 months. One is essentially looking at what happens to 3 species of grass after 2 growing seasons (2013 and 2014) with no maintenance.

The greens were creeping bentgrass. The tees and the collar immediately around the greens were (and still are) korai. Korai is Zoysia matrella – the common name is manilagrass. Everywhere else, the fairways, the roughs, and so on, are noshiba. Noshiba is Zoysia japonica – the common name is japanese lawngrass.

Noshiba in foreground at the edge of the bunker. Korai border immediately surrounding the green. The green surface was formerly creeping bentgrass.

In these photos we see the characteristics of these grasses as they are adapted to this environment. Creeping bentgrass on the green surfaces has been overtaken by weeds. Clearly, creeping bentgrass in this environment seems to require mowing and supplemental irrigation and fertilizer and probably some pesticides in order to persist. It dies quickly without those inputs, or at least it becomes thin, with many voids in the turf that allow for invasion by other species.

The photo above shows a sand bunker in the foreground. Then comes some noshiba with the characteristic autumn symptoms of the wonderfully-named elephant's footprint disease caused by Rhizoctonia cerealis. At the edge of the green surface itself is a band of korai, finer-bladed than the noshiba. And then the green, now a weed patch.

View of an abandoned green complex from a high vantage point, showing the rapid colonisation of a creeping bentgrass putting green by weeds.

The korai and the noshiba both persist at this site for at least two years. It looks like some mowing of the korai and noshiba would get these surfaces back to acceptable condition by next summer. But the bentgrass is beyond saving. Because the korai and noshiba persist, it is evident that they survive without irrigation, and without fertilizer, and that the mowing, and perhaps some weed control, are all that are required to keep them at a minimal level of performance.

The dense korai turf is the most resistant to weed invasion when formerly maintained turf was abandoned for 18 months in the Tohoku region of Japan.

There are various implications of these observations on weed invasion of abandoned turf. This supports something I've written about before: for large areas of maintained turf, it makes sense to use a grass that won't die. Then one will be assured that with minimal maintenance, the quality will be acceptable. And with intensive maintenance, that grass that won't die will be able to tolerate every type of aggressive maintenance, allowing one to produce high performance turfgrass surfaces.

Korai forms a denser turf than noshiba and this is reflected in the relative amount of weed invasion in abandoned turf.

In this case, and at most golf courses in Japan, this good practice of grass selection is used. The creeping bentgrass area is small, less than 5% of the maintained turf area. So the grass that dies, the grass that requires intensive inputs, is planted only on a minimal area. The grasses that don't die, and that require relatively fewer inputs of irrigation, and fertilizer, and pesticides, and mowing – in this climate these are korai and noshiba – are used on more than 95% of the maintained turf area.

Putting green construction and topdressing sand

Figure 1. A putting green being built using the USGA Recommendations for a Method of Putting Green Construction at Krabi, Thailand (January 2006)

When I teach about turfgrass maintenance, much of the discussion involves putting greens or other highly trafficked turf areas, because that is where most of the shots are played. And I am invariably asked questions about the type of sand to use, whether river sand can be used, or what type of amendments should be mixed with sand, and so on.

 These are important questions, and I have six things that I usually talk about when these questions are raised.

 1. Sand is a terrible medium for plant growth because sand has a low water holding capacity and low nutrient content. Plants, including turfgrasses, will generally grow better in soils containing some silt and clay than they will in sand. Of course, with regular maintenance, turfgrass managers are able to produce excellent turf in sand rootzones through the provision of water and nutrients to meet the plant requirements.

 2. However, a sand can be chosen that has two especially useful characteristics for high traffic turf areas. With the right particle size distribution, sands can be used that have a) a rapid infiltration rate, so that the surface is usable soon after a heavy rain, and b) resistance to compaction, even though there is a lot of traffic on the area. Infiltration rate and resistance to compaction — those are the reasons sands are used for high traffic areas.

 3. There are very specific recommendations for putting green construction provided by the USGA. This document, USGA Recommendations for a Method of Putting Green Construction, is freely available ( These are sometimes called the “USGA specifications” and they outline everything from the depth of sand to the type of drainage to the sand particle size and various physical properties that the sand must have if the green is to meet the specifications set out in the USGA Recommendations document. Make variations from these Recommendations, and the green may still perform well, but please don’t call it a “USGA” green if the Recommendations are not followed.

 4. For topdressing sands, a good starting point is to look for sands that have physical properties that meet USGA Recommendations.

Figure 2. The same green, 8 years later, still performing well, which is what one expects when a green is built following the USGA Recommendations (May 2014)

 5. I don’t think the Method outlined in the USGA Recommendations is necessarily the best way to build a green, but it is one that works, and it is a way to build a green that many people understand and know how to manage. Figure 1 shows the construction of a USGA green in Krabi, Thailand in 2006, and Figure 2 shows the same green still performing well in 2014. That type of predictable result is what we expect when building a green to USGA Recommendations.

 6. If I were building a putting green for myself, and if I knew that I would be the one to manage it, I would probably build a green with some soil in it, with lots of surface drainage, with a slower infiltration rate than in the USGA Recommendations. But if I were building a green for someone else, and I knew that I would not be responsible for maintaining it, I would choose the USGA Recommendations. That way, the risk of unexpected problems is much reduced. 

 I encourage everyone to download a copy of the USGA Recommendations and to be familiar with the document. Many problems and confusions could be avoided by a broader understanding of this Method.

 I wrote this as part of a series for the Indian Golf Industry Association (IGIA) newsletter. For more about turfgrass information specific to India, see the ATC site