Turfgrass nutrient guidelines, peer review, andpotassium
Micah Woods
20 June 2014
I’ve long considered conventional soil guidelines to be broken.
By that, I mean the guidelines1 are either substantially different
from the amount required to produce good turf or are impossible toreach.2 It just doesn’t make sense to apply nutrients in an attempt
to chase target levels in the soil, when those nutrient applications
don’t have an effect on turfgrass performance.
Soil testing can be a bit of a numbers game. But it doesn’t haveto be that way. There is a large amount of published research showing turf response to various nutrient levels in the soil. It is also
possible to estimate how much of each element the grass may use,
as a function of nitrogen use and growth. I worked with Larry
Stowell and Wendy Gelernter from PACE Turf3 to develop a new
set of guidelines – guidelines that were simple, based on research,4
environmentally responsible, and that accounted for the amount of
nutrients used by the grass.
When the minimum levels for sustainable nutrition (mlsn)
guidelines5 were announced in 2012, I expected there to be moreobjection to them. I was looking forward to some detailed discussions about turfgrass soil testing and soil nutrient guidelines. The
discussion at that time never materialized, at least not publicly.
This year, however, some pointed questions and concerns about
these guidelines have reached me, with thoughts that the nutrient guidelines may be too low, or that managing turf using theseguidelines may have negative long term ramifications.6 These areimportant questions, and this is a good discussion to have.
Last week, a turfgrass scientist wrote to me with some specificconcerns about the guidelines and the methodology used to move
forward with the mlsn guidelines. I welcome this chance to provide more explanation of the reasoning behind the guidelines, andto elaborate a bit on turf nutrient guidelines in general, and potassium (K) specifically.

Radical changes and the scientific method
The first concern is on how we have developed and introduced
the mlsn guidelines.
I think we need to be very careful that radical changes in soil test
recommendations, such as you are proposing, are backed by thescientific method. You have a hypothesis and you have collected
data to support that hypothesis. However, you have not vetted thosemethods, results, or interpretation of the results through the peerreview process.

The author is Chief Scientist
at the Asian Turfgrass Center.
www.asianturfgrass.com

There is not just one set of “conventional” guidelines. An example of the conventional approachis these of guideline ranges given
by Carrow et al. in their 2003 paper: http://ticpass.lib.msu.edu/
cgi-bin/flink.pl?recno=93213.
1

The Penn State guideline forpotassium on putting greens is
an outstanding example (http://bit.ly/psu_green); the cationexchange capacity (CEC) of most
putting green rootzones are too low
to ever reach the “optimum” level, nomatter how much K one would apply.
2

3

http://www.paceturf.org

Much of which suggests older guidelines were higher than necessary forsome nutrients.
4Micah Woods, Larry Stowell, andWendy Gelernter. Just what the grassrequires: using minimum levels forsustainable nutrition. Golf Course
Management, pages 132–138, January
2014. URL http://bit.ly/gcm_mlsn
5

6

For more on this topic, see http://bit.ly/K_supplyturfgrass nutrient guidelines, peer review, and potassium

Soil nutrient guidelines and the resultant fertilizer recommendations are not usually peer-reviewed. A comprehensive article
on this topic by Carrow et al.7 is in GCM. At the state level I haveseen many extension bulletins or fact sheets with soil test interpretation guidelines. Guidelines for turfgrass, as best as I can tell, areusually put together by reviewing some peer-reviewed researchand by looking at the normal range of soil nutrients in soils from aparticular region.
Guidelines will be based to some degree on the results of multiple research projects that have been peer-reviewed, but the guidelines themselves will not be peer-reviewed. In that sense, the mlsn
guidelines are the same as any other guidelines, with a large improvement. That is, we have reviewed existing guideline levels, wehave made extensive studies of peer-reviewed research to ensure
the mlsn guidelines are reasonable, and we have supplemented
this with data from the soils at hundreds of good-performing
turfgrass sites to ensure that high quality turfgrass can indeed
be grown on soils with nutrient levels at or above the mlsn guidelines.
Although the mlsn guidelines might seem like “radical changes,”
I would consider them an update and significant improvement on
conventional guidelines that themselves were not peer-reviewed.
Doug Soldat, a soil scientist from the University of Wisconsin, consider the mlsn guidelines as “likely more accurate”8 than traditional soil test interpretation.

2

R.N. Carrow, L. Stowell, W. Gelernter,
S. Davis, R.R. Duncan, and J. Skorulski. Clarifying soil testing: III. SLAN
sufficiency ranges and recommendations. Golf Course Management, 72(1):
194–198, January 2004
7

Douglas J. Soldat. How reliable is soil
testing? The Grass Roots, pages 8–11,
May-June 20138

On introduction of new ideas
In order to get the scientific community to back your work andideas, the expectation is that you publish the work in a journal such
as Crop Science or Soil Science so that your methods, analysis, andinterpretation are adequately critiqued to judge their validity. This
will undoubtedly lead to further testing by others to confirm yourresults. Once it has gone through this process, then most scientists
will embrace the results and apply them.

I agree with this in general, but as mentioned in the previous
section, soil nutrient guidelines are not typically peer-reviewed. We
do want these guidelines to be critiqued, scrutinized, and tested; tothat end, we have communicated both formally and informally atconferences and in various media about the mlsn guidelines andthe methods used for development.
The guidelines and the methods used to develop them were
presented in 2012 as “Minimum Levels for Sustainable Nutrition(mlsn)” at The Bouyoucos Conference on the Advances in Researchon Soil Biological, Chemical and Physical Properties for Sustainable
Constructed Rootzones.9 At the 2012 Crop Science Society of America conference, we presented “Development of minimum levels forsustainable nutrition soil guidelines using data mining and distribution fitting software.”10 We wrote “Just what the grass requires:

The conference proceedings canbe downloaded at this link: http://bit.ly/pace_bouyoucos.
9This is the abstract: http://ticpass.
lib.msu.edu/cgi-bin/flink.pl?
recno=213256
10

turfgrass nutrient guidelines, peer review, and potassium

using minimum levels for sustainable nutrition”11 for GCM thisyear to further explain how to use the mlsn guidelines. The mlsn
project Global Soil Survey12 started last year, with samples being
submitted from good-performing turf areas in multiple countries, totest, validate, and refine the mlsn guidelines. This project will be
discussed in a presentation at the 2014 CSSA annual meeting.
I agree with my correspondent that for the mlsn guidelines tobe widely-adopted13 by scientists, publication in peer-reviewed
journals will be required. And that is our intention. Not because it
is a requirement of soil testing guidelines, but because we want tocommunicate in the standard scientific way.
As an aside, just because something is published in a peerreviewed journal does not mean it is correct. For an interesting
discussion about this, see Andrew Gelman’s blog, where he discussed “a larger problem with the scientific communication system,
the idea that once something is published in a journal, it is presumed to be true and it takes a lot of work to dislodge even grosserrors.”14
While I acknowledge that writing about the mlsn guidelines
in peer-reviewed publications is the way to get the idea accepted
and adopted by scientists, the publication of something in a peerreviewed journal is not a confirmation that something is right orwrong. The scientific method involves making hypothoses, testing
them, trying to prove them wrong, and trying to find a better explanation for a phenomenon. Publishing in peer-reviewed journals is
the standard form of communication of scientific results and amongscientists, but it is not, in and of itself, the scientific method.

What, really, is the effect of potassium?
After those introductory remarks, which are more on thephilosophy of this type of work, we turn to the specific topic of K
and the amount required.
I have no idea if the mlsn guidelines you are proposing for K or
any other element are too low. I have never studied potassium fertilization during my career, so do not claim to be an expert and do
not want to debate you about the topic since I do consider you anexpert on K. My questions . . . were just cursory evaluations of yourpublications and what little I know about potassium fertilization, so
they were admittedly simplistic. I did do a quick literature search
and found numerous publications (see attached) where high tissue
K had a positive effect on stress tolerance in turfgrass and I did noteven get into the literature on other crops. So, there is data out there
suggesting that higher K levels can impact performance above andbeyond turfgrass quality or crop yield.

Before considering the publications that describe experiments
in which more K had a positive effect on stress tolerance, I needto make an important distinction. Grass grows in soil and obtains

113

The article can be read at this link:

http://bit.ly/gcm_mlsn
12

http://www.paceturf.org/journal/
global_soil_survey

I would like to see this method ofsoil test interpretation and calculationof nutrient requirements taught inturfgrass classes, and for that tohappen, some peer-reviewed articles
are going to be necessary.
13

This quote is at http://bit.ly/
gelman_tols; for more, about dis14

agreement with authors that “appear to have the attitude, so naturalamong those of us who do science
for a living, that peer-reviewed publication is a plateau or resting place:the idea is that acceptance in a journal – especially a highly selective
journal such as PNAS – is difficultand is a major accomplishment,
and that should be enough,” see
http://bit.ly/gelman_hurricanesturfgrass nutrient guidelines, peer review, and potassium

most of its nutrients from the soil. The amount of K available to
the grass, or with alternative wording, supplied to the grass, is theamount in the soil combined with the amount added as fertilizer.15
So when we consider the results of experiments that investigate
the effect of K application, one must consider not only the fertilizer
supply, but also the supply from the soil.

Figure 1 shows what a profound difference there can be in grass
performance with different levels of soil K. The dead grass at right,
if it had been supplied with K, would have given a great response.
The grass at left, which was not supplied with any K fertilizer, butwhich had an ample supply of K from the soil, would not havemuch response to K fertilizer, if any, because the grass already hasenough K.
As seen in Figure 1, a K deficiency has disastrous effects. The
mlsn guidelines have been developed to ensure that such deficiencies are avoided, by keeping the grass supplied with enough K at
all times.
In that context, considering how much K is available to the grass,
let’s look at the “numerous publications where high tissue K had apositive effect on stress tolerance in turfgrass.”

I’ve written about this for the caseof K and rooting (http://bit.ly/k_roots). Although a number of papers
were cited showing that K increases
root development and branching,when one reads the papers, it isapparent that an elimination of a K
deficiency provides the benefit.
15

Figure 1: These are plants of Penn A-1
creeping bentgrass grown from seedin a greenhouse at Cornell University
during my graduate school research.
The amount of K in the soil is highest
at left, and lowest at right. No K wasapplied as fertilizer.

Water relations of two Cynodon turf cultivars as influenced by potassium
In this experiment16 by Miller and Dickens, the turf was grownin a soil with 8 ppm K. This is very low. Nitrogen was applied at
9.8 g m−2 mo−1 creating a high plant requirement for K in a soil
deficient in K.17 Naturally, adding K had a positive effect. The authors conclude that “under conditions of low soil K concentration,
K fertilization can have a significant impact on leaf water potential
and the plant’s ability to prevent leaf tissue damage brought on bydrought.”
Application of the mlsn guidelines ensure that the soil K remains above a low level. In the experiment described, the mlsn
guidelines approach would have supplied equivalent amounts of K
as were used to produce the beneficial effects in this experiment.

416

http://ticpass.lib.msu.edu/
cgi-bin/flink.pl?recno=61150

For more about nitrogen-driven
nutrient demand by turfgrass, see
the paper on this by Kussow et al.:17

http://bit.ly/161KyWm

turfgrass nutrient guidelines, peer review, and potassium

5

Nutrient Accumulation and Associated Root Characteristics in Response
to Drought Stress in Tall Fescue Cultivars
Huang18 grew tall fescue in a soil with 176 ppm K.19 Then drought
stress was applied to some of the plants. The treatments were theapplication of drought stress, or continued irrigation. Nutrient
contents of shoots and roots were then measured. This experiment
did not test the effect of increasing or decreasing K supply to thegrass and it is not clear from this experiment that changes in K
supply would have an effect on drought tolerance.

18

http://ticpass.lib.msu.edu/
cgi-bin/flink.pl?recno=72616

The soil nitrogen was reported as
106 ppm, which is extremely high andis equivalent to a single application of
fast-release N at a rate of 15.8 g m−2 . In
the 2 mo of growth before the drought
treatment was initiated, the grass was
supplied with N at 17 g m−2 .
19Effects of Mineral Nutrition on High Temperature Induced Growth Retardation of Kentucky Bluegrass
In this experiment20 by Pellet and Roberts, grass was grown in solution cultures, not in soil, at two levels (high and low) of N, P, and
K. Note that the low level was “designed to be below” the range required for good turfgrass development. Even so, the level of K “hadlittle influence on the degree of resistance of Kentucky bluegrass tohigh temperatures” and “neither P nor K levels affected resistanceof plants to high temperatures.” There was an interaction with N;
at high levels of N, more K was required. The mlsn guidelines, bypredicting expected plant use of K based on site-specific N rates,
account for this interaction, ensuring the grass is always suppliedwith enough K.21

20

http://ticpass.lib.msu.edu/
cgi-bin/flink.pl?recno=12707

This is demonstrated in the K
requirement calculator (https:
21//asianturfgrass.shinyapps.io/
mlsn_K/ where the K fertilizer require-ment is shown to change when the N
application rate is changed.

Nitrogen, Potassium, and Irrigation Effects on Water Relations of Kentucky Bluegrass Leaves
22
Carroll and Petrovic22 grew Kentucky bluegrass in soil with 9 ppm
http://ticpass.lib.msu.edu/
cgi-bin/flink.pl?recno=20167−
2−
1
K. This is low. Nitrogen was applied at rates from 3.5 to 17.5 g m mo .
K was applied at 1.75 to 17.5 g m−2 mo−1 ; this gave N/K fertilizer
ratios of 10:1, 2:1, 1:1, and 1:5. “A small change in the N/K ratio,such as from 2:1 to 1:1, did not appreciably change [bulk leaf osmotic potential at full turgor]; however, larger N/K ratio changes
substantially altered [bulk leaf osmotic potential at full turgor],”they wrote. “Increasing the supply of K 10-fold caused [bulk leaf
osmotic potential at full turgor] to decrease 0.2 MPa under daily
irrigation, indicating the influence of K addition alone on [bulk leaf
osmotic potential] is limited.”
The two objectives of this experiment were “to obtain an empirically based equation that would predict stomatal resistance as afunction of leaf turgor, and to determine the effect of N, K, and irrigation frequency on the turgor maintenance characteristics of Kentucky bluegrass leaves.” This was needed “before physiologically
based turfgrass water-use and stress-index computer simulation
models can be developed.” Although at the extreme differences in
N/K ratios, in turf watered daily, the bulk leaf osmotic potential atfull turgor was different, the practical effect of this wasn’t detected.
For the plants receiving these same treatments, but irrigated on aturfgrass nutrient guidelines, peer review, and potassium

5 d schedule, “moisture loss and stress development were clearlyrapid . . . with loss of leaf turgor and leaf folding apparent in allcontainers by the fourth day.”
Note that if the mlsn guidelines were applied to Kentucky bluegrass grown in this soil, substantial amounts of K would be usedbecause the soil is so low in K.

Effects of Nitrogen and Potassium Fertilization on Perennial Ryegrass
Cold Tolerance During Deacclimation in Late Winter and Early Spring
In this experiment23 by Webster and Ebdon, the soil was high in
K, at 121 ppm, and the treatments were five rates of N and threerates of K. At the high N rates, adding more K reduced survival. Another negative effect of K at high N was more disease. Higher ratesof K “increased the severity of low temperature disease (Typhulaincarnata) by as much as 25 % to 35 % over low K.”
The positive effects of adding K occurred at the lower rates ofN. Let’s look just at the lower rates of N, when N was applied at
4.9 , 14.7 , and 24.5 g m−2 yr−1 . At those N rates, for plants exposed
to cold temperature in late winter, the lethal temperature at which
50 % of the plants died was −12.4 ◦C when the high rates of K
were applied; the lethal temperature was −11.2 ◦C when the lowest
rate of K (4.9 g m−2 yr−1 ) was applied. So that is a 1.2 ◦C effect – apositive one.
But for plants exposed to cold temperature in early spring, theeffect goes away, even at the low N rates. In early spring, 50 % of
the plants died at −9.5 ◦C when the high rates of K were applied;
the lethal temperature was −10.3 ◦C when the lowest rate of K wasapplied. So that is a 0.8 ◦C effect – a negative one.
I don’t see that this is a convincing case for adding K. And thedata here show there are substantial risks to adding K, especiallywhen the N rates are high.

23

http://ticpass.lib.msu.edu/
cgi-bin/flink.pl?recno=111993

Irrigation and Potassium Effects on Poa pratensis L. Fairway Turf
Shearman et al.24 applied K at 0 , 20 , 40 , and 60 g m−2 yr−1 at sites
with soil K of 318 and 343 ppm. Irrigation treatments were appliedat both sites at 100 , 80 , and 60 % of evapotranspiration. “There
were no significant irrigation or potassium nutrition treatment
effects on turfgrass color or quality under the conditions of thisstudy, and no data are presented for turfgrass color or quality,”
they wrote. There were effects of K on plant stiffness and ball rolldistance and leaf temperature.
This project was conducted at two sites over two years and there
was not a K effect on turfgrass quality.

24

http://ticpass.lib.msu.edu/
cgi-bin/flink.pl?recno=106300

Potassium fertilization responses as affected by sodium
Snyder and Cisar25 applied N at 10 g m−2 mo−1 to a soil green andto a USGA sand green. K was applied at rates up to 5 g m−2 mo−1 .

25

http://ticpass.lib.msu.edu/
cgi-bin/flink.pl?recno=105421

6

turfgrass nutrient guidelines, peer review, and potassium

7

They concluded that “when fertilized with N at the rate of 10 g m−2 mo−1 ,bermudagrass visual quality generally was reduced by K fertilization rates below 5 g m−2 mo−1 , both on a native soil and on a USGA
green, and clipping yields were reduced occasionally.”
Application of the mlsn guidelines at this site, for the reported
soil test K levels and N application rates, would have recommend
application of K rates exceeding 5 g m−2 mo−1 .

The Effects of Fertilization on Recovery of Kentucky Bluegrass Turf from
Summer Drought
At Blacksburg, Virginia, Schmidt and Breuninger26 grew Kentucky
bluegrass in a soil with 48 ppm K. After a drought in the fifth yearof this study, the effects of adding K at 6 or 12 g m−2 yr−1 were measured, as clipping yield after the drought (recovery). Adding K increased the recovery of yield as measured on clipping yields taken
on 12 August 1966 following a summer drought. Averaged acrossall N and P treatments, the yield was 12.6 , 16.8 , and 16.6 g m−2 forannual K rates of 0 , 6 , and 12 g m−2 yr−1 .
At a starting soil test level27 of 48 ppm, and at the N rates usedin this experiment, the mlsn guidelines would recommend K application at rates from 5.6 to 13 g m−2 yr−1 at this site. That is, themlsn guidelines would recommend the same amount of K that wasused to achieve the maximum drought recovery.

26

http://ticpass.lib.msu.edu/
cgi-bin/flink.pl?recno=1174

The site is described as having soil
K of 95 kg ha−1 which is equivalent to48 ppm; however, data collected during
the experiment show soil K at aboutfour times that level, which would be
impossible to achieve at the rates of K
applied in the experiment. Thus, there
is some confusion about these results.
27

Na+ and K+ Accumulation in Perennial Ryegrass and Red Fescue Accessions Differing in Salt Tolerance
Krishnan and Brown28 grew perennial ryegrass and red fescue “inan ebb-and-flow water bath filled with half-strength Hoagland solution and NaCl.” This experiment is not measuring the effect of K; it
is looking at salinity tolerant accessions and what ions accumulate
in the leaves. K was not a treatment.

28

http://ticpass.lib.msu.edu/
cgi-bin/flink.pl?recno=150951

Response of ‘Captiva’ St. Augustinegrass to Shade and Potassium
Cai et al.29 grew St. Augustinegrass in different amounts of shade
and with different K rates in sand and potting mix in a glasshouse.
Clippings were removed. No fertilizer was applied at establishment. The soil K for this study is not reported. Adding K in thisexperiment increased root weight and turf quality. Based on the
known effects of K, it seems likely that the K added as treatments
was necessary to correct a soil deficiency.

29

http://ticpass.lib.msu.edu/
cgi-bin/flink.pl?recno=192077

Concluding notes
Because K is used by grass in such large quantities, second only
to N,30 it is to be expected that there will be effects of adding K as
fertilizer, usually positive in the cases where the soil cannot supply

Except for seashore paspalum, which
often uses K at similar rates to N.
30

turfgrass nutrient guidelines, peer review, and potassium

enough K to meet the grass requirements. The benefit of K comesfrom eliminating a deficiency, in most cases. The mlsn guidelines
have been developed to ensure that enough K is supplied to meetthe grass requirements and to ensure there will never be a deficiency.
When we look at the results of K experiments, it is common
to see an effect when one is correcting a K deficiency. The addition of K beyond the amount to correct the deficiency gives one ofthree results. It improves turf performance, it causes problems, ormore often, no effect is observed. Because the effect can go in either
direction – good or bad – and because there often is no effect, a reasonable explanation is that there is no consistent benefit to additionof K beyond the amount that is required to supply the plant withthe amount that it will use.31
As a practical matter, adding more K than the grass can use is awaste. Eventually the extra K will leach, unless one increases cationexchange capacity (CEC) to match the amount of K added.

8

A notable and particularly interesting exception is the recent
research at Rutgers showing anthracnose suppression when K isapplied at rates more than 3 times
higher than the grass can use: http://usgatero.msu.edu/v13/n2-2.pdf. A
number of questions arise from this result: what is the mechanism by which
this occurs, would it occur if othersalts were added when grass is supplied with low rates of N, and would
the result be the same if the grass were
supplied with recommended rates ofN?
31