Micah Woods, Ph.D.
Larry Stowell, Ph.D.
Wendy Gelernter, Ph.D.

Just what the grass requires:
Using minimum levels forsustainable nutrition
Good turf performance can be achieved at lower nutrient levels.
In 2012, we introduced the minimum levels
for sustainable nutrition (MLSN) as an alternative to conventional soil nutrient guidelines (7).
Conventional guidelines are epitomized by thelow, medium, high and very high classifcation
scheme described in the third part of the “Clarifying Soil Testing” series published in GCM
10 years ago (1).
In light of recent trends in reduced inputs
and increased sustainability, and taking newly
published data into account, the conventional
approach requires scrutiny and signifcant revision. Conventional guidelines are not only
complex, they are also relatively static, without
regular or systematic updates.
However, regular updates seem like a good
idea, because many research projects suggestthat high-quality turf can be produced at levels
below the conventional guidelines (2,3,4,6,8).
As an alternative to the conventional guidelines, the MLSN guidelines are an attempt toidentify not the optimum levels for soil nutrients, but rather the minimum levels of soil nutrients at which we can be confdent of good
turf performance.
You may have seen the same thing yourself: high-quality turf with no problems,
growing in a soil classifed as low in one ormore essential elements. The question thenarises, if the soil is lacking in these elements,
why is the grass performing so well? Adding
a nutrient may change the soil test result tomove the level up to a desired range, but if thenutrient addition has no effect on the grass
performance, is it necessary?

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How to use the MLSN guidelines
at your facility
The MLSN guidelines (Table 1) take a new
approach to soil test guidelines. Turfgrass managers will have two questions about fertilizer
application, and the MLSN guidelines answer
both of them. The frst question is, “Does thiselement need to be applied as fertilizer?” As afollow-up to the frst question, one also needs
to ask, “If this element is required, how muchshould be applied?”
To answer the frst question, simply com-pare the MLSN guideline value for an element
to the soil test level for that element. If the element is below the MLSN guideline, or if theestimated use of that element will drop the soil
to the MLSN guideline during the course of
the growing season, then that element should
be applied. If the element, as measured by thesoil test, is above the MLSN guideline, and if
estimated use of that element during the growing season will keep the soil above the MLSN
guideline, then that element is not required
as fertilizer.

In experiments with creeping bentgrass at Cornell University, a wide range of soil potassium levels were
established in these research plots, but no beneft to applied potassium was observed even when the soil potassium
levels were well below the conventional guidelines. Photos by Micah Woods

Penn A-1 creeping bentgrass grown in soils with decreasing levels of potassium from left to right; keep-

ing soils at or above the MLSN guideline provides a level of safety that such defciency symptoms will not occur.

This creeping bentgrass green at Takarazuka GC near Osaka, Japan, has calcium, magnesium and potassium levels
not only below the conventional guidelines, but also below the MLSN guidelines, yet still has produced excellent
turfgrass conditions since the soil was frst tested in 2009.

To answer the second question, regarding
how much of an element to apply, simply add
enough of that element to keep the soil at or
above the MLSN guideline at the end of thegrowing season. To calculate that, compare thesoil test result to the MLSN guideline and toan estimate of how much of that element thegrass will use.
To try the MLSN approach, you will needsome recent soil test results from tests doneusing the Mehlich 3 extractant. You will alsoneed an estimate of how much nitrogen will
be applied to your turf in the upcoming year.
Because nitrogen controls the uptake of othernutrients (5), we can use the nitrogen estimate
to predict the grass’s use of other elements.
Before making the calculations, we will
make some assumptions about grass growth
and the relationship between fertilizer applied
to the two-dimensional soil surface and the soil
test levels within the three-dimensional rootzone. These include:• The grass cannot use more of an element
than it harvests.
• The growth and consequently the nutrient
uptake are determined by the amount of
nitrogen applied.
• The concentrations of macronutrients andsecondary nutrients in the leaves will be estimated as in Table 2.

Minimum Levels for Sustainable
Nutrition guidelines
Nutrient

Analytical test

Conventional guideline (ppm)

MLSN guideline (ppm)

Phosphorus

Mehlich 3>50

18

Potassium

Mehlich 3

>110

35

Calcium

Mehlich 3

>750

360

Magnesium

Mehlich 3

>140

54

SulfurMehlich 3

15-40

13

Table 1. Minimum Levels for Sustainable Nutrition (MLSN) soil guidelines for macronutrients and secondary nutrients. A full
copy of the current MLSN guidelines is available at www.paceturf.org/PTRI/Documents/1202_ref.pdf• One gram of an element spread over 1
square meter on the surface is equivalent to
4.4 ppm of that element in the root zoneof 1 square meter to a 15-centimeter depth,
and vice versa.
• One pound of an element spread over
1,000 square feet on the surface is equivalent to 22 ppm of that element in the rootzone of 1,000 square feet to a 6-inch depth,
and vice versa.

Example 1: Potassium
Let’s say the potassium soil test level is 52
ppm, and we plan to apply 3 pounds of nitrogen/1,000 square feet in the upcoming

year. How do we determine the potassium requirement to ensure we stay above the MLSN
guideline for potassium of 35 ppm? As shownin Table 2, the grass is expected to use half (0.5)
as much potassium as it does nitrogen. Thatis, we predict the grass will use 1.5 pounds ofpotassium/1,000 square feet, which is equivalent to a depletion of 1.5 * 22 = 33 ppm fromthe soil. Because we want to keep the soil at or
above the MLSN guideline, the total amountof potassium required is the plant use (33 ppm
or 1.5 pounds) added to the amount we wantto ensure remains in the soil (35 ppm or 1.6
pounds). In our example, this is 68 ppm or 3.1
pounds. The amount of potassium in the soil
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133

Expected leaf nutrient content
Nutrient

Expected % in leaf dry matter

Amount in proportion to nitrogen

Nitrogen4

1

Potassium2

0.5

Phosphorus

0.5

0.125

Calcium

0.5

0.125

Magnesium

0.2

0.05

Sulfur0.2

0.05

Table 2. Expected leaf nutrient content and proportion relative to nitrogen. These values are suitable as a starting point formost turfgrass species. If site-specifc data are available, those values can be substituted to further refne these calculations
for a particular site.

From 2006 to 2009, more than 50 varieties of warm-season grasses were grown at the Asian TurfgrassCenter research facility north of Bangkok in soils with nutrient levels below the conventional soil guidelines, yet the turf
still met all performance goals.

test is 52 ppm (2.4 pounds). The amount required as fertilizer is the difference between theamount required (68 ppm or 3.1 pounds) andthe amount actually present (52 ppm or 2.4
pounds), which comes to 16 ppm or 0.7 pound.
Thus, the fertilizer requirement for potassiumin this situation using the MLSN guidelines is
1 pound potassium/1,000 square feet.

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Example 2: Magnesium
If the soil test level for magnesium is 75
ppm and we plan to apply 3 pounds of nitrogen/1,000 square feet in the upcoming year,
how do we determine the magnesium requirement to ensure we stay above the MLSN
guideline for magnesium of 54 ppm? As shownin Table 2, we expect the grass to use 20 times

more nitrogen than magnesium. That is, wepredict the grass will use 0.15 pound of magnesium/1,000 square feet (3 pounds nitrogen *0.05 = 0.15), which is equivalent to a depletionof 0.15 * 22 = 3.3 ppm from the soil. We want tokeep the soil at or above the MLSN guideline,
so the total amount of magnesium required isthe plant use (3.3 ppm or 0.15 pound) added tothe amount we want to ensure remains in thesoil (54 ppm or 2.5 pounds). In our example,
this is 57.3 ppm or 2.6 pounds. The amounton the soil test is 75 ppm (3.4 pounds). The
amount required as fertilizer is the difference
between the amount required (57.3 ppm or 2.6
pounds) and the amount actually present (75ppm or 3.4 pounds). Because the amount present is more than the amount required, we do
not need to apply any magnesium to keep thesoil above the MLSN guideline.

How the guidelines weredeveloped
We started with soil test data from the
PACE Turf database. This consisted of datafrom more than 17,000 individual soil samples,
each drawn from a stand of turf that was performing well. Because the data in those samples
were from sites where turf performance wasgood, we could expect that whatever the nutrient levels were at those sites, those levels would
be suffcient to produce turf that performed
well. Then we fltered the data, selecting only
the data from sites with a cation exchange capacity (CEC) less than 6 cmolc /kilogram.
This flter removed all the soils with highnutrient-holding capacity from the working
data set. We wanted to look at only the soils
that had a relatively low nutrient-holding capacity, yet still produced good turf conditions,to investigate and identify the individual nutrient levels in those soils. For the MLSN
guidelines, we assume that if there is enoughof an element to produce good turfgrass in a
low-nutrient-holding soil (such as a sand rootzone from a golf course putting green), thenthe same amount of that element will be suffcient to produce good turfgrass conditions in amore nutrient-rich soil that has a higher CEC.
We think that if there is enough of an element
to produce good turfgrass in a sand root zone
on a golf course putting green, then the samelevel of that element in a soil-based green or ona golf course fairway will produce good turfgrass as well.
We added one more flter to the data. Thiswas for pH. We selected only those samples

MLSN potassium data
1
0.9
0.8

Probability

0.7
0.6
0.5
0.4
0.3
0.2
0.1
0

0

20

40

60

80

100 120 140 160 180 200 220 240 260 280

Mehlich 3 potassium (ppm)
Figure 1. The cumulative distribution function for the potassium data was used to identify the MLSN guideline. At the 0.1
probability level, 10% of the samples report potassium values lower than 35 ppm (red line). This is the potassium MLSN
guideline. The blue line indicates the conventional guideline of 110 ppm for potassium. Seventy-two percent (probability =0.72 = sustainability index) of the samples report values lower than the conventional potassium guideline.

MLSN phosphorus data
1
0.9
0.8
0.7

Probability

with soil pH from 5.5 to 7.5. The purpose ofthis was to develop guidelines that would be accurate for a range of elements using the widelyused Mehlich 3 soil test extractant. When soil
pH is less than 5.5, we recommend applicationof liming materials to reduce soluble aluminum, to increase soil microbial activity and to
reduce the risk of toxic soil-soluble ammonium
levels. Because of that, there was no reason to
include soils with a pH of less than 5.5 in thedata set.
In soils with a pH above 7.5, there is a highprobability that the Mehlich 3 extractant may
dissolve some soil minerals that contain calcium or magnesium. Such dissolution would
have introduced error into the guidelines,
which we avoided by selecting for a pH range
at which mineral dissolution is minimal, andabove which magnesium and calcium would
not be defcient.
After the two flters were applied, we wereleft with a working data set of more than 1,500
soil samples. These were from turf that performed well, had a relatively low CEC typical of golf course putting greens or relatively
sandy soil, and a pH of 5.5 to 7.5. Because all ofthese soils were producing good turf, one could
conclude that all the soils had suffcient nutrients, so anything at or above those nutrient levels would be fne. Rather than divide the datafrom these soils into low, medium and highclassifcations, we took a different approach, inwhich we modeled the distribution of the data
for each element (7).
Nutrient concentrations in the soil are acontinuous random variable with a minimum
possible value of zero and a virtually unlimited
maximum possible value. We analyzed the fltered data set using EasyFit distribution-ftting
software from Mathwave (www.mathwave.
com) and found a good ft for each element inthese soil test results with a three-parameter loglogistic distribution. From this modeled distribution, based on the actual data from turfgrasssites that had good performance, we identifed
the MLSN guidelines. A visual representationof the cumulative distribution function is shownfor the potassium data in Figure 1 and for thephosphorus data in Figure 2.
If we look at the data for potassium, for example, we see the cumulative proportion of thesamples at any particular level as we go from0 to 280 ppm. The conventional guidelines
would seem to be taking a number of sites withgood performance and then choosing to target the higher end of that range as a guideline.

0.6
0.5
0.4
0.3
0.2
0.1
0
0

40

80

120 160 200

240 280 320

360 400 440

480

Mehlich 3 phosphorus (ppm)
Figure 2. The cumulative distribution function for the phosphorus data was used to identify the MLSN guideline. At the 0.1
probability level, 10% of the samples report phosphorus values lower than 18 ppm (red line). This is the phosphorus MLSN
guideline. The blue line indicates the conventional guideline of 50 ppm for phosphorus. Fifty-nine percent (probability =0.59 = sustainability index) of the samples report values lower than the conventional phosphorus guideline.

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135

High-performance turf at Keya GC near Fukuoka, Japan, is maintained in soils with sulfur and magnesium near the MLSN guideline and potassium below
conventional guidelines. Adding data from sites like this helps to improve the accuracy of the guidelines as they are updated.

With MLSN, we take a different approach,taking the data from thousands of sites withgood performance, assuming that there must
be enough nutrients available to produce good
turf because the sites are already performing
well, and then selecting a conservative value atthe 0.1 level at the lower end. Because we havealready omitted the sites with bad performancefrom our data set, we can have some confdence
that these apparently low levels are suffcient tomeet the requirements of the grass.

Four advantages of thisapproach
1. The guidelines are based on real datafrom actual turfgrass sites. We worked onlywith a data set from sites with good performance, omitting soil test results from problem
areas and nutrient-defcient soils. The modeled
distribution is a mathematical representation of
the soil nutrient levels as they are distributedon actual turfgrass sites. Because the data arecarefully selected from soils that are alreadyproducing good turf, there is a layer of safety

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in the model. That is, any clearly defcient soils
were not included in the model, so the results
are not skewed lower by nutrient-defcient soils.
2. Once the model has been ft to the actual
data, we can select a base level we wish to stay
above. Again, this model and the associatedlevel are based on the actual nutrient levels inthe soil at sites where turfgrass performs well.
We chose the nutrient level coinciding with the10th percentile to defne the MLSN guideline
for each element. At this level, 10% of the samples in the data set would have a lower soil nutrient level than the selected MLSN guideline.
3. We can calculate a sustainability index
for each element, based on a comparison of theconcentration of that element on a soil test withthe modeled MLSN distribution for that element. The sustainability index is the proportion of the modeled distribution that reports
values greater than the sample soil test value.
This is a metric that assists turf managers in
the evaluation of soil nutrient levels over time.
It also provides a guide for the development of
nutrient management programs. Perhaps mostimportant, the sustainability index identifes
and rewards the restriction of nutrient inputs
when they are not necessary to meet turf performance goals.
4. The MLSN guidelines are easily updated as we add new data from turfgrass sites
with good performance (see the sidebar on
page 138).These guidelines are self-correcting.
Using this method and continuously adding to
the reference data set with soil test data from
turfgrass sites that perform well, we will see theguidelines move up if they are too low or down
if they are too high. In short, these guidelines
are designed to be updated as the core data set
grows, and the MLSN guidelines will adjust
based on samples added to the data set from
turfgrass that performs well on various soils
and across a wide geographic range.

Additional Information
For more about these guidelines, videos explaining the guidelines, and a link to the mostcurrent version of the guidelines, see: www.
paceturf.org/journal/minimum_level_for_Global Soil Survey for
Sustainable Turf
PACE Turf and Asian Turfgrass Center have teamed up to administer a citizen scientist
project known as the “Global Soil Survey for Sustainable Turf.”
The survey hopes to enlist the participation of superintendents from around the globe
in an effort to validate and expand upon the Minimum Levels for Sustainable Nutrition(MLSN) soil guidelines described in this article, and in so doing, contribute to positivechanges in the way turf is fertilized.
Participants in the survey will receive a sampling kit that allows them to collect soil
samples from three areas of turf that performs well at their facility. These samples will beanalyzed by Brookside Labs for nutrient content, and the data will be added to the PACE
Turf/Asian Turfgrass Center database and analyzed. The result will be new and improved
sustainable guidelines for turf nutrition that will be publicly shared with the turf community.
If you are interested in learning more about the Global Soil Survey, read more at: www.
paceturf.org/journal/global_soil_survey

RESEARCH SAYS
• Turf soil nutrient requirements for several key
elements may be much lower than previously
thought.
• The Minimum Levels for Sustainable Nutrition(MLSN) guidelines are a new, more sustainableapproach to turfgrass nutrition.
• Use of the MLSN guidelines can result indramatic cost savings, and an improved environmental profile, without a loss in turf quality.
• The guidelines were developed using data collected from thousands of soil samples from turfthat performed well.
• The concept of a “sustainability index” — a tool
for measuring progress toward reduced inputs— is introduced.

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sustainable_nutrition. To join other turfgrassmanagers from around the world in a discussion of these guidelines or to pose questions
about the guidelines, go to the MLSN page on
Facebook: www.facebook.com/mlsnturf. For
even more examples of how the MLSN guidelines ft into turfgrass nutrient requirements
and how these requirements can be calculated,
download Understanding Turfgrass Nutrient Requirements at: http://calendar.asianturf
grass.com/understanding_turfgrass_nutrient_requirements_5june2012.pdf.

Acknowledgments
We gratefully acknowledge the hundredsof golf course superintendents who have submitted soil samples over the past 20 years, andwhose soil test results were the basis for development of the MLSN guidelines.

Literature cited
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Clarifying soil testing: III. SLAN suffciency ranges
and recommendations. Golf Course Management72(1):194-198.
2. Dest, W.M., and K. Guillard. 2001. Bentgrass
response to K fertilization and K release rates fromeight sand rootzone sources used in putting green

construction. International Turfgrass Society Research
Journal 9:375-381.
3. Fulton, M. 2002. Creeping bentgrass responses tolong-term applications of nutrients. Golf Course Management 70(2):62-65.
4. Kreuser, W.C., P.H. Pagliari and D.J. Soldat. 2012.
Creeping bentgrass putting green Mehlich-3 soil test
phosphorus requirements. Crop Science 52:13851392.
5. Kussow, W.R., D.J. Soldat, W.C. Kreuser and Steven
M. Houlihan. 2012. Evidence, regulation, and consequences of nitrogen-driven nutrient demand by
turfgrass. Online. ISRN Agronomy Vol. 2012, Article ID
359284. doi:10.5402/2012/359284 (www.hindawi.
com/isrn/agronomy/2012/359284/). Verifed Dec. 9,
2013.
6. Raley, R.B., P.J. Landschoot and J.T. Brosnan. 2013.
Infuence of phosphorus and nitrogen on annual bluegrass encroachment in a creeping bentgrass putting
green. International Turfgrass Society Research Journal
12:649-655.
7. Stowell, L., and M. Woods. 2013. Minimum levels forsustainable nutrition. Proceedings: Constructed Rootzones 2012. Online. Applied Turfgrass Science (www.
plantmanagementnetwork.org/pub/ats/
proceedings/2013/rootzones/8.htm). Verifed
Dec. 9, 2013.
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Micah Woods (micah@asianturfgrass.com) is chief scientist at the Asian Turfgrass Center and an adjunct assistantprofessor in the department of plant sciences at the University of Tennessee, Knoxville. Larry Stowell and Wendy Gelernter are the principals of Pace Turf LLC, San Diego, Calif.