By James H. Baird
Few would dispute that there are both an art and a science to
growing high-quality turf. However, these days it seems that soil
fertility and turfgrass nutrition practices are becoming less
scientific and more illogical than artistic.
While science continues to move forward, it appears to me that
most of the new theories or so-called
advancements
are professed by companies or individuals who stand to gain by
selling their products or consultation services. Most turf
managers won't hesitate to apply a new product if they
believe that it won't hurt anything and could only help their
situation. Unfortunately, applying the wrong nutrient or too much
of a nutrient can result in deficiencies of other nutrients,
greater potential for disease outbreak due to changes in soil
acidity, or perhaps unfavorable changes in soil physical
properties. Given today's uncertain economy and increased
scrutiny over chemicals applied in the turfgrass environment, all
turf managers need to re-evaluate their fertilization practices
by using science as the foundation upon which personal experience
and
feel
are built.
Soil fertility and plant nutrition are complex subjects, but
they're far from incomprehensible. An article of this length
cannot begin to address all of the basic principles of soil
fertility and turfgrass nutrition. Rather, the objective is to
help simplify several concepts that are critical to ensuring turf
health and both environmental and fiscal responsibility. Emphasis
will be placed on soils and turfgrass nutritional needs in the
Northeast, although the principles will apply more broadly. For
more information, please see the references that follow.
Let's begin our lesson.
TAKE CHARGE OF YOUR SOIL TESTING PROGRAM
Before applying any nutrient, it's important to determine
which ones are deficient and in what amounts. Nutrient
deficiencies, including nitrogen (N), iron (Fe), and phosphorus
(P), are sometimes visually detectable to the well-trained eye,
although quantification of the supplemental amount required is
difficult if not impossible. Tissue testing provides a much more
objective and quantitative evaluation of the nutritional status
of the plant. However, more research is needed to correlate
nutrient levels in tissue with turfgrass response. Tissue testing
is best used as a diagnostic procedure since a plant must be
under nutrient stress for a deficiency to show.
 |
Although far from perfect, soil testing remains the most
common and best method of determining the nutrient availability
to the turfgrass plant since it attempts to identify potential
problems before they occur. Judging by the number of turf
managers who hire soil consultants or the number of times I have
been asked to interpret reports, I gather that many turf managers
are uncomfortable with deciphering soil test results. In the
reference section, several articles address soil testing in one
capacity or another. The four principal components of soil
testing are: 1) sampling, 2) laboratory analysis, 3)
interpretation of results, and 4) recommendations for chemical
changes, if needed.
DON'T UNDERESTIMATE THE IMPORTANCE OF PROPER
SAMPLING
| |  |
| Soil test results are likely to generate
very different results when samples are taken at varying
depths. In the case of a longer soil sample, separate and
analyze the upper sandy portion of the profile separately
from the mineral soil below. |
Improper sampling for soil testing can be one of the greatest
sources of error in soil testing programs. A few things to keep
in mind about soil sampling are: 1) take at least 20 sub-samples
(cores) of a representative area to be pooled, mixed, and sampled
for testing; 2) sample at a uniform depth (e.g., usually 2 to 4
inches for putting greens; 3) if a true thatch or topdressing
layer is present, consider subdividing each core into thatch or
mat and underlying soil to determine chemical and nutrient
properties of each component; and 4) sampling time and frequency
are important for determining consistency of test results and
effectiveness of fertilizer applications. Chemical change
following fertilization can occur within days or weeks in sandy
soils compared to months or years in clay soils. In the Northeast
on sand-based greens or tees, consider sampling in spring, prior
to aeration, and again 6-8 weeks after fertilization with
granular formulations as a follow-up analysis. Sample once again
6-8 weeks following aeration and fertilization in late summer.
BE CONSISTENT WITH LABORATORY ANALYSES
Several university and commercial laboratories are available
for soil sample analysis. Be cautious about analyses and
recommendations that are offered free of charge from fertilizer
manufacturers or turf distributors. Also, it is important to know
that results are likely to vary from laboratory to laboratory due
to different extraction methods and chemicals used for analyses.
See the articles by Carrow et al. (2003 and 2004) that describe
differences among soil analytical procedures. For the sake of
your soil testing program, it is important to choose a laboratory
that uses procedures and nutrient ranges that are appropriate for
the soil types on your golf course. Once that information is
gathered, the important thing is to use the same laboratory year
in and year out to analyze trends in nutrient availability and
deficiencies.
YOU TOO CAN INTERPRET A SOIL TEST REPORT
Interpretation from the laboratory or a consultant aside,
every turf manager should feel comfortable with understanding
soil test results. The following is a description of information
likely to be found on a soil test report in the Northeast.
Soil Acidity or pH
Soil acidity or pH is the negative logarithm of the hydrogen
ion concentration on a scale from 0 to 14, with 7 being neutral
(concentration of hydrogen ions equals hydroxide ions). Table 1
shows a diagram of nutrient deficiencies and other turf problems
that are likely to occur at varying pH levels. In general, soil
acidity at or near neutrality ensures maximum availability of all
essential nutrients in the soil. This pH range favors the
nutrients being in a plant-available form. This is one of the
simplest and most important principles to remember about soil
fertility and plant nutrition.
Lime Requirement
Lime requirement is the quantity of limestone (CaCO
3
) required to raise the pH of an acid soil to a desired level. A
buffer solution is added to the soil to determine buffer pH. The
value itself is not significant to the turf manager, but it is
instead used by the lab to determine liming rate recommendations,
when necessary. The ability to lower pH of alkaline soils with
the addition of sulfur or acid is largely dependent upon free
lime present in the soil, with higher quantities providing
greater buffer capacity against pH change. Thus, it is not
recommended that pH reduction be attempted in soils with even a
low percentage of lime due to the very large acid quantities
required and the potential for turf injury.
SOLUBLE SALTS
Measurement of soluble salts is especially important for
determining salinity on salt-affected soils. Electrical
conductivity (ECe) is reported in units of decisiemens/meter
(dS/m) or millimhos/centimeter (mmhos/cm). An ECe above 4.0 dS/m
is considered saline. The saturated paste extract (SPE) is
considered to be the standard procedure for measuring ECe, sodium
absorption ratio (SAR), and boron (B) concentration. Although not
typically reported on a test in the Northeast, the SAR is a
measure of the potential for excess sodium (Na) to cause
structural deterioration of soil. SAR levels above 12 are
considered problematic for soil and plant health, whereas ideal
levels should be 3 or lower. If soil tests reveal problems with
soluble salts or Na, it is important to have the water source
tested and seek help from a qualified consultant or university
specialist.
ESSENTIAL NUTRIENTS
Laboratories use chemical extractants to estimate the levels
of soil nutrients that are readily available to plants. Values
are reported in parts per million (ppm) or pounds per acre
(lbs/A). In addition, most labs will categorize each nutrient in
terms of availability to the plant from below optimum to above
optimum, or very low to very high. This method is referred to as
the sufficiency level of available nutrients (SLAN), which
attempts to correlate plant response to extractable soil
nutrients. Although it could be said that there are limited data
directly correlating soil nutrient levels with specific and
desirable responses of all of the turfgrass species, overall SLAN
has been the most tried and true method for estimating
plant-available nutrients.
 |
Remember, the numbers that you see on your report and the
associated sufficiency levels are based upon factors such as type
of extractant used and the specific sufficiency index chosen for
interpretation. The articles by Carrow et al. (2003 and 2004)
contain information about what are considered medium ranges for
various nutrients based on the extractant used. It is possible
that the recommended range provided in your report is so high
that almost every situation would indicate fertilizer need. It is
all right if a lab uses a slightly different range as long as it
brackets the ranges provided in the articles. Your decision,
whether or not to apply fertilizer based on these results, should
take into account the likelihood for nutrient deficiencies to
occur in your situation (see Table 2) as well as existing
turfgrass health and performance.
Cation Exchange Capacity and Base Cation Saturation
Soils have a net negative charge, which attracts positively
charged ions. Thus, cation exchange capacity (CEC) is a measure
of the amount of cations that a soil can hold at a given pH that
are potentially exchangeable for plant uptake. CEC is often
expressed on a weight basis as milliequivalents (meq) per 100
grams of dry soil or centimoles per kilogram (cmol/kg). A 100 g
sample of soil with a CEC of 1 meq (considered very low) contains
6.02 ´ 10
20
(602,000,000,000,000,000,000) negative charge sites. Without
other information about a sample, knowledge of the CEC can
provide some indication of the soil texture. Sands with low
organic matter by weight (1-2%) typically have very low CEC
values ranging from 1-3 cmol/kg, whereas most clay or clay loam
soils are 20 cmol/kg or greater.
The CEC is the sum total of basic or base (K
+
, Ca
+2
, Mg
+2
, and Na
+2
) and acidic (Al
+3
and H
+
) cations. The amount of each listed in the report, divided by
the CEC, is the saturation of that ion. It appears that a
majority of turf agronomic consultants (excluding the USGA Green
Section and university scientists) subscribe to the Basic Cation
Saturation Ratio (BCSR) theory for interpretation of soil test
results and fertilizer recommendations. The theory is based upon
having a base saturation of 80% comprised of 65% Ca, 10% Mg, and
5% K. Fertilizer recommendations are made to attain not only
these percentages, but also desired balances between any
combinations of the nutrients. Having listened to presentations
by those who purport this "feed the soil" theory, I am
not surprised that a significant number of turf managers buy into
this theory, as it is an impressive display of pseudoscience and
salesmanship.
Unfortunately, the BCSR theory is largely unfounded, and those
who attempt to balance soil cations on a routine basis are simply
wasting their time and the club's money. To be more specific,
subscribing to the BCSR theory will likely lead to the following:
1) Increased fertilizer recommendations and usage that are not
necessary relative to the SLAN method. 2) Raising base
saturations in sand-organic matter soils to near 80% can result
in a significant increase in soil pH, which may lead to other
problems such as greater incidence of take-all or summer patch
diseases. 3) When relying on percentages rather than quantities
of nutrients present in the soil, it is possible to have a
sub-optimum percentage of a basic cation such as K
+
but sufficient levels of extractable K
+
or vice versa. 4) The theory often overestimates soil Ca and
underestimates soil CEC in greens or other areas containing
calcareous sands or after continuous irrigation with Ca- and
Mg-rich water. 5) It usually results in over-application of one
base cation, which in turn depletes the availability of the
others. Overall, Ca and Mg deficiencies are rare in plants except
in unusual circumstances (Table 2).
Until recently, the BCSR theory has not been tested on
turfgrass. However, research conducted thus far further
substantiates the lack of validity of the theory. When
appropriate amounts of basic cations are applied, based on
sufficiency data, the percent levels of cations adjust naturally
according to soil type. Does all of this mean that the CEC and
base cation saturation data should be ignored? Not necessarily.
This information can be useful for managing salt-affected soils
(i.e., high Na) and as a supplement to sufficiency levels to help
determine and evaluate fertility programs.
Soil Nitrogen
Your soil testing laboratory may or may not report tests of
soil N because most forms of this nutrient fluctuate too rapidly
in the plant-soil system to be accurate and reliable predictors
of available N. However, there is hope on the horizon with
utilization of the Illinois Soil Nitrogen Test. The test, which
predicts a more stable amino form of N, has been developed for
use in production agriculture and currently is being used to
predict either N fertility needs for turfgrass, or identify
turfgrass areas that have increased potential for nitrate
leaching if N fertilizer is applied. In the meantime, fertilizer
recommendations for N are based on turf response and are adjusted
by the turf manager depending on factors such as turfgrass
species composition (e.g.,
Poa annua
versus bentgrass), traffic, disease susceptibility, and
environmental stress conditions.
ROOTS ARE THE PRIMARY SITE OF NUTRIENT UPTAKE
 | |
| Sometimes it can be difficult to
differentiate between a nutrient deficiency and a disease or
insect problem. Examine the turf thoroughly. In this case,
damage from the annual bluegrass weevil caused yellowing of
the turf. |
These days I hear a lot about foliar nutrient applications and
products touted as being truly foliar in function. While
nutrients can be taken up by shoots, primarily through
trans-cuticular pores, let's not forget that foliar uptake of
nutrients is minor compared to the effectiveness of the root
system. When you think about it, the leaf is engineered to absorb
light and prevent water loss. Factors that are likely to limit
foliar uptake include cuticle thickness, rapid drying before
uptake, removal by mowing or precipitation, and volatility. Last
but not least, true foliar feeding requires a low volume of water
(<1 gallon per 1,000 ft
2
) for retention of spray droplets in the foliage; conversely,
most turf managers that I know use higher sprayer carrier volumes
to distribute turf protectants deeper into thatch or the
underlying rootzone.
There is no doubt that light and frequent nutrient application
is important in turfgrass nutrient management, especially on
putting greens and other intensively managed areas. Call it
semantics, but the term liquid fertilization would better
describe the practice whereby nutrients are sprayed on the
foliage, since uptake can occur by both shoots and roots. The
bottom line is, how much are you spending for your "true
foliar" fertilizer?
NITROGEN UPTAKE
Nitrogen is taken up by the plant primarily in the forms of
ammonium (NH
4+
) and nitrate (NO
3-
) ions and to a lesser extent as urea, which are then assimilated
into amino acids and other important N compounds for growth and
metabolism. The question then becomes, is it better or more
efficient for plants to circumvent this process and absorb amino
acids directly? Although uptake of amino acids is possible, my
search of the literature revealed only a scant reference to amino
acid uptake by arctic sedge! Yet again I pose the question, how
much are you spending for products containing amino acids and
other biostimulants? More research and product testing are needed
to justify both the cost and efficiency of supplying nutrients to
turf using products like these.
GET THE MOST OUT OF LATE-SEASON FERTILIZATION
| |  |
| Disease or over-application of
fertilizer? The granules tell the story. |
Late fall, or what some call "dormant" fertilizer
applications, are typical on cool-season turf in northern,
temperate climates. The ultimate goal of late fall fertilization
is to supply N to the plant for carbohydrate storage, which can
enhance stress tolerance and early spring root growth. Additional
benefits include early spring greenup and reduced need for early
spring fertilization, which can further enhance shoot growth and
increase mowing frequency. Since soil temperatures remain warmer
than the air in the fall, roots are capable of taking up
nutrients even though shoot growth has essentially ceased. At the
same time, photosynthesis can still be active. Thus, proper
timing is achieved between the time of the first hard freeze and
continuous snow cover or ground freezing when true plant dormancy
occurs.
Slow-release forms of N, including natural organics, are
commonly applied in the late fall to avoid an unwanted flush of
growth in the unlikely event that temperatures rise to above
normal. Unfortunately, depending on the carrier, much of the N is
not likely to be available to the plant until the following
spring, which defeats the purpose of promoting root rather than
shoot growth. Furthermore, N may be lost in runoff or leached
into groundwater.
 | |
| Liquid application can be an effective
turf fertilization method, but be skeptical of claims that
hype foliar uptake when root uptake is more common. |
It would be better to apply soluble, readily available forms of N
such as ammonium sulfate to ensure maximum root uptake and
carbohydrate storage in late fall. If slow-release N sources are
to be used, then application should be timed earlier in the fall,
when warmer temperatures permit availability and root uptake.
Less than 1.0 pound of N per 1,000 ft
2
applied when the turf is able to take up and utilize N will help
to avoid potential losses due to leaching or runoff. There is
little evidence that late fall application of N contributes to
low-temperature injury of cool-season turfgrasses as long as
proper rates and timing are followed. On the other hand, late
fall N fertilization may enhance snow mold activity on turf
without a preventative fungicide application; however, the added
N can also help to hasten turf recovery from disease or other
winter damage.
POTASSIUM FERTILIZATION: MORE IS NOT ALWAYS BETTER
In addition to its role in important physiological processes,
K also influences tolerance to drought, cold, high temperature,
wear, and salinity stresses. We also associate the term
"luxury consumption" with K, in that tissue levels
adequate for stress tolerance may be above what is considered
sufficient for growth. Knowing this, it appears that some turf
managers have adopted the "more is better" approach and
apply 2-3 or more times more K than N on an annual basis. With
the exception of situations involving salt-affected soils and
salt-tolerant species, research has demonstrated optimal
turfgrass stress tolerance when soil K is maintained in the
sufficient range. Remember that excessive K can contribute to
salinity stress; suppress Mg, Ca, or Mn uptake; and promote
greater incidence of snow mold diseases.
SUMMARY
Soil fertility and turfgrass nutrition can be daunting
subjects to many turf managers. I hope this article has helped to
clarify and simplify key principles and practices, and has
empowered you, the turf manager, to take charge of your turfgrass
nutrient program. It doesn't require a lot of money or
guessing to meet the nutritional needs of your turf. Let science
be your teacher.
REFERENCES
Carrow, R. N. 1995. Soil testing for fertilizer
recommendations. Golf Course Management. 63(11):61-68.
Carrow, R. N., D. V. Waddington, and P. E. Rieke. 2001.
Turfgrass soil fertility and chemical problems: Assessment and
management. Wiley, Hoboken, N.J.
Carrow, R. N., L. Stowell, W. Gelernter, S. Davis, R. R.
Duncan, and J. Skorulski. 2003. Clarifying soil testing: I.
Saturated paste and dilute extracts. Golf Course Management.
71(9):81-85.
Carrow, R. N., L. Stowell, W. Gelernter, S. Davis, R. R.
Duncan, and J. Skorulski. 2004. Clarifying soil testing: II.
Choosing SLAN extractants for macnutrients. Golf Course
Management. 72(1):189-193.
Carrow, R. N., L. Stowell, W. Gelernter, S. Davis, R. R.
Duncan, and J. Skorulski. 2004. Clarifying soil testing: III.
SLAN sufficiency ranges and recommendations. Golf Course
Management. 72(1):194-197.
Chapin, F. S. III, L. Moilanen, and K. Kielland. 1993.
Preferential use of organic N for growth by a non-mycorrhizal
arctic sedge. Nature. 361:150-153.
Gardner, D., and B. Horgan. 2006. 2006 Turfgrass and
Environmental Research Summary. p. 15.
Happ, K. A. 1994. Tissue testing: Questions and answers. USGA
Green Section Record. 32(4):9-11.
Happ, K. A. 1995. Sampling for results: The methods are
important. USGA Green Section Record. 33(5):1-4.
Kopittke, P. M., and N. W. Menzies. 2007. A review of the use
of the base cation saturation ratio and the "ideal"
soil. SSSAJ. 71(2):259-265.
Kussow, W. R. 2000. Soil cation balance. The Grass Roots.
29(2):58-61.
Marschner, H. 1995. Mineral nutrition in higher plants.
Academic Press, New York, N.Y.
Skorulski, J. E. 2001. Unlocking the mysteries: Interpreting a
soil nutrient test for sand-based greens. USGA Green Section
Record. 39(1):9-11.
Skorulski, J. E. 2003. Digging deeper into soil nutrient
testing. Tee to Green. 33(1):3-5.
Skorulski, J. E. 2003. Micro-managing. USGA Green Section
Record. 41(5):13-17.
St. John, R., and N. Christians. 2007. Basic cation ratios for
sand-based greens. USGA Turfgrass and Environmental Research
Online. 6(10):1-9.
Taiz, L., and E. Zeiger. 1991. Plant physiology.
Benjamin/Cummings. Redwood City, Calif.
Woods, M. S. 2006. Nonacid cation bioavailability in sand
rootzones. Ph.D. dissertation. Cornell University, Ithaca,
N.Y.
Thanks to Drs. Robert N. Carrow, University of Georgia; Paul
E. Rieke, Michigan State University; and James A. Murphy,
Rutgers University; for their assistance.
Jim Baird is a Green Section agronomist in the Northeast
Region, where he visits golf courses in Connecticut, New
Jersey, New York, and Ontario, Canada.