A 7 part series about soil sampling for turfgrass
Micah Woods
9 February 2020
Part 1After I read the article by Lawrence et al. on Guiding soil sampling strategies usingclassical and spatial statistics: A review, I made some calculations, sent a few email
inquiries, looked up some standard recommendations for turfgrass testing, reread another article on this topic, sent out a Twitter poll, and started drafting a blog post.
I soon realized that it might be a long blog post. Too long, probably.1 And the
topic of soil sampling and the number of cores (or subsamples) to collect to then put
together as a single composite sample (Figure 1) representative of the tested area—
that’s a particularly dry subject, isn’t it? So I decided to break that post I’d drafted into
a number of short ones.
In these posts, I’ll be considering soil nutrient analyses. I’ll often use individual
putting greens as an example.
A standard recommendation seems to be what I found in the Soil Sampling Instructions for Turfgrass from Rutgers.
For testing of a single putting green, they recommend a composite sample be madeup of 12 to 15 subsamples.
The instructions read,“Plan to collect multiple subsamples randomly within each defined area toobtain a representative sample. For a uniform sports field or fairway, 20 to30 subsample locations may be needed, whereas 12 to 15 subsamples may
be adequate on a golf green or tee … placing the soil subsamples in a clean
plastic bucket to obtain a composite sample.”

Part 2The instructions from Rutgers are to take from 12 to 15 samples from a single putting
green. I like to check Penn State recommendations too—their websites are a wealth1This document is the 7 blog posts from www.asianturfgrass.com, put together in order.

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Figure 1: How many cores should be taken from a single putting green, or sports field,
or fairway, or lawn, to put together as a composite sample for soil nutrient analysis?
of reference materials. A good starting point is the Professional Turfgrass Management page with its long list of documents. My favorite from that page is Turfgrass
Fertilization: A Basic Guide for Professional Turfgrass Managers.
On the soil sampling instructions page, the instructions for turf soils are:
“Using a soil sampling tube, auger or trowel, and a clean pail, obtain thinslices or borings of soil from 12 or more locations.”
I’m not sure where the 12 comes from, but Penn State recommends a minimum of
12 subsamples be put together as a composite sample for an area, and so does Rutgers.
This seems to be the standard recommendation.

Part 3
There’s an article by Donohue, Evaluation of soil nutrient variability for developmentof turfgrass soil test sampling methods, that describes results from sampling a 2,000m2 (21,520 ft2 ) lawn at the Virginia Tech Turfgrass Center Laboratory.
221 possible sampling locations were identified, on a 3 m (9.8 foot) grid pattern
across this area (Figure 3). I’ll now give some quotes directly from the article:“To determine the optimum number of subsamples required for a representative composite sample for the turf area, the overall landscape was2

Figure 2: Penn State’s websites contain a wealth of reference materials about turfgrassmanagement.

Figure 3: Figure 3 from Donohue (2002) showing Mehlich 1 phosphorus (P) test results;
the units are mg P/dm3 (mg/L).

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first evaluated to identify unusual spots that one should avoid when sampling. This evaluation was made after the grid was marked off, but before
samples were actually collected.”
Donohue then excluded some areas near a road that may be affected by salt application, areas near a flower bed that may have had fertilizer overthrow, and steeply
sloped areas and low areas.
“This left a total of 163 spots for the subsampling study.”“Soil test results from the 163 locations within the uniform area were entered into the Virginia Tech mainframe computer for evaluation. For determining the optimum number of subsamples for a representative composite
sample, 1, 5 10, 15 and 20 subsamples/composite sample were chosen forevaluation. For each subsample number, five separate composite samples
were ‘collected’ by the computer and compared to determine variation in
test results.”
In short, random samples were drawn from the 163 soil tests (for n = 1, 5, 10, 15, and20), then these random samples were averaged as if they were a composite sample, andthe means of those random samples and their variation were compared to the known
mean of all 163 soil tests.
And here’s the final result.
“Within the uniform area selected in the study, the number of subsamples
required for a representative composite sample varied from five subsamples for soil pH to 20 or more for P. Based on the results of this study, 20
subsamples per composite is suggested for a representative sample from a
1,000—2,000 m2 turfgrass area.”Part 4I’ve recommended taking at least 5 subsamples per green, for as long as I can remember.
That’s less than the 12 to 15 recommended by Rutgers, less than the 12 or morein the Penn State instructions, and way less than the 20 subsamples per composite
recommended by Donohue.
The main reason I’ve been comfortable with 5 subsamples per green is the consistency of the rootzone material (Figure 4) on most putting greens. And there is also thethought that I’m collecting soil samples to do soil nutrient analyses, with those results
to be used to make a fertilizer recommendation. The fertilizer recommendation will
be for the course; I rarely make fertilizer recommendations for individual greens. So
I’ve been comfortable having smaller numbers of subsamples per green, realizing that4

Figure 4: Spreading a uniform rootzone mix when building a putting green at
Daitakarazuka Golf Club.
doing so lets me see some of the variability that exists across a property, and at thesame time knowing that the total number of subsamples from the course is probably
going to be 30 or more, because I recommend testing a minimum of 6 greens.
The Donohue article concludes that 20 subsamples are recommended from a 1,000
to 2,000 m2 area. That’s 1 to 2 subsamples per 100 m2 , which works out to a minimumof 5 subsamples per 500 m2 (5,380 ft2 ). That’s about the size of a typical putting green.

Part 5
I’d read the soil sampling instructions from Penn State and Rutgers, suggesting a minimum of 12 subsamples be combined as a composite sample for each area. When I
give instructions for sampling, I suggest a minimum of 5 subsamples be collected per
green (or per area). And the Donohue article I’ve written about suggests 20 samples
be collected from an area about the size of 4 typical putting greens.
I wondered what other people do, so I sent out this poll on Twitter, asking “Whencollecting samples for soil nutrient analysis on golf course putting greens, how many
subsamples do you collect to make a single composite sample for one putting green?”
The results in Table 1, excluding those who answered the survey just to see results,
came to a total of 182 respondents.
It seems a lot of turfgrass managers are following the standard recommendations ofat least 12 subsamples per area, althought more than half the respondents are collecting
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Figure 5: The standard recommendation for turfgrass seems to be a minimum of 12
samples for an area the size of a golf course putting green.

Table 1: Percent of poll respondents collecting this number of subsamples per green
to make a composite sample.
Subsamples Percent
1 to 56 to 1011 or more

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12%
45%
43%

fewer than the recommendation of 12. I was surprised there were so few respondents
collecting 5 or less subsamples—I would have thought more people would do this thefast and easy way.

Part 6
Now this series gets interesting. I’ve reviewed what I do, what a Twitter survey says
people are doing, what Rutgers and Penn State recommend, and what an intensively
sampled lawn in Virginia suggests would be an appropriate number of subsamples tocombine in composite samples for turfgrass.
The article Guiding soil sampling strategies using classical and spatial statistics: A
review by Lawrence et al. recommends doing things differently. In short, they recommend that “soil samples should not be composited.” I recommend reading the fullarticle if you are interested. The article is about testing agricultural fields. It is quite
relevant to turfgrass sites too. Below are some quotes from the article, with notes fromme interspersed. Any italicized or bold text is mine.

They get right to the purpose of soil nutrient analysis in the first sentence sayingthat it is “a key practice to increase the efficiency of nutrient management.” As an aside,
I’ll take the opportunity here to point out that that is how soil testing is supposed towork, but in the turfgrass industry, soil testing seems to actually decrease the efficiencyof nutrient management because of misinterpreted soil tests. But it is easy to fix thiswith a method such as MLSN.
In their review, Lawrence et al. looked at results from other articles, and calculated“an estimate for the range of soil sampling densities that would be required to achieve a
margin of error of 10% of each study’s mean at a 5% precision level.” For K, the mediansampling density is less than 5 samples per hectare; for P the median sampling density
is 8.4 samples per hectare. I think it is interesting to consider these sampling densities
in comparison to what is standard in the turfgrass industry today. To achieve that
aforementioned level of precision with P, for example, one would need on average asingle sample for every 1,190 m2 (12,809 ft2 ). With K, fewer samples would be required.
“Compositing soil samples before analysis is a common method for reducing soil analysis costs … Compositing, which effectively calculates themean of a number of soil cores, is unlikely to represent the true population parameter of interest and will typically present a higher estimate ofnutrient concentration than is accurate … the distribution of a nutrient
would be positively skewed … For fertilizer management, this may cause
the farmer [or the turfgrass manager] to apply an inadequate amountof fertilizer”
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Then in a section on the “Current State of Extension Soil Sampling Recommendations,” this:“54% of extension sources suggested ‘zig-zag,’ ‘Z’, or ‘W’ sampling, with44% mentioning the need to take ‘representative’ samples. This suggestionis at odds with the requirements of a design-based approach in which random selection of locations is paramount. As a result, it is highly likelythat these methods often result in biased results, especially if thesamples are composited before analysis and the soil property is lognormally distributed.”
In the general suggestions and conclusion to the article, more things to think about:“If no prior information on the soil property is available, log-normality
should be assumed, and soil samples should not be composited. If
the collected cores display log-normality, the geometric mean or medianshould be used instead of the arithmetic mean. Only in instances inwhich normality has previously been established should soil samples be composited.”“… it is not surprising that extension recommendations often suggest practices such as compositing and Z-sampling, which do not have a strong
foundation in peer-reviewed literature. Fortunately, there are theoretically simple ways to correct these recommendations, such as suggesting soil cores to be individually analyzed.”

Part 7The summary so far is this:• standard recommendations for turfgrass sampling are to take 12 or more coresfrom each area and composite them into one sample• I’ve been taking 5 cores and compositing them
• Donohue’s research in Virginia led him to recommend taking 20 subsamples andcompositing them
• The article by Lawrence et al. suggests that cores should not be composited
I’ve been wondering if my usual practice of 5 subsamples per green were enough(or too many). Last year I took 30 cores from a 1,092 m2 double green in Bangkok(Figure 6) and tested them individually. That works out to 1 sample every 36 m2 .
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Figure 6: 30 cores were collected from this double green in Bangkok and the cores were
tested individually at Brookside Laboratories.

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Figure 7: 200 simulated means for 1 to 30 cores calculated from the soil test results froma 1,092 m2 green. The lines show a 10% margin above the known mean (calculated fromall 30 cores) and a 10% margin below the mean.
I made some calculations about the number of samples required. I’ll use the Mehlich
3 phosphorus results as an example. These are the soil test phosphorus data for those
30 cores in units of mg/kg.
31 23 29 25 25 29 24 27 26 20 19 30 29 23 24 28 21 24 32 32 22 20 33
24 23 34 24 25 24 21
I took random draws from those results, selecting 1 core, or 2, or 3, all the way up to
30 cores. And from those cores that were randomly drawn, I calculated the geometricmean, as recommended by Lawrence et al. I did that 200 times, and then plotted themeans that were obtained from taking 1 up to 30 cores (Figure 7). This is simulating
composite sampling from 1 core up to 30 cores on the same green.
Looking at the fraction of the simulations that returned a mean within 10% of the
known mean, the fraction goes above 90% (marked with a horizontal dashed line in
Figure 8 below) once there were 6 or more subsamples.
For phosphorus, once 6 cores were composited from this area, the mean soil test P
was within 10% of the known mean more than 90% of the time.
I made the same calculation for other elements, and it was 6 subsamples for K, 5
for Ca, 4 for Mg, and 6 for S.
This green was 1,092 m2 so 6 cores from this area would be 1 core from every 182
2m . Another way to express this is 0.55 cores required per 100 m2 or 0.51 samples
required per 1,000 ft2 . That’s how many samples would be required to get a test result10Figure 8: The fraction of simulated means that were within 10% of the known mean,calculated from 200 random draws from the test data.
within 10% of the true mean of the area, 90% of the time.
The 5 subsamples I’ve been compositing per green is about twice that sampling
density; if nutrient variability within the green I tested in Bangkok is similar to othergreens, then I am probably getting a mean value within 10% of the true mean almost
100% of the time.
The implications of all this include:• a few subsamples per green are probably enough to get a result close to the meanvalue of the green• current recommendations for 12 or more subsamples seem to be excessive, but I
need to check variability on more greens• testing individual cores, rather than composited samples, is probably a betterway to do soil testing11