Bill Kreuser gave a webinar about soil tests for TurfNet, putting a lot of clear information into this one hour discussion, and closed it out by explaining how he would go about making use of soil tests.
If you are interested in this topic, and would like to know more about soil tests and how the data are generated and the results can be interpreted, you'll really enjoy this webinar. Bill has plenty to say about the right way and the wrong way to do this. Some test results are nonsense. As Bill put it, "you can grow really great turfgrass without dealing with this kind of craziness."
3. My approach to turfgrass nutrition specifically, and turfgrass management generally, is that it is simple, but not easy; if we can identify the principle factors influencing turfgrass performance, and modify them, we will be making great progress toward our goal of producing the desired turfgrass surfaces. When it comes to turfgrass nutrition, I suggest making sure that soil nutrient levels are kept above the MLSN guideline levels.
We are very open about how we develop these guidelines, which data are used, what our assumptions are, and we even share the code we use to generate the guidelines. We study this topic intensively, and we only recommend or suggest nutrient application when we are confident it will have an effect on turfgrass performance. This approach reminds me of these comments by Richard Dawkins on facts vs. opinion:
I don't give a damn for anybody's opinion; I only care about the facts. So I'm not an enthusiast for diversity of opinion where factual matters are concerned.
5. I've been writing here since January of 2009, and I've never mentioned BCSR. I make an exception today, to share this point by Max Schlossberg (with counterpoint from Joel Simmons) about how soil tests are done and interpreted and how fertilizer recommendations are made.
Max has an account on Twitter now. I'm excited about that because he has great insight to share about turfgrass nutrition. And a lot of other things too, but I'm especially counting on him to correct me when I err.
6. Turf nutrition doesn't have to be that complicated. Turf maintenance in general doesn't have to be that complicated. You can think it is complicated if you want to. I take the approach that we try to modify the growing environment of the turfgrass to create the desired playing surface, and we take special care to control the growth rate of the grass. If we do those things, which in principle are simple, but which in practice may require a lot of work, we achieve the desired turfgrass conditions.
I am not going to rewrite all on these topics here. If you are interested, read more of the other posts on this blog. But please do contrast the simplicity of the approach I describe, and the MLSN approach, to the more complicated description here:
And it continues here:
Actually, those complicated descriptions of nonsense weren't about turfgrass fertilizer. But you may have heard or read descriptions of turfgrass products and maintenance that were approaching that level of complexity. It really shouldn't be like that.
7. Everyone probably knows about Occam's razor. I had a great discussion about this with John Bladon last month. If you don't know what this is, you can read about it here.
I have a hypothesis about this. I wrote it on Twitter in response to Adam's question, but I got it slightly wrong, in that I wrote max DLI when I meant max PPFD -- or something like that. Here's my hypothesis.
The most important time for full sunlight to reach a turfgrass surface corresponds to the optimum temperatures for photosynthesis for that species. More specifically, the longer the duration of time at which the PPFD on the turf surface is near the maximum the grass can use, while at the same time the air temperature is close to an optimum for photosynthesis, will optimize turf performance.
Turf grows best in full sun. But full sun is a concept -- the reality is there are clouds, trees, buildings, mountains, and whatever other causes of shade that exist. My hypothesis would predict that morning sun, when temperatures are cool, would be better for cool-season grass in the heat of the summer, when afternoon temperatures are better for photorespiration than for photosynthesis. My hypothesis also predicts that morning sun is less important for warm-season grass in hot weather, because it is the afternoon sun combined with high temperatures that produces the maximum carbohydrates for those species.
I've got lots more to write and say about this. For a couple of things you can look at now, if you are interested in this topic:
The most common question I receive is some variation of this: "What about nutrients in the soil that are not available?"
That question throws me for a bit of a loop, because the very purpose of soil testing is to find the availability of nutrients. Let me try to answer this question three times, in progressive order of complexity.
1. "What about nutrients in the soil that are not available?"
Answer: Soil tests already take availability into account. So one doesn't need to consider the term "availability." Just look at the number on the soil test and compare it to the guideline value that you choose. I think the most accurate values to compare to are the MLSN guidelines.
2. "What about nutrients in the soil that are not available?"
Answer: Marschner's Mineral Nutrition of Higher Plants has a chapter conveniently entitled Nutrient Availability in Soils. It begins:
The most direct way of determining nutrient availability in soils is to measure the growth response of plants by means of field plot fertilizer trials. This is a time-consuming procedure, however, and the results are not easily extrapolated from one location to another. In contrast, chemical soil analysis -- soil testing -- is a comparatively rapid and inexpensive procedure for obtaining information on nutrient availability in soils as a basis for recommending fertilizer applications.
Soil testing already provides information on availability.
3. "What about nutrients in the soil that are not available?"
On sand putting greens, it is. The problem caused by sodium is a reduction in the downward movement of water in soils. This is caused by the deflocculation of clay in the soil. It is a real problem in soils with appreciable amounts of clay, and in those soils, an exchangeable sodium percentage (ESP) of 15% or more is indicative of potential problems. The solution? Add gypsum to reduce the ESP, and add water to leach the sodium.
In this experiment, they constructed 6 sand rootzones, with 5 with amendments, and 1 without. The sand was mixed with each amendment in a 4:1 ratio by volume -- 4 parts sand, 1 part amendment.
Sand + peat humus
Sand + Profile
Sand + sphagnum peat
Sand + silt loam
Sand + loam
Then, "the soil cores were placed in plastic tubs and allowed to equilibrate for 48 h in a range of solutions of differing ratios of sodium chloride, calcium chloride, magnesium chloride, and potassium chloride." These solutions ranged in sodium adsorption ratio (SAR) from 0 to infinity, including two solutions with high sodium (37 mmol/L and 185 mmol/L, respectively) and no calcium, magnesium, or potassium.
After these equilibrations in solutions of different SAR, each of the soils had some cores with ESP < 15%, and some with ESP > 15%. How did the sodium influence the saturated hydraulic conductivity (Ksat) of the soils? It didn't do much, except for the sand mixed with loam, which had a clay content by weight of 4.8%. In the unamended sand, sand mixed with peat, or Profile, or silt loam, increasing ESP did not reduce Ksat. In fact, in the unamended sand, the Ksat actually increased after the soil was equilibrated with high SAR solutions.
These are some key results:
In the case of sand-based golf course putting green root zones, which often have very low clay contents, increasing ESP well above the standard sodicity threshold of 15 had no effect on Ksat.
The application of soil amendments for remediation of sodic soils (e.g., gypsum) would only be warranted for sodic soils with higher clay contents and may not provide significant infiltration benefits to sand-based golf course putting greens.
This study also provides evidence that increasing exchangeable Mg2+ [magnesium] does not affect Ksat of sand root zones.
For more about imaginary problems in turf maintenance, see:
... should be removed at a given mowing," reads the Lawn Management Through the Seasons guide from the Penn State Center for Turfgrass Science. "Thus, if the turf is cut at two inches, it should be mowed when it reaches a height no greater than three inches."
Breaking the one-third rule on bermudagrass in Vietnam
Run a Google search on "one third rule mowing" and you will get pages and pages on this "rule." From Cornell University, this explanation:
After raising your cutting height, the next most important thing you can do is to observe the “One-Third Rule” when mowing: never remove more than one-third of the grass blade. That means if your mowing height is 3 inches, you need to mow when the grass is about 4.5 inches tall.
I read this in its entirety last weekend, and was struck by the way the percentage removal was done. It was not by length of leaf. The percentage of cutting was determined as a percent of the verdure volume, not leaf length or plant height. I really liked the paper, and was interested to learn about the root blacking technique and the absolute cessation of root growth that occurred with certain defoliation treatments.
Most interesting to me were the data and discussion on just how much the root system is decreased by mowing. There is also a nice photo and discussion of root system size when grass plants are mown at all, versus left "unmolested" for a growing season:
The roots of the clipped plants [clipped 3 times over 247 days for cool-season species; 2 to 4 times per 146 days for warm-season species] weighed only one-eighth as much as the roots of the unclipped ones. This striking difference in root production by clipped and unclipped plants was manifest as well in the development of the plants as a whole. Compared with [sic] unmolested plants, the mature, clipped specimens were greatly lacking in size and vigor."
I like the thought of trying not to cut grass too short, and trying not to remove too much of the leaf at one mowing. But if the grass must be cut a different way at times, then go for it. Doug Brede's Turfgrass Maintenance Reduction Handbook has a great section on the one third rule in which he explains just how absurd it is, calling "an absolute like the one third rule ... strangely out of place" in a discipline like turf management that usually "deals in shades of gray."
And Brede has a fine replacement for the one third rule too.
So what can we use in place of the one-third rule? What general guideline can be employed to govern mowing frequency?
How about the plugged-up mower rule: "If your mower plugs up when you're mowing, you let it grow too tall." This guideline makes more sense for the turf caretaker who's battling practical limitations of budget, equipment, labor, and weather. This guideline also allows added flexibility for managing low maintenance turf.
The Crider paper on root growth and defoliation is interesting but one can read it and realize a few things:
It is about forage grass more than turf.
It does not measure turf or surface performance, rather it looks at root growth.
It was not based on mowing height and cutting a percentage of leaf length; it was based on grass allowed to grow for two months and then cut to different percentages of verdure volume.
For more about mowing, original research, dogma, and the one third rule, see:
Gelernter et al. wrote about the Global Soil Survey in the December 2014 issue of GCM. The article describes what we are doing with this exciting project, why we are doing it, what is innovative about this, and who is involved. Read the article to get the full details and links to all the info.
Here's a quick summary.
Conventional nutrient guidelines for turfgrass are too high. This explains.
This project is developing new guidelines that more closely match the nutrient levels required to produce good turf. Here they are.
This is an open science project, with the data, code, and results all shared. Here they are.
"These guidelines would literally not exist without the active participation of the turf community." Here's the participant list for the Global Soil Survey.
I use this almost every day. When I am studying something new, I may access the Turfgrass Information File database multiple times per day. When I was a student, I had access through the university. One of the first things I did when I graduated was to purchase a lifetime subscription, to ensure I would always be able to access the information in this database.
You probably already have access, because if you are a turfgrass professional, you really should be a member of at least one of these organizations that provide access to the database as part of their membership benefits package.
For this week only, even if you are not a subscriber to the database or a member of a cooperating professional organization, you can experience the database full of turfgrass information to celebrate its 30th anniversary. See the details at the login page.
And not everyone realizes this, but much of the information indexed in the database, including full text turf books, a searchable database of every Green Section Record article, and much more, are available free whether one is a subscriber or not.
I've always liked this paper. It is about N, P, and K applied or withheld to kentucky bluegrass (Poa pratensis) grown in a loam soil and in a USGA sand rootzone. I was reviewing it recently in advance of an upcoming presentation about turfgrass nutrient use. As turfgrass papers go, this one is especially full of useful data. And it is open access, so you can download and read the full paper.
Here are a few highlights from this paper.
At the start of the experiment, the loam had P and K of 68 and 63 ppm, respectively, by the Mehlich 3 extractant. The sand had P and K of 58 and 39 ppm.
"Added P had no significant effect on clipping yield and underground turf biomass in both sites."
"Despite the low initial soil K levels ... , clipping yield and underground turf biomass showed no significant response to K addition in both sites."
"Potassium showed no significant main effect on shoot density or foliage colour."
I suggest having a look at Figure 2 in the paper, which shows how N rate has a large effect on turf shoot density and turf color, but adding more P and K has negligible effects.
Note that the K in the control plot in the sand would have been below the MLSN guideline for K very early in the study, and yet the addition of K fertilizer had no effect, except to decrease the color of the turf at the highest N application rate. This is another indication that the MLSN guideline for K is not an absolute minimum, but it is a level that if one stays above, there is a high level of assurance that turf quality can be optimized, with no risk of K deficiency.
Bill Kreuser has written an interesting article about the ideal fertilizer ratio for turfgrass. You will want to read the entire article, which discusses turfgrass nutrient demand, nutrient uptake, soil nutrients, and suggested nutrient ratios for various turf situations. Here are a few highlights:
We first need to recognize that nutrient uptake is controlled by plant nutrient demand and not fertilizer applications.
Fertilizing with P and K is a waste of resources when the soil test reports indicate nutrient levels are already adequate.
Think of the soil as a nutrient bank. When fertilization exceeds plant nutrient demand and other mechanisms of nutrient loss (leaching, denitrification, fixation) then soil test nutrient levels increase. Likewise, soil test nutrient levels will decline when plant uptake and nutrient loss exceed fertilization.
Uptake of all soil nutrients is dependent on turfgrass growth rate ... Since turfgrass is chronically N deficient, N fertilization promotes leaf growth and increases demand for other nutrients.
I encourage you to compare your Mehlich-3 soil test results with the Minimum Levels for Sustainable Nutrition (MLSN) guidelines generated by PACE Turf and the Asian Turfgrass Center. Briefly, if soil test P is much greater than 21 ppm and K is much greater than 37, then there is little reason to apply anything other than straight N.
Try to use the diversity of fertilizer ratios to your advantage.