Previous month:
September 2014
Next month:
November 2014

October 2014

"Available" calcium, soil pH, and fearmongering

I did an experiment in a greenhouse in which I grew creeping bentgrass in four different sands. I collected all the clippings and measured what was in them. And I tested the pH of the sands, and I did soil tests to measure the soil nutrient content.

Penn A-1 creeping bentgrass was grown from seed in four different sands. Leaf tissue was analyzed for nutrient content, and soils were tested for pH and nutrient content.

There is almost always enough calcium in the soil, even in sand rootzones with low CEC, to provide all the calcium the grass requires. Tom Margetts asked a good question about "available" calcium, especially in calcareous soils. Calcareous soils have free calcium carbonate. That is, they have some amount of calcium carbonate in the solid phase. And one sometimes hears that the calcium in calcareous soils is abundant, but isn't available. That is a complete misunderstanding of the situation.

Obviously the grass roots don't take up calcium (or any element) from the solid phase. The roots take up elements that are in soil solution. The idea that calcium in calcareous soils is somehow unavailable to grass is ridiculous fearmongering.

The reason there is solid phase calcium in calcareous soil is because the soil solution is saturated with calcium. In fact, it is often super-saturated, having more calcium in solution than would be expected by thermodynamic calculations. Simply put, there is plenty of calcium in the soil solution of calcareous soils.

Data from the experiment with the four different sands shows what we can expect in turfgrass soils. These next three charts show that the calcium availability was highest in the high pH (calcareous) sand.

The amount of calcium in the leaves increased with increasing soil pH.
The amount of calcium extracted by a 1:5 soil:water extract also increased with soil pH, and was highest in the calcareous sand.

And with increasing amounts of calcium in the soil, there was an increase in leaf tissue calcium.

"If you want to use soil test results to develop a fertilizer program, use a different extraction method"

  1. Water_saltIs water (or a saturated paste extractant, or mixing irrigation water with soil) a good way to look at soil nutrients?
  2. What about two tests to look at "available" and "exchangeable" nutrients, is it good to look at both?

The quick answer to both of those questions is no.

The saturated paste extraction is used for measuring soil salinity (ECe). 

I think water extractions, whether with a saturated paste, one part soil with two parts water (1:2 extraction), or one part soil with five parts water (1:5 extraction), are quite interesting and informative for research purposes. But water extraction results are not useful as a decision making tool in turfgrass maintenance.

Carrow et al. wrote about this in Clarifying soil testing: 1. Saturated paste and dilute extractants. They explained that the "saturated paste extraction is not the best method for determining soil fertility levels and can be very misleading."

I wrote about this in Water-based Extraction Methods for Turf Soils. At the time I wrote that article, I was a graduate student, doing lots of research about extraction methods, and I appreciated water as an extractant a bit more then than I do now. It is great for research into soil nutrients. But "it is not possible to take the numbers and decide that they are low enough to justify fertilizer applications ... If you want to use soil test results to develop a fertilizer program, use a different extraction method."

Water use and growth

Turfgrass water use is a real thing. The temperature-based growth potential (GP) is not real in the same sense. GP is a value between 0 and 1 that gives an indication of the potential for the grass to grow, based on how close the actual temperature is to the optimum temperatures for photosynthesis and shoot growth for that species. 

There are many practical uses of GP. I think GP is more useful than water use, or evapotranspiration (ET) as an estimate of water use, in the planning and prediction of turf nutrient use. 

But the water use is important. Nutrients go into the roots with water, so when lots of water is flowing through the plant, there will be more nutrients being used too. In winter, for example, dormant turf won't be using water, and thus there won't be any nutrient use. This has important implications for late autumn N fertilizer and how much of it is utilized by the grass.

As grass grows more, it uses more water. Or, as more water is used, the grass grows more. I'm not sure which is cause, and which is effect, but there is a definite connection between water use and growth. An experiment with grass in Thailand is illustrative on this point.

Bermudagrass, seashore paspalum, and manilagrass were grown in a plastic house in Thailand for 48 days. The water use and the clipping yield of the turf were measured.

This was a fertilizer experiment, with the same amount of N but different amounts of K applied as treatments. Looking at the water use and clipping yield, one can see that with more water use, there was more growth. Or with more clipping yield (growth), the grass used more water. This has obvious implications for how many nutrients are used and required by the grass.


"Would the growth potential model more accurately match the growth of the grass if soil temperatures were used rather than air temperatures?"

A golf course superintendent who wishes to remain anonymous wrote with an interesting question. Here's our exchange.

I have been using the growth potential model for 2 seasons - but very religiously this season. I have some brief thoughts that I wanted to pass along, and was curious what you thought regarding the following:

Would the growth potential model more accurately match the growth of the grass if soil temperatures were used rather than air temperatures?  Or canopy temperatures?

I am sending this email today because of some observations this weekend. We had our first cold snap of the season - average temperatures of 51, 49, 47, 53 [that would be from about 8 to 12 ºC] this past Thursday - Sunday.  However, we had full sun in the afternoons and our soil temperatures at 2" [5 cm] depths and canopy temperatures (in the full sun) both exceeded the air temperatures. It does not appear that our grass slowed its growth much. This is small sampling in a short period of time, and my guess is over a long time frame (the interval between fertilizer applications) the peaks and valleys will smooth out.

Going into and coming out of winter are when I notice the greatest variances.  Honestly, I don't know that the variations will have much impact on N amounts. I have not monitored soil temps religiously enough to cross reference with the growth potential model, but plan to next year.

Ultimately, just curious what you thought about the impact of soil or canopy temps on the growth potential model.

Here's my response.

Real quick answer: the GP optimum temperature and equations are set based on the general AIR temperatures for growth written about in Dr. Beard's Turfgrass: Science & Culture book. So as far as general use, the GP equation is meant to generate a value between 0 and 1 that is calculated from air temperature and related to the optimum air temperature given by Dr. Beard.

The application of GP is not an exact science and is not reality -- it is just a number between 0 and 1 (or between 0 and 100%) that can be put to use in various ways. As you will have seen, and as I and others around the world have found, despite the fact that GP is just a number, and is not in and of itself reality, it can be practically useful in a lot of maintenance work. 

Here's my guess at what happens with weather like this past weekend. I think the variances you see in spring and fall may also be due to the optimum temperatures changing -- grasses adapt a bit when it is hot or cold, so there is just no way that one can model growth exactly with this type of equation. The enzymes in the leaf change, or the activation temperatures for certain reactions, with changes in temperature. So I think as long as things generally seem to even out over say a week -- a week's average of GP should correspond to the way one sees the grass actually growing -- then I would be comfortable that the number is useful. 

Canopy temperatures, I don't know enough to consider.

Soil_tempSoil temperatures -- one could do this type of model using soil temperature, and one would adjust the optimum temperatures to do so, probably. But a lot of research about soil temperatures is related to root growth, not so much shoot growth. So I still like to work with air temperatures and try to estimate shoot growth. My guess is that soil temperature would not add a whole lot to the accuracy of the model. The reason being that soil temperature is a wave function following air temperature. The soil temperature just follows the air temperature and doesn't go to extremes of highs and lows and has a time lag depending on depth. My measurements of soil temperature and air temperature have been in Asia and in California and Oregon. When I've done that, the soil temperature is usually quite similar to the air temperature (see Figure 10 here), and it has been somewhat predictably lagging behind air temperature. So I just don't think there is a huge amount of information in soil temperature that would improve accuracy. 

I think it is awesome you are applying this, and I think it is an excellent idea to try different measurements or adjustments that better make it fit the observed growth or grass conditions. 

One more thing comes to mind regarding GP. I look at it as a passive predictor, not as a driving or controlling force. So the GP for me is very much a "potential" but it is not something that is "causing" the grass to do anything. In that sense, if the grass was healthy last week, and had some ample amounts of nutrients, and the soil temperatures and air temperatures were reasonable and then dropped off, you can see that the driving force would be the inertia of the grass just growing along. The GP as a "potential" would not immediately slam the brakes on growth. But over the long term, the potential makes sense.

Playing with numbers, evapotranspiration edition

I've used equation 52 in Crop Evapotranspiration: Guidelines for computing crop water requirements (FAO Irrigation and Drainage Paper 56) to calculate the predicted ETo for San Digeo, California, and for Madison, Wisconsin.

Rplot01I've also obtained the average ETo for San Diego (Torrey Pines, station # 173) from CIMIS and the average ETo for Madison from UW Extension Ag Weather

RplotPlotting the calculated ETo from equation 52 (predicted ETo) against the ETo average, one sees a pretty close relationship between the two values.

Rplot04But there is not such a close relationship between the temperature-based growth potential of PACE Turf and the ETo.

Rplot03I've suggested that the growth potential can be used to estimate turfgrass nitrogen use, for any grass, anywhere. Doug Soldat has pointed out to me that ET, or the consumptive water use, can be used as an estimate of how much nutrient uptake there may be. Nutrients (including nitrogen) go into the roots with water, and Bill Kreuser has written about this in Rethinking fall fertilization

The ET is one way to look at nutrient uptake and nutrient demand. And it is important to remember that nutrients do go into the roots with water, so there is going to be a huge, dominating role of consumptive water use (ET) in how much of each nutrient gets used by the grass.

Even so, I think the GP is more useful as a predictor of nutrient requirement extended across grass species and locations. These calculations don't show the whole story, but they are a first step at looking into this. And, it is interesting to see just how well equation 52, which requires only temperature and latitude data, predicts the average ET.

Not entirely a surprise

At the Poipu Bay area of Kauai, most hotel lawns are seashore paspalum. This lawn at the Sheraton Kauai is a typical example.

Under the trees, though, closer to the beach, a different species grows. 

This is manilagrass (Zoysia matrella). It is growing under the trees and creeping onto the rocky beach.

In fact, it even grows right on the beach. The manilagrass is salt tolerant, drought tolerant, and has a finer leaf blade than the seashore paspalum. One might expect to see the seashore paspalum growing closer to the ocean, but in this case, it is the manilagrass that grows right at the water's edge.

For more about these grasses, see:

Everything you need to know about turfgrass nutrition in 1 lecture

EverythingWhen I visited Oregon State University, I was the guest lecturer for Alec Kowalewski's Principles of Turfgrass Maintenance class. In just 50 minutes, I wanted to teach the most important things about turfgrass nutrition. 

Here is how I distilled turfgrass nutrition to a single lecture, with all the fundamentals presented and explained.

First, I gave the 1 minute version. If one can keep soil pH above 5.5 and less than 8.3, ensure that soil K is above 37 ppm and P is above 21 ppm (Mehlich 3), then by applying the amount of nitrogen necessary to produce the desired growth rate, the result will be excellent turf. For the next 49 minutes, I discussed this in a bit more detail, explaining how one can determine if enough of an element is available, and how much is required as fertilizer. These slides and associated handout have the details.

"The salesmen all suggest Calcium"

Amino_fert_9-6-5Jason Chennault and I had a conversation about interpreting soil tests. 

I had seen test results for a site and didn't think there were any problems with the soil. Jason wrote back with more information.

An example is the salesmen all suggest Calcium (and yes I know your thoughts about that :) ) but suggest applying foliar P, noting it won't be soil applied nor adding to the soil. They suggest that I'm high in S, my Ca and Mg ratios are concerning and on and on. Nothing that persuades me to do one thing or another exactly, just there's so many interpretations of the sample results.

There are certainly many interpretations of test results, but that doesn't mean all interpretations are correct. I think it makes sense to interpret soil tests in this way.

  1. Make the assumption that if there is enough of an element, then adding more of that element will have no effect.
  2. Don't worry about the specific function of each element. Rather, make sure that enough of each element is supplied to the grass.
  3. Then, find out how much is enough.
  4. This can be done by making sure that the amount of an element in the soil stays above the MLSN guideline
  5. Because the MLSN guidelines have been identified from soils that produce high quality turf, and have a safety margin built in to ensure they are not too low, have confidence that as long as nutrient levels remain above the MLSN guideline, the turf will be supplied with all of that nutrient that it needs. 

After going through that process of interpretation (full explanation and examples here), I like to remember this quote from Wayne Kussow: "How many more times do I have to say that applying nutrients to turfgrass growing on soil already well supplied with the nutrients is a waste of time and money?"

Measuring surface hardness on greens, fairways, and approaches

I measured soil moisture and surface hardness on three fairways, approaches, and greens at a golf course in Thailand last month. The fairways and approaches at this site are seashore paspalum on a sandcap. The greens are ultradwarf bermudagrass on a USGA green.

For some background info with data and charts from previous measurements, see this.

I do this to study a few things:

  • what type of soil moisture levels are normal
  • what type of surface hardness levels are normal
  • what is the relationship between construction method and surface performance
  • what is the relationship between grass species and surface performance
  • how the surface conditions change over time 
  • how the surface conditions change with different maintenance inputs

I used a TDR-300 to measure the volumetric water content of the soil to a 7.5 cm depth. At that same location, I used a Clegg Golf Course Tester with a 500 g hammer to measure the surface hardness.

The surface hardness decreases as soil moisture increases from greens (g) to approaches (a) and fairways (f).

For playability, I would prefer the fairways to be harder. I'm not an advocate for sandcapping of fairways in Southeast Asia. Even though the fairways at this location are sandcapped, the surface is soft. With these data, the golf course superintendent can make some changes to the fairway management and perhaps the surface hardness can be increased.

I also used the TruFirm to measure firmess at the same location. There is a bit more variability with the TruFirm, compared to the Clegg, but the result is the same – the greens are firmer, and the approaches and fairways are less firm. Note that a lower reading by the Clegg means softer, but a lower reading on the TruFirm means firmer.

The surface firmness (lower values are firmer) goes away as soil moisture increases from greens (g) to approaches (a) and fairways (f).

"More K does not promote stronger cells?"

FrostThat question about more K was raised in this discussion about late season fertilizer application. And the answer is, when looking for the beneficial effects of potassium (K), those effects are seen only when a deficiency is corrected.

Adding more K to turf sufficient in K does nothing. Adding more K to turfgrass that is deficient in K will promote stronger cells and healthier plants and all the benefits that are commonly associated with K. But this effect disappears once the grass has been supplied with enough K. When the grass has enough, adding more K will have no effect. I recommend these four papers for a better understanding of this:

Cool-season grasses use nitrogen and potassium in about a 2:1 ratio. With warm-season grasses, it is closer to 3:2 (for Cynodon and Zoysia) or 1:1 (for Paspalum vaginatum). Applying that much K almost guarantees that the grass will be supplied with enough. Adding more K beyond the amount the grass can use doesn't provide a benefit, and does not promote stronger cells.

In fact, one can often apply as fertilizer less K than the grass will use. This is possible because the grass can make use of potassium in the soil. One needs to test the soil to learn just how much potassium can be supplied from the soil, and how much must be added as fertilizer. This GCM article explains that approach.