Salinity

"Don't try to jump on his bandwagon"

Jon Scott wrote to me about my recent post on a poor way to fertilise.

"While this superintendent has solved his problem of nitrogen input by monitoring salinity level that has worked for him, this is probably a very unique situation. It may be relevant to other golf courses where similar salt levels exist, but there are too many variables to draw general conclusions. Thus, I would focus on salt levels as related to this situation and not extrapolate. What he has said may be relevant to similar situations, but it all depends on the salt levels."

I agree, and I meant to make that clear in the original post. Let me try now to explain in clear terms.

DSC_7285

If there is a salinity problem at a site, then one will always want to minimize the salinity in the soil. If one is always trying to minimize salinity in the soil, then it is impossible to use any measure of salinity as a criterion for fertilizer application.

In a case where there is not a salinity problem at a site, it might sound reasonable to try to use salinity as an index of nutrient content in the soil. However, there are three big problems with this, and these I did describe in the original post. First, most turf managers don't want fluctuating nutrient supply; second, salinity says nothing about which nutrients are there; and third, the salinity measurements from soil moisture meters, whether EC or a salinity index, are so affected by the water content of the soil that using the salinity of non-saline soils to make decisions about fertilizer is like chasing a target that moves randomly.

I like using soil moisture meters to measure the water content in the soil. I think it is useful to assess the salinity of the soil with the meter too, if that function is available. But I don't think it is a good idea to make fertilizer decisions based on soil salinity.

I replied to Jon that "I think it is ridiculous but tried to be as polite as possible."

He wrote back:

"You, trying to be polite? Don’t lose your edge ... I think you need to clarify how unique this situation is so that others don’t try to jump on his bandwagon. His premise is flawed when applied outside of his operation."


"Is there a particular reason why you think it's a poor way to fertilise?"

A correspondent wrote:

"I'm hoping to get your thoughts on something I came across today.

I was discussing greens fertilising whilst at a friend's course this morning. He went on to get his new toy, the [...]. He's started to use the salinity level reading as an indicator to fertilise. So he's found a number that he's happy with that the turf looks hungry, applies a granular fertiliser and then waits for the number to drop back down to his threshold number again and repeats.

I'm not sure about this method as I've never come across it before plus I've never really looked into the salinity levels of my soils. I would just prefer to use gp and feel for when the plant needs something and adjust accordingly. But maybe he's onto something.

I would love to get your feedback on this if you're not too busy."

I replied that "I think that is a poor way to decide when to fertilize. Or what to fertilize with."

Then came a few more questions:

"Is there a particular reason why you think it's a poor way to fertilise?

If he's getting the results he desires, does that still make it poor? A reason he gave me about fertilising with a granular is [...] that by fertilising this way, he will encourage his perennial poa rather than poa annua."

First, the idea of deliberately managing soil nutrients to fluctuate up and down seems like the opposite of what most turf managers would like to accomplish.

I think most would ideally try to keep nutrient supply and growth as consistent as possible, rather than trying to cause them to fluctuate.

Salinity_spoon_feed

Second, it's changes in N that make grass grow, and then P and K and Ca and Mg and all the rest get taken up by the grass according to how much the grass is growing. So it makes sense to know the quantities of nutrients supplied, and also the quantities of the nutrients in the soil. But measuring the salinity of the soil doesn't tell which nutrients are there. It just gives the total quantity of salt.

Third, I have some concerns about the salinity number itself. The soil moisture meters that measure electrical conductivity at the same time are measuring the electrical conductivity of the water in the soil, and that measurement is strongly influenced by the amount of water in the soil. When the soil is drier, the meters give a low electrical conductivity reading, and when there is more water in the soil, even though there is no salt added, the electrical conductivity goes up.

DSC_4007

This chart shows some measurements I made over a four day period on test plots on a golf course nursery green (pictured above). On a Sunday, I measured the soil VWC and the EC. Then I added irrigation water with 137 ppm salt, and I measured soil VWC and EC again. On Tuesday, there was a typhoon with 121 mm rain. On Wednesday, I measured the VWC and the EC again.

The EC as measured by the soil moisture meter is influenced by the water content of the soil.

EcPlot

One might say that is useful, because it gives some idea of the EC as the plant sees it. But if one makes that argument, then it is difficult to simultaneously make the argument that the EC is a useful criterion for determining when to supply fertilizer, because it is clear that the EC measurement is affected by the soil water content independently of the quantity of nutrients in the soil.

It is possible to adjust the EC measurement by incorporating the soil water content and the EC into a unitless measurement. The salinity index can be obtained by taking the EC, dividing it by the VWC, and multiplying by 100. This value takes into account both the EC and the amount of water in the soil at the time the EC was measured. There is not such a direct relationship between the VWC and the salinity index. But for those same data as shown in the previous chart, the salinity index also shows higher values with more VWC, even though no salt was added. In fact, after the typhoon's 121 mm of rain, one might expect leaching of nutrients, and a lower salinity index. But the opposite happened here, as shown in this chart.

SalIndex

And fourth, the follow-up question about if he's getting the desired results, is that still a poor way to fertilize? If he is getting the desired results, then fine, keep doing it. At some point it comes down to personal preference, because one can get good results in a lot of different ways. My preference, and what I think is a better way to determine when to supply fertilizer, involves monitoring the grass conditions, supplying N to produce the desired growth rate, and ensuring the grass is supplied with enough of each nutrient to meet the grass requirements. I expect such an approach is easier and will result in lower nutrient applications.

And about perennial Poa vs annual Poa, I'd be looking to supply a consistent amount of nutrients to the grass, rather than a fluctuating amount, because I expect the more ruderal biotypes of Poa annua would be more competitive with fluctuating nutrient supplies and with periodic granular fertilizer applications.


My presentation on irrigation water quality

Yesterday I taught a seminar about irrigation water quality.

Here are some links related to that presentation.


Field day poster: 5 grass varieties, 3 levels of salinity, and a month to grow-in -- or not

This poster for today's field day describes what happened when we planted five grass varieties as stolons and then supplied irrigation with different amounts of salt in the water. I think two things might surprise you. First, some of the grass varieties, including a particular manilagrass (Zoysia matrella), can reach full coverage in about one month after planting. Second, irrigation with salty water, so long as enough is applied, doesn't slow down the growth of many warm-season grasses.

image from c1.staticflickr.com

When the grasses can grow like this, it shows that managing the problems with water quality are closely tied to the quantity of water supplied. I will be talking more about this tomorrow when we are back for another day of seminars.


After 28 days, grow-in and salinity differences

Annotated

I've been growing grasses in a plastic house with a lot of help from colleagues at the Thailand Institute of Scientific and Technological Research (TISTR). The idea was to see how these grasses grow in after being planted as stolons, and to see what happens when salt is added in the irrigation water. I'll be discussing this experiment at the field day in Chonburi next week.

The picture above shows the grasses that receive the irrigation with 330 ppm total dissolved solids (TDS), 28 days after planting. The seashore paspalum looks the best, and the nuwan noi manilagrass has grown-in almost as fast. The hosoba korai, which is a beautiful grass once established, still hasn't covered much of the pot.

Another thing I've found interesting is measurements of salinity in the soil with the new TDR-350. All the pots are supplied with the same quantity of water. But different sets of pots get different amounts of salt in the water.

Index

The soil salinity in these pots is changing depending on which irrigation water is applied. That's just as expected.

For more about the TDR-350, see this webinar.

 


Grow-in potential

These pictures were taken 28 days apart. Here's what the grasses looked like yesterday, on February 24. That was 4 weeks, exactly 28 days after planting.

Medium_salt_28_days

On 27 January, five different grass varieties were planted from stolons. The grasses, shown from left to right, are:

  • manilagrass (nuwan noi)
  • tropical carpetgrass (yaa malay)
  • seashore paspalum (salam)
  • manilagrass (hosoba korai)
  • bermudagrass (Tifway 419)

For the first 10 days after planting, all the grasses were irrigated with 330 TDS (total dissolved solids, in units of ppm) water. For the next 18 days, the grasses shown above were irrigated with 4,500 TDS water.

The planting rates for the stolons ranged from 99 g/m2 for the nuwan noi to 312 g/m2 for the yaa malay. This is the mean mass for the stolons planted in the pots. We cut the stolons into 10 segments with 3 nodes each and then weighed them and planted them; each 0.02 m2 pot was planted with 30 nodes (1,500 nodes per square meter).

This is what the pots looked like immediately after planting, on January 27.

Planting_jan27

I think this is interesting for two reasons. One, this gives some indication of the grow-in rate (and relative rates) of various grass varieties. Second, this shows the tolerance or not of the grasses to different salt levels in the water.

One set of grasses is getting water with salt (TDS) at 330 ppm, the one pictured are getting 4,500 ppm, and another set are being irrigated with 9,000 ppm.

I'll be talking about this, and showing some of these grasses, at the upcoming Sustainable Turfgrass Management in Asia conference.


Read these articles, but disregard the subtitles

When I saw there was a new article at Golfdom about sodium causing agronomic challenges on sand putting greens, I clicked the link to see what this was about.

That link took me to the article by Obear and Soldat in which they explain that sodium does not cause agronomic challenges in sand putting green soils:

"Sand putting green soils have low clay contents and are therefore unaffected by sodium ... The findings from this study suggest that sodium will not negatively affect putting green soils with low clay content, including those constructed to USGA recommendations ... In the case of sand-based putting green root zones, which often have very low clay content, increasing exchangeable sodium percentage well above the standard sodicity threshold of 15 percent had no effect on hydraulic conductivity."

But the link is www.golfdom.com/sodium-causes-agronomic-challenges-for-sand-putting-greens, which seems the opposite of what the article is about. Sodium causes agronomic challenges for sand putting greens? Maybe if the sand putting green is made of clay.

That reminds me of the subtitle for the article Frank Rossi and I wrote about the Park Grass experiment for the Green Section Record. One of the things I thought was amazing was how soon the botanical composition on the Park Grass field changed in response to fertilizer treatments. We wrote about that in the article, quoting Lawes and Gilbert from their first paper on the botanical composition of the experiment, published just a few years after the first treatments were applied:

"the plots had each so distinctive a character in regard to the prevalence of different plants that the experimental ground looked almost as much as if it were devoted to trials with different seeds as with different manures [fertilizers]."

The fertilizer treatments began in 1856. We didn't put these quotes in the article, but it is clear the effects were noticed immediately. More from Lawes and Gilbert:

"So striking and characteristic, indeed, were the effects produced in this respect, that, in 1857 and 1858, the subject was thought of sufficient interest to induce us to request the examination of the plots by Professor Henfrey, to which he kindly assisted.

An endeavour was also made in the second year, 1857, to separate, and determine, the proportion of the different plants in carefully averaged and weighed samples, taken from the several plots as soon as the grass was cut."

So I was surprised that the subtitle of our article, when I saw it published, was Sometimes the value of a turfgrass management practice takes a long time to become apparent. That's not quite what we were trying to say.


High soluble salts, K, and extractants

Earlier this year Brad Shaver and I had a discussion about salinity and extractants.

I had written previously this post explaining that a saturated paste extract is not a good way to look at soil nutrients and that it is not a good idea to look at a saturated paste extract and compare it to a standard soil test.

Brad asked about potassium (K) in saline soils, about acid extracts overestimating exchangeable K in saline soils, and alluded to a continuing confusion about the combination of high soluble salts in soil, potassium, and different extraction methods.

I’ll explain this in two ways. First briefly, without all the details.

Saturated paste (I’ll abbreviate as SPE for saturated paste extraction) is not useful to evaluate K in soils with high soluble salts because the problem with saline soils is too many soluble salts. The solution to this is leaching of the salts. The K measured by the SPE will be deliberately leached, and depending on how saline the soil is, a large portion of the K measured by a standard soil test, because it measures soluble and exchangeable K, will be deliberately leached as well.

Because one is going to deliberately leach soluble salts from a saline soil, as part of the standard management of saline soils, it doesn’t make sense to use the soil test K, from any extraction method, to determine how much K to apply as fertilizer to saline soils.

What does make sense? There will be some K in the soil. There will be some K added through irrigation water. And in a saline situation one can supply K as fertilizer in the quantity that the grass can use, disregarding the soil K and the K added in irrigation water. This guarantees the grass will be supplied with more than enough K, and one doesn’t need to test the soil for K at all.


Now explained the second way, with a few more details, and some data.

The purpose of soil testing is to determine if an element is required as fertilizer, and how much of that element should be applied. Or, in the case of salt-affected soils, the purpose of testing is to identify the problem and to determine what actions should be taken to solve the problem. Of course, if there is a problem with soluble salts, and one leaches them, it doesn't make sense to try to make a fertilizer recommendation from something one is going to be removing from the soil.

There are two forms of plant-available K in the soil: soluble and exchangeable. A SPE measures the soluble K and a small amount of exchangeable K. A standard soil test, such as the Mehlich 3 or normal ammonium acetate extractions, measures the soluble K plus the exchangeable K. Both the SPE and the standard soil test measure the soluble K, and the standard test will additionally measure exchangeable K.

Here are data from nine sites with the electrical conductivity of the saturated paste extract (ECe) labeled as (ec), the K in ppm by SPE labeled as (kh2o), the K in ppm by Mehlich 3 labeled as (km3), and the location of the sample.

ec kh2o km3 location
4.5 59.0 89 Thailand, fairway
17.4 110.0 118 Thailand, fairway
0.2 6.8 51 Philippines, green
0.1 4.4 215 Philippines, fairway
0.2 10.5 82 Philippines, green
0.3 14.8 55 Philippines, green
0.3 17.1 74 Philippines, green
0.9 45.0 174 Thailand, green
0.9 20.8 38 Philippines, beach

I've marked the ECe = 4 dS/m level with a red line, to show in which cases a soil would be considered saline, and in which it would not. Note that one will try to maintain a site-specific ECe depending on the species being grown and the irrigation water salinity -- the 4 dS/m level is included here as a reference level. These samples represent a range of soil salinity levels, most not saline, and two of them saline. Let's look at what happens with soil K across this range of soils and salinities.

Kh20_vs_ecThe K extracted by SPE, which I have labeled as KH2O to indicate it was extracted by water, is low when the ECe is low, and it increases when the ECe is higher. That is to be expected, because the quantity of soluble K is expected to be a function of the soluble salt content of the soil.

Now we can look at the Mehlich 3 K (KM3) for these same samples.

Km3_vs_ecThis looks a bit different, as it should, because the Mehlich 3 test is measuring both the soluble K and the exchangeable K. When the soil salt content (the ECe) is low, then the KM3 is going to be influenced by the cation exchange capacity of the soil and the quantity of K on the exchange sites, and when the ECe is high then there will be a greater proportion of soluble K as part of the the K measured by Mehlich 3.

This next chart demonstrates that. In each of these samples, the KM3 is a larger value than the KH2O. That is because the Mehlich 3 test measures soluble and exchangeable K, while the SPE test measures only the soluble K. By looking at the difference between the KM3 and the KH2O, we can see that the more salt there is in the soil, the smaller the difference is between these two quantities.

Difference_km3_kh2oIs this making sense? When salt in the soil is low, which is what we want, there tends to be a big difference between the quantities of K extracted. As the salt in the soil increases, the difference gets small, because the quantity of soluble K is very high compared to the amount on exchange sites -- at least in a sandy rootzone.

Slight tangent for a moment -- this is something I've written and talked about before, as something that one should not be bamboozled by.

One wants to have low soluble salt content in the soil. When there is low soluble salt content, it is normal to have a large difference between the water soluble and the exchangeable nutrients. But that doesn't mean the grass won't be supplied with enough nutrients. From Environmental Chemistry of Soils (McBride, 1994): "Ion exchange reactions at surface sites exposed to solution are extremely fast."

Back to the data, now looking not at the difference between KH2O and KM3, but the ratio between them. Remember, KM3 in these data is always larger than KH2O, because KM3 contains both the water soluble (KH2O) and the exchangeable K.

Kh20_vs_km3_by_ecWith this proportion, when it is close to 0 (on the y-axis), that means the KH2O by saturated paste is only a small amount of the KM3. At low soil salinity, that's just what we see. And with increasing ECe, as expected, the proportion of soluble K increases.

This can also be represented in a linear relationship for these data by showing that same proportion of $(K_{H2O}) / (K_{M3})$ across the natural logarithm of ECe.

Kh2o_vs_km3_by_log_ecFrom this chart, it seems that knowing ECe and KM3 is enough to predict KH2O. Not only is the KH2O value not useful in making a fertilizer prediction because one will try to leach it away with the other soluble salts in a saline situation, but it can be predicted from other measurements, meaning it isn't adding any new information.


Managing salt by leaching

Selection_010My turfgrass talk column in the May-June issue of GCM China explains how to calculate the amount of irrigation water to apply when one is trying to keep the soil salinity (ECe) from exceeding a threshold value.

The article is available in both Chinese and English.

If the salt is not leached, and accumulates in the soil, the grass can die. To prevent the accumulation of salt, more water than the grass can use must be applied. This causes leaching as the extra water moves below the rootzone, carrying some salt with it.

Good drainage is essential when salt in the irrigation water requires leaching to be done. In the photo below, there is a low area below the drain, and salt accumulation in the soil at that spot prevents grass from growing.

Salt_accumulation

For more on this topic, see the preceding article in this series: Do you know how much salt is in your irrigation water?


Do you know how much salt is in your irrigation water?

Selection_084My column in the March-April issue of GCM China is about salt. The salt in water is invisible, so one needs to test the water to find out how much salt is in it.

As I wrote in the article, water with total dissolved solids (TDS) of 800 ppm would add 56 g salt/m2 (11.2 pounds salt/1000 ft2) in a 2 week period if irrigation is applied at 5 mm/day. Being aware of how much salt is in the irrigation water is the first step in determining if leaching will be required.