Monthly Turfgrass Roundup: November 2014
"We knew that the results of the survey and of the new nutritional guidelines that resulted from it might attract attention, and even controversy, because they represent a significant departure in the philosophy and practice of turf nutrition"

The importance of irrigation water testing

Brad Burgess wrote:

I would appreciate your thoughts and comments re this water test. I just read your PACE Turf Article and thought I would run this by you. It could be a nice study for you re salt tolerance in Zoysia …

Never seen water this bad before and tested it after the fact. 

Others have said this water is not even suitable for Paspalum … 

Look forward to your comments. Also attached some photos … at 90 days after planting.

This water had electrolytic conductivity (EC) of 1.4, pH of 9, calcium at 4.7 ppm, magnesium 3.7 ppm, and sodium 314 ppm.

I responded:

Thanks for the photos and the water test.

Photo of turf at the site being grown-in with this irrigation water.

The grass looks great. And that is a pretty poor water. It would be an interesting site to do some tests.

My thoughts on the water -- the 2 most important things to look at are total amount of salt (EC) and SAR [sodium adsorption ratio]. 

EC is what one looks at to see the effect salt in the water is going to have on the grass, how that may accumulate in the soil, and how much extra water will be required to keep the soil salts at a level the grass can tolerate.

For the salt content, it isn't too bad. The leaching requirement [for more about leaching requirement, and how to make these calculations, see this handout] for zoysia using that water, if I use a soil EC tolerance level of 8 ds/m, is 0.037, so the amount of extra water required is minimal, ET / (1 - 0.037). But if the soil structure would deteriorate, then one couldn't leach to maintain the soil EC at 8, and then the salt would damage the grass. As a comparison, the irrigation water at [golf course name redacted] has had 4 times as much salt as this water, and Tifeagle can still be maintained to a high level, as long as one leaches properly.

SAR is what one looks at to see how the sodium may cause a problem with soil structure.

I think [this lab’s water test is flawed] because it does not provide the SAR directly, forcing the customers to calculate it themselves, while emphasizing [less relevant data]. 

For this particular water, the SAR is about 26, which is especially bad for soil structure, especially because the water doesn't have a high salt content. One expects the regular use of this water to cause problems with soil structure (unless it is a sand rootzone) over time, exhibited by slowing of water infiltration. This problem can be addressed by regular applications of gypsum. The amount of gypsum to apply is based on the amount of sodium added in the water, or based on the ESP of the soil. Gypsum can be applied at pretty high rates, like 200 to 400 g/m2. I make a rough calculation that for every liter of water added, one should apply 1.5 g gypsum/m2 to prevent soil structure problems. So if 150 mm of water were added in a month, that would be a 225 g/m2/month gypsum requirement.

To summarize, I'd be concerned about soil structure with this water, would apply 1.5 g gypsum/m2 for each L of water that was applied, with that being done to prevent soil structural problems (disregard that advice on a sand rootzone), and I would make sure that slightly more water was applied than ET, to prevent salinity problems.

I’d like to emphasize three things.

1. It is really important to test the irrigation water. Because Brad had this water tested, he can identify and prevent potential problems. What is in the water is invisible. Many sites have water that is perfectly fine, and a test will confirm that. For locations with high salinity or high SAR, that problem is invisible in the water, until there are visible problems on the turf, and by then it is way too late.

Seashore paspalum has died at this site where salt has accumulated in the soil. This problem can be prevented by knowing what is in the water and then carefully managing the salinity through leaching.

2. Make sure the water is being tested for the right things. One needs an irrigation water suitability test. A comprehensive guide for this is Harivandi’s Interpreting Turfgrass Irrigation Water Test Results. In that, he writes:

When irrigation is applied to the soil, the best indicator of sodium effect is a water’s Sodium Adsorption Ratio (SAR), a value which should be provided in all laboratory water analyses. 

3. If for some reason SAR is not reported, one can calculate it from this equation:

\[SAR = \frac{Na}{\sqrt{\frac{Ca + Mg}{2}}}\]


SAR is sodium adsorption ratio

Na is the sodium concentration of the water in milliequivalents per liter

Ca is the calcium concentration of the water in milliequivalents per liter

Mg is the magnesium concentration of the water in milliequivalents per liter


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