On average, irrigation water contains 7 times more Ca than grass uses

The normal range of calcium (Ca) in irrigation water is 20 to 60 ppm. That comes right from Penn State’s Irrigation water quality guidelines for turfgrass sites.

Were you surprised by the previous post that worked out daily Ca use by the grass, and Ca added in irrigation water, to find that irrigation water was supplying 26 times more Ca than the grass was using? The irrigation water at that site only had 18.62 ppm of Ca—that puts it in a low classification.

Here I’ll redo the calculation, for waters with a normal range of Ca, and for creeping bentgrass. This will be an annual calculation too, because irrigation isn’t added every day.

### Ca in irrigation water

The normal range is 20 to 60 ppm Ca. That is 20 to 60 mg/L. It is convenient to remember that 1 L of water applied to 1 m2 is a 1 mm depth of water.

I’ll take the middle of the normal range and say the average water has 40 mg of Ca in each liter.

How much water might be applied? A recent nationwide survey, with results reported by Gelernter et al. in Documenting trends in water use and conservation practices on U.S. golf courses, provides an answer. The median water use was 105.7 acre-feet per year and the median irrigated area was 80.3 acres. That works out to a median irrigation amount of 381 mm.

If that median amount of 381 mm (or L/m2) has an average amount of Ca in it—40 mg/L—then the Ca added by irrigation is 15,240 mg/m2

### Ca use by the grass

How much Ca does the average grass use? I’ll look to another recent survey, also reported by Gelernter et al. This is Documenting trends in nutrient use and conservation practices on US golf courses, and the survey found the median nitrogen (N) use on putting greens was 3.7 lbs/1000 ft2/year.

The grass won’t grow more than the amount of N supplied. In fact, it will grow less because there isn’t 100% efficiency of applied N. But for the sake of this calculation, I’ll use 3.7 lbs of N use as an upper limit on growth.

That 3.7 lbs of N per 1000 ft2 works out to 18.5 g N/m2. In monthly measurements of bentgrass putting green leaf N last year, I measured a median N content of 5%.

The 18.5 g of N, in leaves with 5% N, could produce an upper limit of 370 g of grass yield per m2/year. Now we are getting somewhere.

Those monthly bentgrass tissue tests from last year had a median Ca content of 0.58%. The 370 g of clippings, with 0.58% Ca, would contain 2.1 g of Ca/m2. That is 2,100 mg/m2.

### Ca in irrigation compared to Ca used by the grass

The 15,240 mg of Ca supplied by irrigation water happens to be 7 times more than the 2,100 mg of Ca used by the grass.

You might like to make this calculation for your site, based on the concentration of Ca in your irrigation water, and based on how much your grass grows.

• Nutrient profile: Calcium by Doug Soldat estimates that annual irrigation and rainfall in Wisconsin supplied 2 to 3 lbs Ca/1000 ft2 (10 to 15 g Ca/m2)

• Calcium leaches? in which I make this calculation on a daily basis for an irrigation water low in Ca

• Frank Rossi and the imaginary problem of calcium deficiency