Potassium is often required as fertilizer because the grass uses more potassium than is available. The situation for calcium is quite different. The amount of calcium available in the soil is almost always much more than the grass requires.
This waterfall chart for calcium takes a look at the calcium in the soil and shows the various additions and subtractions of calcium over the course of a year.
- A blue horizontal line is drawn at 360 ppm. That is the MLSN guideline for calcium, meaning we want to keep soil calcium at or above 360 ppm, and when we do that, we have a high level of confidence that turfgrass will be supplied with ample calcium.
- The initial soil test level of 519 ppm is a typical level for sand rootzones. This is based on the median value of 100 soil samples taken from sand rootzones in five countries of Southeast Asia.
- Annual plant uptake is expected to decrease the amount of calcium in the soil by 27 ppm. This is based on an estimated annual harvest of 900 grams dry matter per square meter, with an average leaf tissue calcium content of 0.45%. This is typical of bermudagrass turf with a twelve month growing season. Use of calcium by cool-season grasses in a temperate climate would be about half of the amount shown here.
- No calcium is applied as fertilizer because it is not necessary. The amount in the soil is much higher than the MLSN guideline.
- I estimate an addition of 80 ppm calcium to the soil through irrigation water. This is based on an average calcium concentration in the irrigation water of 20 ppm, and an annual irrigation amount of 600 L/m2. This is a conservative estimate of irrigation application at Bangkok based on 150 days of irrigation at 4 mm per day. Note that the amount added by the irrigation water in this conservative case is three times the amount of calcium actually used by the grass.
- I estimate a leaching loss of 67 ppm. Normally calcium won't leach. But in this case it absolutely has to. Why? Because the soil cation exchange capacity (CEC) does not change. The exchange sites in the soil already have cations reversibly adsorbed to them at the time the irrigation water is applied. So some of the calcium added through irrigation must leach. There is no place for it to remain in the soil.
- Taking the initial amount of 519 ppm in the soil, subtracting the amounts used by the grass and lost by leaching, and accounting for the amount of calcium added in the irrigation water, we are left with 505 ppm calcium in the soil after one year. This is still well above the MLSN guideline for calcium and this demonstrates why none is required as fertilizer.
For more information about calcium for turfgrass, see: