A new look at calculating calcium requirements

Soil,

We are continuing our efforts to refine the new Minimum Levels for Sustainable Nutrition (MLSN) guidelines in cooperation with Dr. Micah Woods at the Asian Turfgrass Center. In the process, we have received some questions about managing sodium.

The MLSN guideline identifies 110 ppm (milligrams/kilogram, mg/kg) sodium as the maximum threshold for cool season grasses. To maintain sodium below 110 ppm in the soil, supplemental calcium is sometimes needed. To determine how much calcium needs to be applied to reduce sodium if levels exceed 110 ppm, follow the calculations below. For a similar method using metric units, refer to the Asian Turfgrass Center calcium requirements in metric units:

1. Determine how much excess sodium is present in your soil by subtracting 110 ppm (mg/kg) sodium (Na) from the value on your soil report.

   For example, if your soil contains 150 mg/kg sodium, your soil contains 40 mg/kg more (150 mg/kg - 110 mg/kg) sodium than is desired.

2. To compute how much calcium is needed to displace the sodium, we need to convert sodium units from mass into units of positive charge. To make the calculations easier, we use millimoles (mmol) of charge.

   For example, (40 mg sodium/kg)/(23 mmol sodium charge/mg) = 1.7 mmol sodium charge/kg soil

3. To displace 1.7 mmol sodium charge/kg with calcium, we will need 1.7 mmol calcium charge/kg soil

   For example, (1.7 mmol calcium charge/kg) X (20 mmol calcium charge/mg) = 34 mg calcium/kg (ppm) are needed to displace the sodium

4. To convert this value to pounds per acre, multiply by 2

   For example, 34 ppm calcium X 2 = 68 lbs calcium/acre are needed to displace sodium

5. To convert pounds per acre to pounds per 1000 sq ft, divide by 43.56

   For example, (68 lb calcium/acre)/43.56 = 1.6 lbs calcium/1000 sq ft needed to displace 40 ppm sodium

6. A shortcut to steps 2 - 5 is to multiply the excess sodium ppm value by 0.04 to obtain pounds calcium/1000 sq ft needed to displace excess sodium

For example 40 ppm excess sodium x 0.04 = 1.6 lbs calcium needed/1000 sq ft

7. To determine how much of a calcium amendment is needed to provide the desired amount of calcium, divide by the proportion composition of the product.

   For example, gypsum typically contains about 23% calcium. Therefore a requirement for 1.66 lbs calcium/1000 sq ft will require: (1.6 lb calcium/1000 sq ft)/0.23 = 7 lbs gypsum/1000 sq ft required to displace 40 ppm sodium

It is important to remember that calcium is also required as a plant nutrient (at levels of 360 ppm in the soil), in addition to its value as a tool for managing sodium. When calculating calcium requirements, you will need to keep both the sodium management and plant nutrition requirements in mind.