Calcium and Magnesium

Like potassium (see Soils - Part 6), the sources of calcium and magnesium are primary minerals such as hornblende, mica, feldspar, calcite and dolomite. Considerable research has been conducted to determine the relationship between the amounts of potassium, calcium, and magnesium that should be present on the exchange complex. The University of Missouri Agricultural Experiment Station established ranges for the percentages of these elements’ saturation of the soil complex that provides a “balanced soil saturation.” (See Table 7.2.)

  Acceptable % Saturation1
Calcium (Ca) 50 to 75
Magnesium (Mg) 10 to 35
Potassium (K) 2 to 5

1Applies only to neutral to acid soils. Free lime in soil can lead to erroneous saturation relationships due to high calcium levels.

Table 7.2.  Range of K, Ca, and Mg saturation as  suggested by University of Missouri Agricultural Experiment Station

Nebraska soils all contain calcium on the soil complex in the top soil, as well as in the subsoil. Many soils contain free lime, even in the surface soils. It is doubtful from a nutrient standpoint that calcium deficiency will occur. A calcium saturation percentage of 40, should it occur, would probably not cause any problem. Calcium deficiency as a nutrient should not be confused with the need for lime as a correction of low pH. There are situations where acid soils will respond to adding limestone, as discussed in Soils - Part 4.

Most research has shown that a soil test reading of 40 to 50 ppm (80 to 100 pounds/acre) of magnesium is adequate for crop production. Some laboratories report percent magnesium saturation, which is the relative amount of exchange capacity that is satisfied by magnesium. This is a valid approach, unless the soil contains free lime (pH > 7.2).  A pH above 7.2 indicates that the soil contains calcium in addition to that found on the exchange complex. Excessive amounts of either potassium or calcium ions influence the uptake of magnesium. However, calcium and magnesium levels have little effect on potassium uptake.

The only soils in Nebraska where magnesium deficiency may be a problem will be the extremely sandy soils which have received excessive amounts of calcitic lime and/or potassium, either from potassium fertilizer or excess manure applications high in potassium.

Irrigation water in Nebraska contains calcium, magnesium, and potassium. The relative amounts of each can vary depending on location and whether the source is ground water or river water. The amounts of calcium, magnesium, and potassium vary little over time. A complete analysis of ground water every 10 to 15 years provides the crop producer with valuable information concerning the plant nutrients applied with each irrigation.

Note: PPM in water x 2.72 = pounds of nutrient per acre foot water applied.