Calcium (Ca++)

Calcium in the Soil

Calcium is present in adequate amounts in most soils. Calcium is a component of several primary and secondary minerals in the soil, which are essentially insoluble for agricultural considerations. These materials are the original sources of the soluble or available forms of Ca. Calcium is also present in relatively soluble forms, as a cation (positively charged Ca++) adsorbed to the soil colloidal complex. The ionic form is considered to be available to crops.


Calcium is essential for many plant functions. Some of them are

Calcium is transported in the xylem via an ion exchange mechanism. It attaches to lignin molecules and exchange must occur with calcium or another similar cation (e.g. Mg++, Na+, K+, NH4+, etc.). Calcium is not very mobile in the soil, or in plant tissue, therefore a continuous supply is essential.

Factors Affecting Ca Availability

Calcium is found in many of the primary or secondary minerals in the soil. In this state it is relatively insoluble. Calcium is not considered a leachable nutrient. However, over hundreds of years, it will move deeper into the soil. Because of this, and the fact that many soils are derived from limestone bedrock, many soils have higher levels of Ca, and a higher pH in the subsoil.


Balances and Ratios

For many years, there have been a few people who claim that there is an "Ideal" ratio of the three principal soil cation nutrients (K, Ca, and Mg). This concept probably originated from New Jersey work by Bear in 1945 that projected an ideal soil as one that had the following saturations of exchangeable cations 65% Ca, 10% Mg, 5% K, and 20% H. The cation ratios resulting from these idealizes concentrations are a Ca:Mg of 6.5:1, Ca:K of 13:1, and Mg:K of 2:1.

It is generally accepted that there are some preferred general relationships and balances between soil nutrients. There is also a significant amount of work indicating that excesses and shortages of some nutrients will affect the uptake of other nutrients (see later sections of this paper). However, no reliable research has indicated that there is any particular soil ratio of nutrients.

Over the years, a significant amount of conversation and salesmanship has revolved aroung the concept of the ideal soil Ca:Mg ratio. Most of the claims for the ideal ratio range between 5:1 and 8:1.

Some of the claims are that the correct soil Ca:Mg ratio will

According to Dr. Stanley Barber, Purdue Univ., "There is no research justification for the added expense of obtaining a definite Ca:Mg ratio in the soilResearch indicates that plant yield or quality is not appreciably affected over a wide range of Ca:Mg ratios in the soil."

Wisconsin research found that yields of corn and alfalfa were not significantly affected by Ca:Mg ratios ranging from 2.28:1 to 8.44:1in all cases, when neither nutrient was deficient, the crops internal Ca:Mg ratio was maintained within a relatively narrow range consistent with the needs of the plant. These findings are supported by most other authorities. A soil with the previously listed ratios would most likely be fertile. However, this does not mean that a fertile soil requires these specific values (or any other). Adequate crop nutrition is dependent on many factors other than a specific ratio of nutrients. It will rarely be profitable to adjust the soil Ca:Mg ratio.

In later sections of this paper, you will find references to nutrient ratios. However, in most cases there will not be specific numerical ratios associated with these relationships. The intention is to indicate that as the relative abundance of the nutrients changes significantly, it could affect the availability of the other nutrient. This concept is much less specific than claiming that there is a value to a specific numerical ratio.

High Response Crops

While Ca is an essential element for all plants, the following crops have been found to be especially responsive.

apples, broccoli, brussel sprouts, cabbage, carrots, cauliflower, celery, cherries, citrus, conifers, cotton, curcurbits, melons, grapes, legumes, lettuce, peaches, peanuts, pears, peppers, potatoes, tobacco, and tomatoes.

Deficiency Symptoms

Calcium deficiency symptoms can be rather vague since the situation often is accompanied by a low soil pH. Visible deficiency symptoms are seldom seen in agronomic crops but will typically include a failure of the new growth to develop properly. Annual grasses such as corn will have deformed emerging leaves that fail to unroll from the whorl. The new leaves are often chlorotic. Extremely acid soils can introduce an entirely new set of symptoms, often from different toxicity's and deficiencies. Many fruits and vegetables demonstrate dramatic symptoms such as Black heart in celery and broccoli, Tipburn in lettuce and cabbage, White heart or Hollow heart in cucurbits, Blossom End Rot in tomatoes and peppers, and Pops in peanuts. Tree fruit with low calcium will exhibit increased storage problems such as bitter-pit in apples, cork-spot in apples and pears, cracking in cherries, and other degradation of the fruit while in storage. Deficiency in all crops often also impairs root growth and lead to additional symptoms as a secondary effect. Calcium deficient conifer trees will have exhibit yellowing then death and dropping of the needles on the new growth. The new growth may also be deformed.


Calcium, for all practical purposes, is not considered to have a directly toxic effect on plants. Most of the problems caused by excess soil Ca are the result of secondary effects of high soil pH. Another problem from excess Ca may be the reduced uptake of other cation nutrients. Before toxic levels are approached in the plant, crops will often suffer deficiencies of other nutrients, such as phosphorus, potassium, magnesium, boron, copper, iron, or zinc.

Using Calcium in A Fertility Program

Calcium sources can serve either, or both of two functions.

Correcting calcium problems is usually not difficult. Liming to the proper pH is the first consideration to supply Ca to the crop. If additional Ca is needed, and the soil pH is already correct, neutral amendments such as gypsum (CaSO4.7H2O) or other fertilizer products are available. Gypsum can also be used to correct high salt conditions in the soil. Such conditions may be a natural condition of the soil, the result of brine water around present or past oil wells, or due to the use of winter de-icing salt.

Recommended rates of Calcium: (follow soil test or plant analysis recommendations)

Liming Material

Approx. % Ca*.

Recommendation Rate

Calcitic Limestone


1,000 to 15,000lb./A

Dolomitic Limestone


1,000 to 15,000 lb./A

Hydrated Limestone


750 to 10,000 lb./A

Precipitated Lime


500 to 10,000 lb./A

Blast Furnace Slag


100 to 2,000 lb./A


Approx. % Ca.

Recommended Rates of Product



500 to 1500 lb./A



5-8 lb./A Foliar

Ca(NO3) 2


10-15 lb./A Foliar



0.25-3 gal/A Foliar

* Calcium content is not the same as neutralizing value. Neutralizing value is determined by the combined amounts of calcium carbonate (CaCO3), magnesium carbonate (MgCO3), and other neutralizing constituents in the liming material.

Calculating Gypsum Requirement

There are various purposes for applying gypsum and each has a specific method for developing a recommendation. There may also be more than one legitimate method used to make recommendations for each purpose. The following are some of these methods.

Gypsum is recommended for two primary purposes. They are

  1. To remove excess sodium (Na)
  2. To build soil calcium (Ca) levels when a pH change is not desired.

Reducing Soil Sodium (Na)

  1. Reducing Na to a generally acceptable level: Lb. gypsum/acre = C.E.C. x (%Na sat. - 5) x 18
  2. Reducing Na to a particular saturation percent:
  3. Example: Assume that the soil CEC is 20 (meq/100 grams) and the Na concentration is 40%. You want to lower the Na concentration to 10%, or eliminate 30% of the Na saturation (30% of 20 meq/100 grams = 6 meq of exchangeable Na/100 grams of soil). Multiply the milliequivalents of exchangeable Na by 0.85 tons of gypsum to get the required application of gypsum ( 6 x 0.85 = 5.1 tons of gypsum/acre). Typically, commercial gypsum is not 100% efficient in displacing Na, and some authorities suggest using an 80% efficiency factor. Doing this results in our example changing as follows... 5.1 divided by 0.80 = 6.38 tons per acre. If your irrigation water has a gypsum content, or your soil contains gypsum, you can deduct these amounts from the required rate of gypsum to apply.
  4. Calculating gypsum to offset Na in irrigation water:Gypsum requirements can be calculated from the residual sodium carbonate (RSC) value of the irrigation water from the following equation.
  5. RSC x 234 = pounds of gypsum required to offset the excess sodium in 1 acre foot (325,852 gallons) of irrigation water

Remember, gypsum alone does not solve a high Na problem, you must apply adequate irrigation water to leach the displaced Na out of the root zone.

Increasing Soil Calcium (Ca) Saturation

Lb. gypsum/acre = C.E.C. x (desired %Ca sat. - present %Ca sat) x 18