Calcium (Ca⁺⁺)

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

• Proper cell division and elongation
• Proper cell wall development
• Nitrate uptake and metabolism
• Enzyme activity
• Starch metabolism

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.

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.

• Soil pH: Acid soils have less Ca, and high pH soils normally have more. As the soil pH increases above pH 7.2, due to additional soil Ca, the additional “free” Ca is not adsorbed onto the soil. Much of the free Ca forms nearly insoluble compounds with other elements such as phosphorus (P), thus making P less available.
• Soil CEC: Lower CEC soils hold less Ca, and high CEC soils hold more.
• Cation competition: Abnormally high levels, or application rates of other cations, in the presence of low to moderate soil Ca levels tends to reduce the uptake of Ca.
• Alkaline sodic soil (high sodium content): Excess sodium (Na) in the soil competes with Ca, and other cations to reduce their availability to crops.
• Sub-soil or parent material: Soils derived from limestone, marl, or other high Ca minerals will tend to have high Ca levels, while those derived from shale or sandstone will tend to have lower levels.

• Other cations: Being a major cation, calcium availability is related to the soil CEC, and it is in competition with other major cations such as sodium (Na⁺), potassium (K⁺), magnesium (Mg⁺⁺), Ammonium (NH₄⁺), iron (Fe⁺⁺), and aluminum (Al⁺⁺⁺) for uptake by the crop. High K applications have been known to reduce the Ca uptake in apples, which are extremely susceptible to poor Ca uptake and translocation within the tree.
• Sodium (Na⁺): High levels of soil Na will displace Ca and lead to Ca leaching. This can result in poor soil structure and possible Na toxicity to the crop. Conversely, applications of soluble Ca, typically as gypsum, are commonly used to desalinate sodic soils through the displacement principle in reverse.
• Phosphorus (P): As the soil pH is increased above pH 7.0, free or un-combined Ca begins to accumulate in the soil. This Ca is available to interact with other nutrients. Soluble P is an anion, meaning it has a negative charge. Any free Ca reacts with P to form insoluble (or very slowly soluble) Ca-P compounds that are not readily available to plants. Since there is typically much more available Ca in the soil than P, this interactions nearly always results in less P availability.
• Iron (Fe⁺⁺) and Aluminum(Al⁺⁺⁺): As the pH of a soil decreases, more of these elements become soluble and combine with Ca to for essentially insoluble compounds.
• Boron (B^-): High soil or plant calcium levels can inhibit B uptake and utilization. Calcium sprays and soil applications have been effectively used to help detoxify B over-applications.

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 around 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

• Improve soil structure.
• Reduce weed populations, especially foxtail and quackgrass, plus improve forage quality.
• Reduce leaching of other plant nutrients.
• Generally improve the balance of most soil nutrients.

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 soil. Research 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 8.44:1 in 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.

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.

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.

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

• As a nutrient source
• As lime (CaCO₃), to neutralize soil acidity

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.

* 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.

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. 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.

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