Calcium is the most abundant mineral element by weight in citrus trees, accounting for approximately 1% of tree dry weight. Most Ca resides in the leaves, but fruit also contains Ca at a level of approximately 4.4 lb. per 100 boxes of oranges. Calcium is involved in cell division and cell elongation, is an important constituent of cell walls, and plays a major role in cell membrane integrity.

Calcium is also an important element for root development and functioning. Root growth is severely restricted in Ca-deficient plants, and the roots become more prone to bacteria and fungi infections. Calcium is required for chromosome stability and cell division. It activates several enzyme systems and neutralizes organic acids in plants. An inadequate Ca supply can reduce plant growth and fruit yield long before deficiency symptoms become evident. Soil Ca content is rarely low because occasional applications of lime (calcium carbonate, CaCO3) are used to control soil acidity and because Ca is present in irrigation water. Florida’s alkaline soils have an abundance of Ca because they contain free calcium carbonate (limestone). Despite the abundance of soil Ca, citrus trees can suffer from a range of Ca-deficiency disorders that affect plant tissue function. For example, the “creasing” disorder in navel and Valencia oranges may be caused by Ca deficiency in the albedo of the rind.

Calcium Deficiency

A Ca deficiency in citrus is expressed as a fading of the chlorophyll along the leaf margins and between the main veins during the winter months (Figure 1). Small necrotic (dead) spots can develop in the faded areas. Calcium deficiency produces small, thickened leaves and causes loss of vigor, thinning of foliage, and decreased fruit production. Severely deficient trees can develop twig dieback and multiple bud growth of new leaves. Trees with a Ca deficiency produce undersized and misshapen fruit with shriveled juice vesicles (Figure 2). Fruits from Ca-deficient trees are slightly lower in juice content but higher in soluble solids and acids.

Figure 1. 

These leaves showcalcium deficiency symptoms, includingfading of the chlorophyll along the leaf margins and between the main veins during the winter months.

ca-deficiency2
Figure 2.
These fruit show calcium deficiency symptoms; they areundersized and misshapen with shriveled juice vesicles.
 
Calcium deficiency usually occurs on acidic soils where native Ca has leached. Continuous use of ammonium-containing fertilizer, particularly ammonium sulfate, accelerates Ca loss from soils. Using muriate of potash and S causes similar losses of soil Ca. Soil testing is a widely used method of making liming recommendations. In the long term, liming is the most effective and economic practice to supply Ca to crops in Ca-deficient acid soils because liming the soil neutralizes soil acidity and supplies available Ca. Calcium deficiency can also occur in highly saline soils because of the excessive sodium (Na) concentration. Under such a situation, gypsum can correct the deficiency and reduce the deleterious effect of Na. Short-term calcium deficiency can be addressed by foliar spraying with a water-soluble Ca source such as calcium nitrate. Calcium efficiency in crop production can also be improved by using farmland manures.
 

Footnotes

The information presented above is part of document SL382, one of a series of the Department of Soil and Water Science, UF/IFAS Extension. A version of this article originally was published in Citrus Industry magazine. Original publication date: July 2013. Please visit the EDIS website at http://edis.ifas.ufl.edu.

Mongi Zekri, multicounty citrus UF/IFAS Extension agent; and Tom Obreza, professor and interim associate dean for Extension; UF/IFAS Extension, Gainesville, FL 32611. Photo Credit: Dr. R. C. J. Koo

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