Millet,
See below for some extracts on Calcium deficiency in citrus. It is more common in areas where soils are acid and high in Na and aluminium which is common in my area. I had splitting of fruit and a citrus grower said to me don't assume weather and soil moisture issues as calcium is naturally low in my area of the humid tropics. It is not as crazy as you may think and the foliage signs are subtle.
Calcium deficiencies in citrus
There are few visual signs of calcium deficiency in the leaf. Leaves may show interveinal symptoms similar to iron or manganese deficiencies but these are rare. Leaves with calcium levels below 1.6% are deficient, however, trees lack vigor and start defoliating when the leaf calcium level falls below 3%.
Calcium deficient trees are stunted due to poor root development, with dark green foliage. The main impacts of calcium deficiency are root systems that are more susceptible to root diseases and fruit which is more prone to albedo breakdown and splitting.
Calcium (Ca)
A deficiency of calcium in citrus is expressed as a fading of the chlorophyll along the leaf margins and between the main veins during the winter months. 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 grown under Ca deficiency produce undersized and misshapen fruit with shriveled juice vesicles. Fruit from Ca-deficient trees are slightly lower in juice content but higher in soluble solids and acids.
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. Use of muriate of potash and sulfur cause similar losses of soil Ca. Liming the soil not only neutralizes soil acidity but also supplies available Ca. Calcium deficiency can also occur in highly saline soils due to the excessive sodium (Na) concentration. Under such a situation, gypsum can correct the deficiency and reduce the deleterious effect of Na. Calcium deficiency can also be corrected by foliar spraying with a water-soluble Ca source.
Made worst by
Acidic soils. Sandy or light soils (leaching). Acid peat soils. Soils rich in sodium. Soils rich in aluminium. Drought conditions. Fruit high in nitrogen or potassium. Large fruit.