Crop Requirements
Calcium
In contrast to nitrogen, phosphorus, and potassium, calcium is immobile in the phloem i.e., it does not move from older to younger leaves. Calcium moves mostly through the xylem, along with water, and its uptake is reduced by low temperature, drought, high salts in the growth medium, or high atmospheric humidity. If, for any reason, the supply of calcium to meristems of young shoots (or roots) is interrupted even for a short period, localized deficiency, and dieback, may occur. Since little Ca is translocated from leaves to fruits, leaf analysis is of limited use as diagnostic tool for preventing calcium shortages in the fruit that result in BER.
Deficiency
Localized calcium deficiency shows up as dieback of the growing tips (Fig.
2) and fruit blossom-end rot, a physiological disorder discussed in
greater detail below. Typical appearance of the top of a calcium deficient
plant includes: restriction of growth; loss of turgor; dark green small
leaves turning later yellow or orange or purplish; short internodes; leaves
curling downwards and inwards; scorched leaf margins; and, dieback of
the growing tip. Roots are poorly developed and brown, with few root hairs,
and dying back tips. Clusters are weak with poor fruitset and fruit ripening.
In most cases, the calcium deficiency is not in the soil but is induced
by cultural or environmental factors. The most likely cause is water stress
on the plant resulting from inadequate or uneven watering, frequent and
large variations in relative humidity, or a high level of salts (Papadopoulos,
1991).
Toxicity
Calcium, magnesium, and potassium compete for the same sites of absorption
by the plant; so increasing one may decrease another.
Although magnesium deficiency is common, it rarely results in yield reduction and is usually related to competition with other cations, low soil pH, or poor conditions in the rootzone. Adverse root conditions could include soil compaction, waterlogging, water stress, or poor aeration of hydroponic solutions. Magnesium deficiency also sometimes appears at times of heavy fruit load. Under any of these conditions, the plant might not be able to take up sufficient magnesium. Since magnesium is mobile, reserves may be moved from old leaves to new. Magnesium problems might also develop in soilless culture if the magnesium in the nutrient solution drops below the minimum recommended level or becomes unbalanced relative to the other cations (i.e., K+, Ca++, NH4+, H+).
Deficiency
Initially leaf margins on the lower leaves turn yellow. These yellow areas
quickly develop in the interveinal areas of the lower leaves while the
large veins remain green. Magnesium deficiency first appears in the lower
or middle leaves, while manganese and iron deficiencies first appear in
young leaves. The yellowing later spreads to the top of the plant while
affected areas of the lower leaves form necrotic spots between the veins.
In severe situations, the lower leaves dry out completely and the whole
plant turns yellow, withers, and dies. Fruit production is the least affected
and is affected only when magnesium deficiency is serious enough to limit
the photosynthetic productivity of the plant. Magnesium deficiency is
promoted by high pH, high K, or low N concentration in the growth medium.
Excess
High magnesium levels may cause and potassium and calcium deficiency because
of competition in uptake.