Kinds of Fertilizer
The most common application of calcium to soils is as calcium carbonate in chalk or lime. This practice occurred in Britain and Gaul before the Romans (Pliny, quoted in Ref. (105)). It does not come strictly under the definition of fertilizer, as the main functions of the calcium carbonate are to make clay particles aggregate into crumbs, thereby improving drainage, and to lower soil acidity.
Following an addition of lime, Ca2+ from the calcium carbonate (CaCO3) exchanges for Al(OH)2+ and H+ ions on the cation- exchange sites. The Al(OH)2+ ions give rise to insoluble Al(OH)3 that precipitates; the H+ ions react with bicarbonate (HCO3)- that arises during the dissolution of calcium carbonate in the soil water. This reaction leads to the formation of carbon dioxide, lost from the soil as a gas, and water, both of which are neutral products (107).
In very acid soils, there is a shortage of available calcium, and application of calcium carbonate will help rectify this problem. One of the outcomes of adding calcium would be to displace Al3+ and H+ ions from the root plasmalemma, where they would otherwise be displacing Ca2+ ions (108). Experiments with alfalfa (Medicago sativa L.) grown on acid soils showed that while application of lime increased calcium concentrations in the shoots, it also decreased concentrations of aluminum, manganese, and iron. As those cultivars that were the least sensitive to the acid soil had lower concentrations of these three elements anyway, it seems as if the beneficial effect of the lime was in modifying soil pH rather than supplying additional Ca (109).
The more neutral or alkaline pH brought about by liming gives a more favorable environment for the microorganisms of the nitrogen cycle, enhancing the cycling of nitrogen from organic matter. It also increases the availability of molybdenum, and it restricts the uptake of heavy metals (107).
Another action of lime is to decrease the concentration of fluoride in tea (Camellia sinensis L.) plants. This crop accumulates high concentrations of fluoride from soils of normal fluoride concentration. The action of liming in limiting fluoride concentrations in tea plants is surprising given that the uptake of fluoride is higher from more neutral soil than from acid soil and given that liming may increase the water-soluble fluoride content of the soil (110). In this case, it appears that the Ca2+ in the lime either affects cell wall and plasmalemma permeability or changes the speciation of the fluoride in the soil.
In some instances calcium sulfate (gypsum) may be applied as a fertilizer, but this application is more for a source of sulfur than calcium or to improve soil structure. Apatite (applied as rock phosphate) and superphosphate contain twice as much calcium by weight as the phosphorus that they are used primarily to supply, and triple superphosphate contains two thirds as much calcium as phosphorus (98). One situation where gypsum is particularly useful is in the reclamation of sodic soils, where the calcium ions replace the sodium on the cation-exchange sites and the sodium sulfate that results is leached out of the soil (107).
Calcium nitrate and calcium chloride are regularly used as sprays on developing apple fruits to prevent bitter pit (111). Of the two calcium forms, nitrate is less likely to cause leaf scorch, but some varieties of apple are susceptible to fruit spotting with nitrate. Dipping the fruit in CaCl2 immediately after harvest supplements the regular sprays (111). Spraying apple trees with calcium nitrate during the cell expansion phase of fruit growth increases the nitrogen and the calcium concentrations in the fruit at harvest and gives firmer fruit at harvest and after storage (112).
Application of calcium salts to sweet cherry (Prunus avium L.) fruits just before harvest may also decrease the incidence of skin cracking that follows any heavy rainfall at this time (43). Multiple applications throughout the summer give better protection, and CaCl2 is better than Ca(OH)2, as the latter can cause fruit to shrivel in hot seasons (113). Recent research has shown that spraying CaCl2 and boron with a suitable surfactant on strawberry plants at 5-day intervals from the time of petal fall gives fruits that are firmer and more resistant to botrytis rot at harvest, or after 3 days storage, than untreated fruits; after the 3 days, they have a higher concentration of soluble solids and more titratable acidity (114). Treating pineapples with lime during their growth seems to lower the incidence of internal browning that arises in the fruit in cold storage, and increases their ascorbic acid content (38). The fruit of tomato cultivars particularly susceptible to BER (e.g., the beefsteak cultivar Jack Hawkins) may be sprayed with calcium salts, although the efficacy of this treatment is doubtful.
There are also calcium treatments for improving shelf life and fruit quality that are used after harvest. For example, dipping cherry tomatoes in 25 mM CaCl2 after harvest increases apoplastic calcium concentrations and decreases incidence of skin cracking (115). Vacuum infiltration of Ca2+ increases the time of ripening of peaches, so that they can be stored for longer periods before sale, and such use of calcium salts is common for tomatoes, mangoes (Mangifera indica L.), and avocadoes (Persea americana L.) (116). The firmness of plums (Prunus domestica L.) is increased by pressure infiltration of 1 mM CaCl2 (117).
There is some evidence that supply of supplementary calcium nitrate partially alleviates the effects of NaCl salinity in strawberry in hydroponic culture (118) and in cucumber and melon (Cucumis spp. L.) in irrigated fields (119).
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