Reference Comments

86 Proximal pericarp tissue

Distal pericarp tissue (healthy)

Distal pericarp tissue (BER-affected)

87 Range of values according to salinity treatment and size of fruit

91 Values from left to right for plants that had 33.2% of leaves with tipburn (plants supplied 1/40th control Ca and 3x K), 9% of leaves with tipburn (plants supplied control Ca and 3x K) 1% of leaves with tipburn (control)

94 For 'deficient' values, first value is for an experiment in which 44.5% of fruit had BER, second value for an experiment in which 18.9% of fruit had BER. For 'adequate' value 0.9% of fruit had BER

95 Fruitlets with 'deficient' concentration showed much higher incidence of physiological disorders in storage

45 Range found in fruit harvested in 3 consecutive years. Fruit with the lower values had higher incidence of bitter pit

96 Minimum level for recommending fruit for controlled atmosphere storage. Below this level bitter pit is common

97 Values of 60 and 67 mg kg-1 fresh weight in fruit from different orchards linked with high incidence of internal breakdown and cork spot

Note: Values in fresh matter, unless shown to contrary.

come from elsewhere and been deposited by glaciers, and furthermore, although calcium carbonate is sparingly water soluble, it can be removed by leaching so that the overlying soil may be depleted of calcium carbonate and be acidic.

Some soils contain calcium sulfate (gypsum), but mostly only in arid regions. A further source of calcium in soils is apatite [Ca(OH2).3Ca(PO4)2] or fluorapatite [Ca5(PO4)3F]. Chlorapatite [Ca5(PO4)3Cl] and hydroxyapatite [Ca5(PO4)3OH] also exist in soils (98). Calcium is also present in the primary minerals augite [Ca(Mg,Fe,Al)(Al,Si)2O6], hornblende [NaCa2(Mg,Fe,Al)5(Si,Al)8O22(OH)2], and the feldspar plagioclase (any intermediate between CaAl2Si2O8 and NaAlSi3O8) (98).

Within the fraction of soils where particles are as small as clay particles, calcium occurs in gypsum, calcite, hornblende, and plagioclase. Sherman and Jackson (99) arranged the minerals in the clay fractions of the A horizons of soils in a series according to the time taken for them to weather away to a different mineral. These calcium-containing minerals are all early in this sequence, meaning that calcium is lost from the minerals (and becomes available to plants) early in the weathering process, but has been entirely lost as a structural component in more mature soils (98). Any calcium present in these more mature soils will be present attached to cation-exchange sites, where it usually constitutes a high proportion of total exchangeable cations, so the amounts present depend on the CEC of the soil.

Concentrations of Ca2+ in soils may be affected by ecological disturbance. Acid depositions are known to decrease Ca2+ concentrations in soils, which while not necessarily affecting plant yields directly may have a big impact on ecosystem dynamics. Acid deposition on the coniferous forests of the Netherlands has been shown to give rise to fewer snails, and the birds that feed on the snails have fewer surviving offspring due to defects in their eggs (100). This effect seems to be related largely to the abundance of snails being depressed by low calcium concentrations in the plant litter. In terms of how serious this problem might prove to be, it should be noted that changes in soil Ca2+ concentration caused by acid rain are less than 1 g Ca2+ m"2 year1. This change is small compared with a transfer of 3.3 to 4.7 g Ca2+ m"2 year"1 from mineral soil to young forest stands (101).

Experiments on the Hubbard Brook Experimental Forest in New Hampshire, USA, have shown that calcium is lost from ecosystems following deforestation. This loss is true for other cations and also for nitrate. In the Hubbard Brook experiment, during the 4 years following deforestation, the watershed lost 74.9 kg Ca2+ ha"1 year"1 as dissolved substances in the streams, compared with 9.7 kg Ca2+ ha"1 year"1 in a watershed where the vegetation had not been cut down (102). This increased loss was attributed partly to increased water flows due to decreased water loss by transpiration, but more importantly through the breakdown of the plant material enhancing the turnover of the nitrogen cycle and the consequent generation of H+ ions, thereby releasing cations from the cation-exchange sites of the soil (102). Recent studies have shown that calcium loss continues for at least 30 years, with the longer-term loss possibly occurring because of the breakdown of calcium oxalate in the forest soil after removal of the trees (103).

5.4.2 Soil Tests

The main test for soil calcium is to calculate the amount of the limestone required for a particular crop on a particular soil (see 5.5.2 below).

5.4.3 Tabulated Data on Calcium Contents in Soils

Concentrations of Ca2+ in soils typical of a range of soil orders are shown in Table 5.3.

5.5 FERTILIZERS FOR CALCIUM 5.5.1 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

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