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The total Ca2+ in the earth’s crust is approximately 3.5%. Anorthite is ‘n rock mineral and the most important natural resource of Ca2+ in soil.
In damp regions with soils that are non-calcareous, the calcium index is between 0.5 and 0.7%. In tropical soils that are eroded the calcium would be between 0.1 and 0.3%. The dryer, arid climates would have a higher Ca2+ index (1 – 30%) because of less calcium leaching and calcium precipitations in the soils.
The trend is that rough (sandy) soils in damp regions have exchangeable calcium layers, thus lower plant available calcium. Soils in damp regions have finer textures (clay) that have higher Ca2+ and would have more exchangeable calcium and more plant-available calcium, but in acidic soils, there would be no plant-available calcium.
Figure 1 Calcium cycle in soils
The calcium cycle in figure 1 explains how calcium moves in soil and how it is taken up by the plant. Natural minerals weather in soil and release calcium, ending up in the soil solution. The calcium can leach and because of mass flow (movement of calcium to the roots in water) the calcium can be incorporated into the soil. Plant residue and alternative Ca2+ resources via fertilizer and lime will end up in the soil solution that could possibly leach, binding on clay particles and absorbed by the plant. Calcium in the soil solution could also precipitate as primary and secondary minerals. The largest loss of calcium is by leaching, which can total 400kg Ca2+ per hectare per year.
The most economic yield is achieved when the Ca2+ dominates the TEC (total exchangeable cations) in the soil. Magnesium will suppress calcium if the magnesium levels are high, emphasizing the importance of the correct balance of these two elements. Ca2+ is also important to control the Na levels, as a 5-15% exchangeable sodium percentage would affect the penetration of water in the soil. Ca2+ would move the sodium from the exchangeable complex, creating Na+ salts, resulting in the leaching of sodium salt. Calcium is also important for the soil structure as calcium causes soil particles to flocculate and form a structure.
The needed Ca2+ corrections will yield better long-term results (2 – 3 years), but it is regarded as an expensive correction as producers are more concerned with short-term seasonal results.
The plant uptake of calcium has a relative low potential as calcium is only absorbed by the roots of young plants. Furthermore, potassium (K+) and ammonium (NH4+) competitively suppress calcium and are favourably absorbed by the roots. The translocation of calcium in the plant is upwards in the transpiration system, therefore, calcium is translocation is determined largely by the intensity of transpiration. If calcium is deposited in older leaves, mobilization will not be possible and does not translocate to the growing points of the plant.
Calcium plays an important role in cell extensions and cell division in plants. It is also needed for the stabilization of new call membranes. It creates a steep gradient of K+ between the apoplast and cytoplasm. Calcium enhances the permeability of the cell membrane, allowing inorganic and organic compounds via diffusion.
Calcium-phosphate precipitations can form when the calcium concentrations is too high, resulting in a shortage of both calcium and phosphate. The high calcium concentrations will suppress the magnesium, resulting in a shortage of magnesium as well. Monitoring the calcium concentrate is very important, at the right time, at the right place and using the right method.
Calcium corrections are usually made with:
