Calcium - Wonder or Waste?

Calcium is an essential nutrient, required in small amounts for metabolism and cell nucleus formation. Much larger amounts are tied up in the cell walls, bound to pectin. This is not an essential role, but it does help the plant by stiffening the cell walls. The amount of calcium taken up by crops is relatively small, with most of it in the vegetative parts of the plant rather than the grain. For example, a corn crop yielding 150 bushels per acre will take up between 25 and 50 pounds of calcium per acre, but only one pound per acre is removed with the grain.

Calcium is carried to the roots with the soil water as it moves to the roots. In most soils, more calcium is carried to the roots than the plant needs, and the excess is filtered out of the water and accumulates around the roots. This is not evident in annual crops, but take a look at tree roots if you get a chance. A casing of lime is often found around the roots, where the excess calcium reacted with carbonates released by the roots, to form limestone.

When calcium deficiency occurs in plants, it is generally due to a lack of water rather than a lack of calcium in the soil. Calcium is only moved through the xylem, carried from the roots to the rest of the plant with the water stream, and stays where it is deposited. During periods of moisture stress, the fruits and newly opening leaves don't transpire as much as the rest of the plant, and so don't receive as much calcium. This can result in pockets of cells with weakened cell walls, which break down when the fruit begins to grow rapidly again. Blossom end rot in tomatoes or bitter pit in apples are examples. It is rarely, if ever, a problem with grain crops.

Calcium in the soil is present as a positively charged ion (cation), like potassium or ammonium. Unlike these cations, calcium has two positive charges instead of one. This means that it balances two negative charges on the soil particles, and can sometimes act to hold clay particles together. In western Canada, this is evident where some soils have large amounts of sodium, and adding calcium will improve the soil structure. Most Ontario soils, however, have calcium as the dominant cation, so adding more makes no difference to the soil structure. Also, organic matter is a much more effective soil particle glue than any cation!

The calcium in soil solution or adsorbed on soil particles is only a small part of the total amount of calcium in the soil, particularly in alkaline (pH >7) soils. Limestone (calcium carbonate) can make up over 30% of the soil! This may have come from limestone ground up and left behind by the glaciers, or from calcium ions reacting with carbonic acid in the soil. Either way, it represents a huge reserve of calcium, as well as a buffer against changes in soil pH. The older soils in areas south and east of us don't have this luxury. That is why they recommend and need calcium, while we don't. So don't expect calcium recommendations from those areas to work here in Ontario.

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