Studies in the 1990s found falling calcium levels in forest soils in the eastern United States. Calcium is vital for plant growth. Calcium taken from the soil by growing plants is replaced by calcium from weathering rocks and rotting plant material. Today, some regions have little weatherable calcium left. Intensive agriculture removes nutrients from the soil without replenishing them. Erosion and acid rain wash calcium away. Deposition of calcium is slowing while rates of depletion are increasing.
Leaching
Calcium is water soluble and will leach -- wash out -- from soils in rain water. Storm water runoff and flash flooding increase calcium loss.
Acid Rain
The increasing acidity of rain increases the leaching of calcium, outpacing its replenishment by weathering of rocks. Acidification also causes increased concentrations of hydrogen and aluminum in the soil. Because of the way chemical bonds are formed and exchanged, the introduction of aluminum and hydrogen into the forest floor displaces calcium, allowing more to leach away.
Monoculture
When farmers or gardeners grow just one crop or plant the same crop on the same ground year after year, mineral depletion occurs. Different plants require different balances of nutrients. Crop rotation helps maintain healthy levels of all minerals. Tomatoes are one crop requiring large amounts of calcium. If tomatoes are grown repeatedly in the same spot without additional fertilizer, calcium levels in the soil will drop. Over production speeds the removal of minerals from the soil. Harvesting prevents the return of minerals through rotting vegetation.
Agriculture
Poor farming practices cause erosion of soils and depletion of nutrients from those soils. The top soils washed away through erosion contain approximately three times the mineral content of the soils left behind. Bare, plowed fields are exposed to erosion. They lack the root systems that bind other soils together. They do not benefit from the return of nutrients to the soil through decomposing vegetation and animal waste. Chemical fertilizers help, but organic matter is still lacking, which reduces soil's capacity to retain moisture. In eroded areas, chemical fertilizers are quickly washed away in runoff water, polluting water supplies.
Forestry
Felled timber represents calcium removed from the land and not replenished.
When land becomes too poor to farm, it is often given over to forestry. Forestry increases calcium depletion because trees consume large amounts of calcium. Calcium is the main component of tree cell structure after hydrogen, carbon, oxygen and nitrogen. Rotting trees return calcium to the soil. If trees are cut and removed, calcium is removed and not replaced.
Forest Fires
The effect of forest fires can be an immediate rush of mineral availability in soils, followed by longer term depletion. Only small amounts of calcium are vaporized unless fires are extremely hot. However, calcium is a substantial component of ash. After large fires, calcium particulates in smoke and ash can be blown many miles away. Further calcium is lost as ashes are washed away by rain and storm runoff.
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