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Calculations of soil magnesium loss by leaching after advent of tilling in the Mississippi River watershed

The prime farmland in the watershed of the Mississippi River may be slowly becoming magnesium depleted. Year after year, crops are harvested, removing magnesium from the top nine inches of tilled soil. Tilling also exposes the soil magnesium to the leaching effects of rain water and runoff. The following calculations suggest that in the last 150 years over eight percent of the magnesium in the top nine inches of soil in the Mississippi River's watershed has run off to the Gulf of Mexico.


1. The Mississippi River contains about 17 ppm dissolved magnesium (1). This does not include magnesium in the undissolved dirt load, which is a different problem.
2. The Mississippi River discharges an average of 640,000 cubic feet of water per second into the Gulf of Mexico (2).
3. The Mississippi River's watershed is 1,257,000 square miles (3).
4. The average magnesium content of the soil in the Mississippi River's watershed is about 250 ppm (4).


1. The annual discharge of the Mississippi River is 20,183,040,000,000 cu. feet of water, which is 571,180,032,000,000 liters.
2. The annual discharge of dissolved magnesium at 17 mg/L is 9,710,060,544,000,000 mg, which is 971,006,544 kilograms of dissolved magnesium discharged annually by the Mississippi River (not counting undissolved dirt load).
3. The Mississippi River Watershed is 1,257,000 square miles, or 804,480,000 acres, which is 35,043,148,800,000 sq. ft. Assuming that in plowed fields, there is very little chance of entry of nutrients from below the plowed strata (top nine inches), the annual runoff of dissolved Mg per square foot is 277 mg.
4. There are 21.2 liters of soil per square foot, nine inches deep. Mississippi River watershed soils average 255 mg/L. Therefore, annual Mg runoff per liter of soil is 277 mg divided by 21.2 liters, which is 13 mg/liter of soil.
5. Dividing the annual Mg runoff of 13 mg per liter of soil, by the average soil content of 255 mg, gives an annual loss of 4.7%.
That loss does not include other losses due to harvesting and removal of magnesium-rich crops.
However, this would not apply to orchards, whose deep roots bring up nutrients from below the top nine inches, nor do these figures take account of deep soils brought to the surface by earthworms or burrowing animals. Some research has been done on the surfacing of deep soils, which will be cited soon in an refinement of these calculations.


Not all soil will leach at the same pace. Forests, grasslands, grazing lands, roadways, and urban areas cannot be expected to lose magnesium nearly as fast as tilled fields do. Geologic eons are needed to leach magnesium from protected soils, else all the magnesium would have leached away eons ago.
Computer analysis of satellite photos can determine the exact percentage of the Mississippi River's watershed that is tilled. If only 33 percent of the land is tilled, that may indicate that as much of 24 percent of the soil magnesium in the tilled areas has leached away.
To test this hypothesis, small remaining patches of virgin prairie should be tested for magnesium content, and compared to adjacent plots that have been tilled during the last 150 years.
If, indeed, a large percentage of soil magnesium has been harvested or leached, that suggests that food grown today may not contain as much magnesium as food grown on the same land would have contained 150 years ago. Magnesium deficiency in consumers may be expected to slowly worsen. Magnesium deficient soil and ground water has been correlated with high rates of cardiovascular disease, suicide, and sudden death (5,6,7,8).
Magnesium deficiency may become a global problem. Asian soils that are fertilized with recycled waste may be richer in magnesium than American soils which receive no magnesium replenishment.
June 18, 1994
Paul Mason, P.O. Box 1417, Patterson, CA 95363


This scatter chart shows a steady decline in the magnesium content of American streams, 1974-1994. (Ref: U.S.G.S. National Stream Water Quality Monitoring Network supplied about 90,000 representative stream water samples. We used the Epi-Info epidemiology program from the National Center for Disease Control (CDC) to create this scatter chart. 9/97