The Q&A Archives: What calcium-magnesium ratio is best?

Question: I'm confused about the proper calcium to magnesium ratio for garden soil. One source says a high ratio is best, the other a low ratio. Which is right? Eugene Shirk Barnett, MO

Answer: Don't get hung up on the ratio numbers, says Daryl Buchholz, an agronomist at the University of Missouri in Columbia. A normal calcium to magnesium ratio can range from 8:1 up to 15:1, so knowing the exact number isn't too helpful. Having adequate levels of available calcium and magnesium is what's important. In our area, if your soil test indicates you have at least 150 parts per million (ppm) of magnesium and the pH is between 6.0 and 7.0, the magnesium and calcium levels are probably adequate, he says. Here in Missouri if the pH is outside the 6.0 to 7.0 range, adjusting it should make plenty of native soil calcium available. If magnesium levels are low, add dolomitic limestone or Epsom salts in quantities recommended on a soil test. Low magnesium or calcium levels can also indicate a potassium imbalance. When the soil potassium gets above 400 ppm, the potassium competes with the uptake of calcium and magnesium in plants, explains Buchholz. Gardeners who add large amounts of wood ash, which is high in potassium, to their soil can cause this deficiency.

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