By Jim Isleib, Crop Production Educator
The question has been raised over and over by Michigan’s Upper Peninsula farmers, but it’s not only an Upper Peninsula issue. Farmers in all areas where soil pH is naturally low, or where magnesium levels are low or high, have concerns about getting their soil out of “calcium/magnesium balance.”
In some areas, the local and most economical source of agricultural lime is from a dolomitic limestone quarry. Calcitic lime may need to be trucked a longer distance, or vice versa. Either way, one source of lime may be cheaper than the other. In this case, it may have been applied repeatedly over many years. The end result of repeated applications of dolomitic lime can be a build-up of soil magnesium level shown in soil test reports. The basic question is: Is magnesium build-up from use of dolomitic lime a problem?
The short answer? Very unlikely.
Calcitic lime is derived from deposits of primarily calcium carbonate. Dolomitic lime is derived from deposits of calcium carbonate combined with magnesium carbonate and contains much higher levels of magnesium. The key factors in deciding which of these types of lime should be applied to your soil is the soil pH and magnesium level.
There is little difference between lime types in their respective ability to neutralize soil acidity. Also, as long as the amount of each is adequate, the balance of magnesium and calcium can vary quite a lot and have little or no impact on crop performance. Making the decision based on the calcium-to-magnesium ratio can be a mistake.
Recommendations from University of Wisconsin Extension’s publication, “Soil Calcium to Magnesium Ratios – Should You Be Concerned?,” include the following:
- Calcium deficiencies in Wisconsin are rare in soils above pH 6.0. However, if a crop requiring a low pH is being grown and liming is not recommended, gypsum can supply calcium to the crop.
- If liming is required, a dolomitic or calcitic liming material will supply sufficient calcium to maintain crop growth. Dolomitic lime sources have the added benefit of increasing available magnesium.
- Choose the most economical liming material when liming is required. Do not apply gypsum or calcitic limestone to Wisconsin soils simply to increase soil calcium to magnesium ratios.
- If you choose a liming material low in magnesium, be careful to avoid magnesium deficiencies. High calcium applications alone can decrease soil and plant magnesium levels. If the soil is acid and originally has a low magnesium content, adding a calcitic (low magnesium) liming material or high rates of gypsum could induce a magnesium deficiency.
Wisconsin research also indicates that, as long as soil magnesium levels are adequate, variations in the calcium-to-magnesium ratio are unlikely to affect alfalfa yields.
If you have serious concerns about which type of lime to use based on soil test report information, feel free to contact your local Michigan State University Extension educator for research-based information before you invest in the application.
Additional resources on this topic can be found at: