Soils high in magnesium (Mg) often suffer from potassium (K) deficiency. High levels of magnesium also make it difficult for plants to successfully utilise the bit of potassium that exists in the soil. Plants growing in such a situation often lack vigour and suffer from stunted growth. The high magnesium content also causes the soil to lose its structure and drain slowly.
High levels of magnesium occur in soil that was once beneath the ocean. As the continental and oceanic plates collided, the impact and friction caused the oceanic crust to crumble. The serpentine rock created by the collision contains high levels of magnesium. When soil contains 300 ppm (parts per million) of ammonium acetate extractable magnesium, it has the designation of being high in magnesium, according to Calcium Products Incorporated. The soil will also often have less than 100 ppm of potassium. Soils with high magnesium content can also contain dangerous levels of nickel, chromium and cobalt.
Soil that suffers from high levels of magnesium can form a hard crust, which usually takes on a cracked appearance. Farmers often have a difficult time successfully tilling this type of soil. Adding abundant organic matter such as aged manure to the top 12 inches of the soil can make it viable, so crops will grow successfully. Organic matter will also help improve soil structure and drainage. Testing the soil each year will offer insight into the effectiveness of the treatment.
Magnesium and Calcium
Soil should have a calcium-to-magnesium ratio of 4:1 to 7:1, according to Calcium Products Incorporated. Magnesium causes the soil particles to bind together, but calcium causes the particles to separate. A good ratio of calcium to magnesium allows the soil to have adequate aeration and drainage. Soils with high magnesium levels lack sufficient calcium. High-magnesium soils form a hard coating, and water will run off the soil's surface.
Balancing the Soil
Adding lime to the soil can help displace the magnesium in the soil and facilitate drainage. Dolomitic limestone contains high levels of magnesium and may cause the soil's magnesium imbalance to become worse. Calcitic limestone, on the other hand, contains high levels of calcium but extremely low levels of magnesium, so it will help balance out the soil. Adding calcium sulphate, also called gypsum, to the soil will help move the magnesium away from plants' root zones. Also consider adding calcium carbonate. Adding small levels of potash can also help improve the soil.
- Calcium Products Incorporated; Calcium Magnesium Ratio; Craig Dick; October 2009
- A & L Great Lakes Laboratories: Calcitic or Dolomitic Limestone?
- Lake County Winegrape Growers; High Magnesium Soils; Erica Lundquist
- Spectrum Analytic Inc.: Magnesium in the Soil
- Agri-Food Laboratories; Measure and Manage High Magnesium Soils; Dale Cowan