Magnesium versus Manganese: Supplemental sources and application methods – Part II

First soil test, then consider these options for correcting Mg and Mn deficiencies.

Soil testing offers the best opportunities to correct both Magnesium (Mg) and manganese (Mn) deficiencies. Where Mg is needed on acid soils, the recommendation is for at least half a ton of dolomitic limestone (MgCO3+CaCO3 with 11 percent Mg ) per acre before planting. For non-acid soils requiring Mg, either broadcast application of 50 to 100 lbs. of actual Mg/A, or 10 to 20 lbs. of Mg/A banded at planting is recommended.

Magnesium sulfate (MgS04.7 H20 - Epsom salt) is an excellent soluble source containing 10 percent actual Mg. Other soluble sources are Kieserite (MgS04.H20) with 18 percent Mg, potassium-magnesium sulfate (K2S04 + 2MgSO4) with 11 percent Mg and finely ground magnesium oxide (MgO) with 55 percent Mg. For foliar application, 10 to 20 lbs. of Epsom salt in 30 gallons of water per acre can be used.

High rates of potassium (K) fertilizer can induce magnesium deficiency. This can lead to grass tetany disorder in livestock that feed on Mg-deficient grass. Grass tetany is a serious, often fatal metabolic disorder characterized by low levels of magnesium in the blood serum of cattle. In areas where grass tetany is a concern, Michigan State University Extension recommends feeding cows with supplemental magnesium. Also, applying high rates of K in a single application (more than 200 lbs/a) should be avoided.

In the case of Mn, crops vary in their response to supplemental Mn. In Michigan, soybean, wheat and oats are considered highly responsive crops. Corn, alfalfa, grass and sugarbeet are medium responsive. Rye is considered a low responsive crop.

Research has shown that it is difficult to build up available Mn in the soil. As such, if Mn deficiency occurs in a field, it will likely reoccur each year, especially when highly sensitive crops are grown. Even though various micronutrient formulations containing Mn and other micronutrients are available in the market for use as starter fertilizer, the best option to correct Mn deficiency is to foliar spray during the season. Manganese sulfate (MnSO4.3H2O) with 28 percent Mn can be used to provide 1 to 2 lbs. actual Mn in 30 gallons of water.

In the past, studies have shown that tank-mixing micronutrients with glyphosate can cause antagonism, resulting in loss in effectiveness in weed control and reduced nutrient absorption. It is best to make separate micronutrient and herbicide applications to avoid antagonism. If a farmer decides to tank-mix the two, then the synthetic chelated form Mn-EDTA (5 to 14 percent Mn) should always be used along with ammonium sulfate to reduce adverse interactions.

Part I of this article series, “Magnesium versus Manganese: What’s the difference? – Part I," addresses the differences between Mg and Mn in relationship to crop uptake, deficiency symptoms and soil pH.

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