AgWatch

Should I Apply Red Or White Lime?

DR. A.J.FOSTER

BLOOMFIELD, MO.
   In the past few weeks I have received a number of calls about the use of dolomitic (red lime) vs. calcite (white lime). Clearly, there exists some confusion on the use of liming material containing Mg. There is a concern that our soils are high in Mg and application of liming material with Mg will increase the soil Mg concentration. Before I can offer my opinion on the subject, let’s first define a few key terms to ensure we are all speaking the same language.
   Dolomitic lime: Liming material used to raise soil pH composed of calcium magnesium carbonate, ideally CaMg(CO3)2
   Calcite lime: Liming material use to raise soil pH composed of calcium carbonate, ideally CaCO3.
   Calcium to Magnesium Ratio: The relative proportion of the percent base saturation of exchangeable calcium and magnesium in the soil.
   Why plants need Ca and Mg:
   The primary function of calcium in plants is to provide structural support to cell walls. Calcium also serves as a secondary messenger when plants are physically or biochemically stressed. While, magnesium is the central atom in the chlorophyll molecule, so it is involved in photosynthesis. It serves as an activator for many enzymes required in plant growth processes and stabilizes the nucleic acids. Plant requires Ca in larger amount than Mg.
   Origin of the Ca:Mg Ratio Idea:
   The origin of the concept of an ideal ratio for Ca:Mg in soil for good plant growth derived from work by Bear and colleagues in the 1940s. They recommended an ideal Ca:Mg ratio of 5:4 for good plant growth. However, their work did not differentiate between crop response (alfalfa) due to pH improvement from lime application to acid soils and the change in Ca:Mg.
   My Perspective:
   In my opinion, providing adequate amount of available Ca and Mg to promote good crop growth is most important. Let’s say this favorable basic cation ratio does exist, it would be like chasing wind to find and maintain in most soil. One reason why we don’t need to spend our time chasing wind to find this ratio is a plant will excrete excess Ca and Mg at the root surface. So in the saying of the legend Bob Marley “A man can only be what he is meant to be, if he takes in too much he will un-filter.” Thanks Bob, it is the same with plants. So, maintaining soil pH in the range suitable for plant growth (5.5-7.5) and sufficient amounts of available nutrients should be our focus.
   Others Perspectives:
   McLean and colleagues in 1983 conducted a series of studies where ratios were manipulated by application of calcite lime, magnesium oxide, and magnesium sulfate and yield response measured. Their results showed the highest yielding treatments and lowest yielding treatments both occurred in soil with similar Ca:Mg ratios, thus indicating that Ca:Mg ratio was not the reason for measured yield differences.
   Webb and colleagues in 1978 in Iowa also addressed the issue of over application of Mg. Webb applied potash and potassium-magnesium sulfate (K-Mag) annually to a Webster soil (total of 784 lb Mg/acre over an 8-year period). The results indicate a response to applied potassium, but no effect of applied Mg.
   Dr. Gene Stevens and colleagues in southeast Missouri studied the effect of Ca:Mg ratio on potassium uptake in rice. They applied different rates of epsom salt (MgSO4), red lime (dolomite), and white lime (calcite) to create different soil Ca:Mg ratios. The results of their study similar to all the previous mentioned studies also show no difference in tissue nutrient concentration and yield.
In closing, the Ca:Mg ratio concept is unproven and should not be used as a basis for fertilization or liming practices. Having sufficient levels of Ca and Mg is the proper method of evaluation, rather than trying to manipulate ratios. For more detail the links below offers great explanation on the subject or you may contact Dr. AJ Foster at the Stoddard County Extension Office. ∆
   DR. A.J. FOSTER: Agronomy Specialist, University of Missouri


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