Calcium and Magnesium For Mississippi Crops
Calcium (Ca) and (Mg) magnesium are positively charged secondary nutrients. They are generally adequate in most Mississippi soils with favorable pH and organic matter levels. They affect acidity when applied to the soil, and form the backbone of liming materials used to raise pH. Materials containing Ca and Mg often are used as soil amendments rather than as fertilizers.
The primary functions of calcium in plant growth are to provide structural support to cell walls, and to serve as a secondary messenger when plants are stressed from physical or biochemical reasons.
Calcium deficiencies rarely occur in Mississippi soils. Soils with favorable pH levels are normally not deficient. Acid soils with soil test Ca of 500 pounds per acre or less are deficient for legumes, especially peanuts, alfalfa, clovers, and soybeans. At this level, limited root system crops such as tomatoes and peppers would also need additional Ca.
Soluble Ca is available as the Ca2+ ion and is needed for peanuts at pegging time and for preventing blossom end rot in peppers and tomatoes.
Calcium may be lost from the soil when it is (a) dissolved and removed in drainage water, (b) removed by plants, (c) absorbed by soil organisms, (d) leached from the soil in rain water, or (e) absorbed by clay particles. Deficiency symptoms are indicated by death of growing point, abnormally dark green foliage, weakened stems, and shedding of flowers and any combinations of these.
Limestone is the primary source of calcium in Mississippi. Other common sources include basic slag, gypsum, hydrated lime and burned lime, which contain more readily available forms. Gypsum, hydrated lime, burned lime, and basic slag are recommended for peanuts, pepper, and tomatoes to prevent pops and encourage pegging in peanuts, They also prevent blossom end rot in tomatoes and peppers. Gypsum (calcium sulfate) does not affect soil pH despite containing calcium.
Magnesium is adequate for crop production in most Mississippi soils except the coarse sandy soils of the Coastal Plains and the heavy dark clays of the Blackbelt Prairie. Magnesium is absorbed as the Mg2+ ion and is mobile in plants and moves from the older to the younger leaves. It leaches from the soil like Ca and potassium (K).
Magnesium is the central atom amid four nitrogen atoms in the chlorophyll molecule, and therefore involved in photosynthesis. It activates many enzymes required in plant growth processes, and stabilizes the nucleic acids.
Deficiencies are likely seen in crops such as legumes, corn, sorghum, cotton, and certain leafy vegetables as interveinal chlorosis (yellow in older leaves, but veins remain green). The leaves may become pink to light read in color, and may curl upwards along the margins.
To correct soil deficiency in Mg, use dolomitic lime when lime is needed, and soluble sources of Mg when lime is not needed. It may be generally needed for cotton production in the Blackland Prairie.
Cattle are often affected by grass tetany when forage Mg content is low. Other factors include N, Ca, and K plant nutrition, stage of growth (usually in spring), lactating cattle, and seasonal conditions. Dolomitic limestone is recommended as a liming material where grass tetany has been a problem. Supplemental Mg and should be provided to grazing animals where and when grass tetany is an issue. For more detailed information on these issues, see Mineral and Vitamin Nutrition for Beef Cattle, MSU Extension Service Publication 2484.
The most common soluble sources of magnesium to use as fertilizer are magnesium sulfate (containing 10% Mg and 14% S, also known as Epsom salt), sulphate of potash magnesia (containing 11.2% Mg, 22% S, and 22% K2O, commercially sold as K-Mag), and magnesium oxide (containing 55% Mg, also known as magnesia).
Calcium magnesium ratios in soils
Occasionally there are inquiries about locating gypsum (calcium sulfate) or dolomitic lime (magnesium) from places in Mississippi where soil testing history indicates soil amendments with either calcium or magnesium are unlikely to be recommended.
Soil test recommendations from soil analysis rely on basic philosophies. Most land grant university recommendations seek to provide sufficient crop nutrients for a crop to be grown this year based on critical values of the nutrient in the soil. These recommendations are based on field calibration and correlation studies.
Other in the recommendation universe use a Basic Cation Saturation Ratio philosophy that an ideal ‘mix’ of nutrients can exist in the soil. This concept originated in a series of papers in the 1940’s from New Jersey that said the exchangeable cation saturation in a productive soil is 65% calcium and 10% magnesium. Other formulations have been proposed as well.
Trials in Wisconsin, Ohio, and more recently Missouri have not shown the advantages reported in the earlier New Jersey papers. Cotton yields over two years in the Delta section of Missouri in a soil which had been amended to attain Ca to Mg ratios ranging from 2.5:1 to 7.6:1 were not a function of the ratio. (Complete information is available in Stevens et al., Journal of Cotton Science 9:65-71 available online.)
In this study, whole plant cotton potassium uptake measured at first square over three seasons was not different across the range of Ca/Mg ratios. There was no effect on fiber quality.
Other Magnesium Issues
Within plant cells, magnesium does not bind as strongly as calcium or potassium to the cell walls. This competition can lead to lower uptake of magnesium in situations with high calcium or potassium. Potassium induced magnesium deficiency has been diagnosed in a few situations in Mississippi over the past thirty years. One situation in the south Delta in a corn field a few years ago was unique because soil test potassium levels (i.e. exchangeable potassium) were very high. Since the soil was almost pure sand with a very low cation exchange capacity, this was not a base percentage issue but a sheer quantity problem.
This is not to say categorically that no Mississippi soil fertility situation exists which could not benefit from gypsum or dolomite amendment. Gypsum is useful when additional calcium is needed for some crops. It also helps with some structural issues, and is the amendment of choice on salt affected soils such as those affected by hurricane storm surge. Dolomite is an excellent liming material, and the material of choice in some parts of the state not close to water or rail transportation hubs, or where the magnesium is actually needed.