In the late 1990s, I was called to northwest Missouri to evaluate several cornfields. Poor corn growth in these fields occurred on steep, eroded areas on the hillside shoulder approximately one-third of the way down the hill slope. My first thought was that corn on this landscape was suffering from nitrogen, phosphorus or zinc deficiencies that often occur in eroded areas--where plant roots must grow in low nutrient- containing exposed subsoil. However, these corn plants expressed magnesium deficiency symptoms (severe interveinal chlorosis), something I had never seen in Missouri outside of the Bootheel. Tissue samples confirmed that the corn was magnesium deficient.

This year, Lyndon Brush, MFA staff agronomist, documented corn magnesium deficiencies in southwest Missouri. Unlike the locations in northwest Missouri, these fields are in a region where soil magnesium content is commonly low. However, this was the first time that we documented visual symptoms, confirmed by low magnesium corn tissue analysis. Other southwest Missouri crops, especially rapidly growing forages, commonly respond to magnesium fertilization.

The optimum magnesium soil test level is based upon the percent magnesium saturation of the soil cation exchange. In general, it is approximately 240 pounds of magnesium per acre. Soil test levels less than 150 pounds per acre are considered low. A 1995 summary of soil test information conducted by Manjula Nathan from the University of Missouri Soil Testing Laboratory provides some interesting information. Twelve percent of all soil samples analyzed by the lab were low in magnesium. Fifty-seven percent of the soil samples from southwest Missouri were low in magnesium. A much smaller percentage of state and regional acreages receive magnesium fertilizer.

Row-crop producers have not observed visual deficiencies, and therefore, have become complacent concerning magnesium. Many animal producers have the philosophy that low magnesium forage is not a concern if they use magnesium feed supplement. This philosophy is not logical; magnesium is an essential plant nutrient. Regardless of nutritional composition, poor crop magnesium nutrition will reduce crop yield, production and profitability.

Why are we experiencing an increase in crop magnesium nutrition problems? Most causes are based upon crop management. Recent research by Dr. Dale Blevins at the University of Missouri has shown a phosphorus/ magnesium relationship in which plants require adequate phosphorus to optimize magnesium uptake. How complete has your phosphorus nutrition program been lately?

Another issue is the interaction among calcium, potassium and magnesium. Calcium, potassium and magnesium are positively charged ions (cations). Therefore, high levels of calcium and/or potassium can interfere with plant magnesium uptake and function. What happens when high levels of calcitic limestone are used to correct soil pH? The soil/plant system becomes saturated with calcium. This calcium saturation can lead to a calcium-induced magnesium deficiency, especially in the year following lime application on low-magnesium soils. But this phenomenon rarely occurs. Potassium is routinely added to soils as a base fertilizer product. Over time, with no concurrent magnesium inputs, the potassium to magnesium ratio will widen.

Corn removes approximately 0.05 pounds of magnesium from the soil/crop system per bushel produced. Alfalfa hay removes over 5 pounds of magnesium per ton removed. Over time, this removal, combined with continued addition of potassium through fertilization and calcium through liming, may lead to crop magnesium nutrition deficiencies.

Do we need to reevaluate magnesium fertilization programs? Based upon the number of grower concerns (few), I would say no. However, based upon documented deficiencies and soil test reports, we should at least be aware of this issue.

Soil testing is the most accurate and efficient means of predicting when and where crops need supplemental magnesium. Keep good records of liming practices and fertilization programs. Know the calcium to magnesium composition of all liming materials used. Keep records of crop removal and potassium fertilization in your fields.

Magnesium fertilization is not complicated. Magnesium is a non-mobile nutrient that can be applied anytime during the year. The most common magnesium material is dolomitic (high magnesium) agricultural limestone. Before liming, always check the effective-magnesium index for the liming material used. If you have low magnesium soils and are in an area where the limestone does not contain magnesium, consider paying a higher price to transport high-magnesium lime from another area. A more costly, but efficient approach is to apply a magnesium-based fertilizer. You have only one viable option. That would be to apply K-Mag or another potassium-sulfur-magnesium-based material. I recommend applying 20 to 40 pounds of actual magnesium per year until you have raised soil test levels to the 240-pound-per-acre optimum level.

Magnesium is required as a crop fertilizer less often than nutrients such as nitrogen, phosphorus or potassium. However, when magnesium is needed and not applied, crop yield and quality are greatly reduced.