“I’ve seen studies showing a loss of 3 bu. to 5 bu. per acre in soybean fields with magnesium deficiency issues,” says Farm Journal Field Agronomist Ken Ferrie. “In our own studies, the loss has been as high as 10 bu. in soybeans and 7 bu. to 15 bu. in corn.”
A 200-bu. corn crop requires 50 lb. of magnesium per acre. A 60-bu. soybean crop takes up 25 lb. to 30 lb. of magnesium per acre. The secondary nutrient plays several critical roles in plants. “Because the magnesium ion sits in the center of the chlorophyll molecule, it plays a huge part in photosynthesis,” Ferrie says. “One of magnesium’s main functions is to help capture energy from the sun and transfer it into chemical energy. So it is involved with everything energy is used for, such as starch production and leaf and root growth.
“Magnesium also activates several critical enzymes, especially the ones essential to carbon fixation, in which carbon dioxide from the atmosphere is turned into starch. It is a carrier mole- cule for phosphorus; plants can’t take up phosphorus and magnesium unless both elements are present.”
Magnesium influences the earliness and uniformity of crop maturity. “With magnesium issues, crops don’t mature uniformly,” Ferrie says. “This is especially noticeable in soybeans—some plants will be ripe and some will have green stems and lag in maturity.”
Because magnesium is mobile in plants, deficiency symptoms show up first on the older leaves, as magnesium moves into the newer growth. “As magnesium is pulled out of the older leaves, it shuts down the ability of the older leaves to operate photosynthesis,” Ferrie says. “The first symptom we see is pale green leaves. Then interveinal chlorosis develops—leaf veins stay green but the area between them turns yellow. As the deficiency progresses, reddish and purple spots appear on the leaves.
“On soybeans, the leaves fall off, along with the lower pods. You can’t see this from the road—unlike iron or zinc deficiency symptoms that show up at the top of the plant.”
The greater the light intensity, the more magnesium symptoms show up. “If you had wide and narrow rows in the same field of corn or soybeans, the wide rows would show stronger symptoms,” Ferrie says.
On corn plants, leaf striping resulting from magnesium deficiency can be confused with zinc striping. But because zinc does not move inside the plant, zinc deficiency appears in the new leaves, not the older ones.
Magnesium deficiency lowers yield and makes plants more prone to disease. With very severe deficiency, plants can die. Magnesium issues in forage or pastures can lead to grass tetany for livestock.
In any soil, magnesium can become tied up and unavailable if pH rises above 7.4. “But in light, sandy soil, high pH produces a double whammy,” Ferrie says. “Not only is magnesium rendered unavailable because of the high pH, but whatever is available might be leached away by water because sandy soil contains few exchange sites where it can attach to the soil particles.”
Magnesium deficiency often results from competition with other nutrients. “You are likely to have issues with magnesium if your soil contains high levels of potassium, ammonium (NH4) or calcium,” Ferrie says. Magnesium deficiencies are often man-made. “Frequently, failing to implement a balanced fertility program leads to magnesium uptake problems,” Ferrie explains. For example, one farmer applied several years’ worth of potassium when he had extra income. The excessively high level of potassium in his soil interfered with magnesium uptake.
Ferrie has seen magnesium uptake problems result when farmers used ammonium sulfate as their only source of nitrogen fertilizer (usually because it was cheaper). “Ammonium ions in the soil solution compete with magnesium uptake,” he explains. “In addition, when a negatively charged sulfate ion marries up with a magnesium ion, it neutralizes the magnesium ion’s positive charge, and both ions then can be leached out of the soil. In that situation, magnesium is being removed by the plants while it also is being leached out faster than the soil can weather the mineral form into exchangeable magnesium.”
Mistakenly basing lime recommendations for a light, sandy soil on the soil test’s water pH reading, rather than buffer pH, caused another farmer to overapply lime, using a high-calcium limestone. “Because calcium is a competing ion, the high levels of calcium in the soil solution caused poor magnesium uptake and set up the magnesium to be leached away,” Ferrie says.
Another farmer had a similar problem after he overapplied gypsum (calcium sulfate) to a light soil that was low in magnesium.
A soil’s magnesium status often depends on the parent limestone material at local quarries and the soil’s history. “If the quarries in an area produce dolomitic limestone, the farmland in that area usually will be high in magnesium,” Ferrie says. “If they produce calcitic lime, area fields are more likely to be low in magnesium.
“In some areas, where limestone isn’t readily available, farmers use lime byproducts from municipal water plants, which tend to be calcitic.”
Magnesium management starts with a soil test. “See how much exchangeable magnesium your soil has,” Ferrie advises. “Then tissue test during the growing season.
When walking fields, carry a scouting manual to help you recognize symptoms, and examine the older portions of plants. In soybeans, the symptoms might be only 6" to 8" above the soil surface.
Fixing magnesium problems is fairly straightforward.
- Deficiencies are easiest to fix when soil needs lime. Just use the dolomitic form, rather than the calcitic. “Dolomitic lime is 8% to 11% magnesium,” Ferrie says. His studies comparing dolomitic and calcitic lime revealed the importance of using the right type. “In a corn/soybean rotation, the effect showed up in increased yield three soybean crops later,” he says.
- “If the soil doesn’t need lime, use a product such as K-Mag [potassium-magnesium sulfate],” Ferrie advises. “Or add magnesium to your starter or sidedress nitrogen. In sandy soils, replenish magnesium more often.”
- Avoid overapplying lime, especially calcitic lime or potassium. “Put on less product more often and use the 4 Rs—right product, right rate, right time, right placement,” Ferrie says.
- There are differences between crops—corn is less sensitive to low magnesium levels than soybeans. “If magnesium-deficient soil is causing yield and/or harvest problems, consider changing your rotation,” Ferrie says.
- Crop varieties differ in their tolerance to magnesium deficiency, so you might be able to select one that better tolerates deficiencies.
- High magnesium levels inhibit the uptake of phosphorus, potassium and sulfur, so you must aggressively manage those nutrients, Ferrie says.
The Nutrient Navigator series focuses on efficient, environmentally sound management of nutrients. The goal is to provide practical knowledge that helps drive yields and profits higher.