Richard Taylor, Extension Agronomist; rtaylor@udel.edu
A question came up this past week concerning corn tissue analyses showing low magnesium (Mg) concentration in leaves of corn in the early reproductive stage (R1-R3). Low tissue Mg levels in grass crops is a frequent occurrence in forage grasses and is often seen during the rapid growth of grasses in the spring, and sometimes fall, following intervals of temperatures in the 45-60°F. range. In grazing ruminant animals, the condition often leads to grass tetany, a condition involving serum hypomagnesia leading to possible death in the animals. For corn, the period of cool temperatures is not likely to be involved so what else could explain the low tissue Mg levels?
A review of the literature reveals a number of other factors that may help explain the tissue Mg concentrations. Magnesium (Mg2+) is one of the basic cation and essential elements in the soil that compete for uptake by the corn plant. High levels of the other cations, such as ammonium (NH4+), potassium (K+), and calcium (Ca2+), can compete with Mg for plant uptake and effectively reduce the amount of Mg taken up by the plant. Magnesium, NH4+, and Ca2+ ions are primarily taken up via a mechanism called mass flow that is dependent on the amount of water the plant removes from the soil and transpires through the leaf stomata openings to cool the leaves. The cations move with the water flow and are taken up and accumulated by the plant. This year the excessive moisture present in the soil in many fields and the high relative humidity has reduced the amount of water that the plant can transpire. Lower than expected water use has been noted by people who have been monitoring soil water using soil tensiometers or other measuring methods. Reduced transpiration volume affects Mg uptake more than the other cations and could be one of the factors leading to low tissue Mg concentration.
Another complicating factor is nitrogen (N) fertilization. High N fertilization rates are associated with low Mg uptake, especially where soils have high soil test levels of K and/or in acid soils where soil aluminium (Al) levels are high. This is not the end of the explanation, however. The form of N fertilizer that is available for plant uptake is important as well. It is known that the ammonium form of N is antagonistic to the uptake of Mg by corn whereas the nitrate form can enhance the uptake of Mg by plants. However when soil K is available, an interaction can occur that results in nitrate-N stimulating K uptake and this in turn inhibits Mg translocation or movement to the plant tops (leaves).
Most growers are putting on liquid urea ammonium nitrate solution (UAN) as the major source of N for corn. Irrigated growers often fertigate with UAN during the rapid growth phase of corn. UAN contains N in the form of urea (which in soil is quickly converted to ammonium), N in the ammonium form, and N in the nitrate form with the majority of the N initially available to the corn crop as ammonium. With many of our fields at or near saturation with water, the nitrate form of N has been rapidly denitrified to nitrogen gas (N2), nitrous oxide (N2O), or nitric oxide (NO) all of which are loss from the soil as gases. Denitrification requires the presence of a carbon source such as soil organic matter or manure. Fields that received a poultry litter application will be at a slightly greater risk for denitrification since there will be adequate soil carbon present for the anaerobic organisms to use during the denitrification process. On sandy fields, another risk for nitrate N is the loss of the nitrate below the rooting zone. This year the corn root systems are more shallow than normal due to the saturated soil conditions so it wouldn’t take much downward water movement to remove the nitrate from the root zone.
For growers who used manure as the N source or for growers who have been spoon feeding corn N using fertigation, the ammonium N supply is being frequently replenished while the nitrate supply is being continuously depleted (by denitrification and/or leaching). This set of circumstances is likely to lead to low tissue Mg concentration.
One last possible explanation is that soil Mg losses from leaching tend to be greater when large quantities of N fertilizer are applied which is often the case for high yield irrigated corn. The conditions this year have been very conducive to leaching losses especially on sandy soils where much of the region’s irrigated corn is grown.