Spectrum Analytic Inc
1087 Jamison Rd
Washington Court House, OH 43160
(800) 321-1562
1087 Jamison Rd
Washington Court House, OH 43160
(800) 321-1562
Knowledge is key to using your analytic results to their fullest. The Spectrum Agronomic Library provides you with useful information that will help you to better understand the complex science of agronomy. Our agronomists will be continually adding original and reprinted articles, so check the library regularly for new information.
Magnesium (Mg) does not receive much attention in the popular press. However, an excellent article by Dr. Paul Tracy (MFA, Inc., Columbia, MO) in the November, 2001 issue of Today's Farmer reviewed Mg fertility and a potentially increasing problem in Missouri and other areas.
In his article, Dr. Tracy discussed and illustrated the point that growers in several areas of Missouri should to pay more attention to the need for Mg fertilizer. He noted that the “boot-heel”, in the Southeast part of the State was long known to have Mg problems, but that other areas now appear to be a concern. According to Dr. Tracy, a 1995 survey, conducted by the University of Missouri Soil Testing Laboratory found that only 12% of soil samples analyzed by that lab were low in Mg. However, 57% of soil samples from Southwest Missouri (not the boot-heel area) were low in Mg. Reviewing fertilizer practices; it was found that a much smaller percentage of both State and regional acreage received Mg fertilizer. His points illustrate both the regional nature of Mg needs and the typical lack of attention paid to this essential element.
Magnesium is an essential element for both plants and animals. In plants, Magnesium is the central atom in the chlorophyll molecule. This makes it essential for photosynthesis, the basis of plant life. It also plays other critical roles in plant growth. A shortage of Mg in forage can lead to grass tetany in cattle. Dr. Tracy points out that many livestock producers have the philosophy that low Mg in forage is not a concern if they use Mg feed supplements. While this may help the livestock, it does not correct the plant deficiency and perpetuates low yields of low quality forage. Uncorrected Mg shortages contribute to higher production costs and lower farm profits. Dr. Tracy is supported in this conclusion by Sumner (4) where he states “Although supplying Mg as mineral supplements is generally helpful, complete protection is only obtained when adequate bioavailable Mg in forage is consumed daily.”
Magnesium is absorbed by plants as the divalent cation Mg++. It is one of the three major nutrient cations in the soil. The others are calcium (Ca++) and potassium (K+). These three nutrients are involved in a competitive interaction with each other, as well as being affected by a fourth significant cation, hydrogen (H+). Hydrogen is significant because as the quantity of soil H+ increases, the soil becomes more acid and the soil pH decreases. Acid soil can significantly reduce plant uptake of Ca, Mg, and to a lesser extent K. The major nutrient cations occasionally have other, less significant competitors. For a short time after the application of large amounts of ammonium-N, the ammonium cation (NH4+) presents some competition for the other cations. This competition is normally very short-lived, in localized soil zones, and not significant to crop production. Where the soil pH is below 5.5, the soluble aluminum cation (Al+++) can also be found in significant amounts in the soil. Soluble Al not only provides another source of cation competition (Huang 6), but can be directly toxic to most crops.
Several situations that can lead to Mg shortages in plants are
The principle danger to crops from excess fertilizer placed too close to the seed corn comes from excess salts. In fertilizer, the nutrients that are of primary concern are N, K2O, and S. The combined amount of these three elements is used to evaluate the potential risks of a fertilizer rate and placement. To evaluate a particular situation, add the rate per acre of these three elements in the fertilizer, and refer to the table below.
Consult other sources when thinking of placing seed in close proximity of other crops such as soybeans, sugar beets, potatoes, small grains, etc.
| Maximum “Safe” Rate of N+K2O+S (lb./acre) | ||
|---|---|---|
| Fertilizer Location | CEC < 12 | CEC > 12 |
| With Seed | 5 | 8 |
| 0.25” to 0.5” away from the seed | 8 | 15 |
| 1” to 2” away from the seed | 15 | 40 |
| More than 2” away from the seed | 20 | 40 |
Notes:
While a few aspects of irrigation water quality have a direct impact on plants, the primary goal of water analysis is to judge the effect of the water on the soil, and ultimately on the plants grown on the soil. As such, much of the interpretation of the water analysis is based on a prediction of the consequences for the soil. The interpretation of the test results is, in many cases, dependent on the intended use of the water. Some plant species and production systems may have much different requirements or tolerances. The interpretation guide lists some of these conditions and will help you evaluate your results.