Have you Forgotten About Magnesium? by J. Benton Jones, Jr. 2011-09-01 Print Email A doctor of science has the latest word on what determines a magnesium sufficiency in a rooting medium. I began my professional career as an assistant professor of agronomy at the Ohio Agricultural Research and Development Center (OARDC). In order to become acquainted with the agricultural industry in the state, I attended regional and county extension meetings, met with farmers and made visits to the outlying agricultural research stations. With the coming of spring, newly planted crops were emerging, new growth was appearing in pastures and trees were advancing into full leaf. With the coming of summer, however, I observed that not all crop plants were growing very well, with visual evidence of a possible nutrient element deficiencyeven some of the roadside plants were showing signs of stress. The visual leaf symptoms on all of these plants were similar to those present in magnesium (Mg) deficiencies. A graduate student in the agronomy department was concluding a long-term corn fertilizer research project on plots that were located on each of the major soil types in the state and he had observed visual leaf symptoms of a possible nutrient element deficiency on corn plants in some treatment plots. I had him collect corn leaf tissue from all of the plots for elemental analysis and in my visits to the various outlying research stations I also collected soil samples and leaf tissue from plants showing symptoms. That fall and winter was spent assaying the soil and plant samples collected and then making an evaluation of the results. Most of the plant tissues collected were found to be either deficient in Mg or near what would be considered deficient for each crop or plant species. Most of the soils collected with the plant tissues, however, soil-tested sufficient in Mg, based on current soil testing interpretation criteria. A colleague and I then decided to do a more detailed analysis that would require us to collect sufficient quantities of soil in order to conduct a greenhouse study. With the coming of the next crop year, we made preparations to revisit those sites where I had collected plant tissue and soil samples the year before. But as the season progressed, the expected symptoms of Mg deficiency did not appear! No more widespread signs of Mg deficiency were seen in field crops or even among roadside plants. So what had been the cause for the high incidence of Mg deficiency symptoms seen in many plants throughout the state the previous year? The answer was that the previous year was the final year of a succession of summers of low rainfall and high air temperatures, coupled with high light-intensity days due to cloudless sky conditions, meaning that plants had undergone a number of years of moisture and heat stress. Now, with more normal rainfall, moisture stress was at a minimum, while air temperatures were also within their normal expected range and cloud-covered days had returned. It turned out that stress was the factor that had contributed to the widespread appearance of Mg-deficient plant leaf symptoms and that these symptoms had then disappeared when growing conditions became less stressful. At one time Mg was classified as a secondary element along with the elements calcium (Ca) and sulfur (S). Today, that classification has been dropped and Mg, Ca and S are classed as major elements, in the same grouping with the elements nitrogen (N), phosphorus (P) and potassium (K). An atom of Mg is in the structure of the chlorophyll molecular. Magnesium acts as an enzyme activator in various energy-transfer functions in plants, so any deficiency will slow plant growth. The visual leaf symptom of a Mg deficiency is a chlorosis appearing on older leaf tissue, which is an indication that this element is mobile in the plant. For those growing in soil, Mg sufficiency is usually assumed when the soil is adequately supplied with Ca and the soil water pH is within the recommended range. However, Mg soil availability is pH-dependent and if the soil water pH is less than 5.4 Mg deficiency can occur even when soil Mg availability is classed as being sufficient. When liming an acid soil based on a soil test recommendation, the form of limestone recommended may be specified depending on the soil test level for Mg. If the level is less than sufficient, dolomitic limestone would be recommended, since this form of limestone contains Mg. Among the three cations, K+, Ca2+ and Mg2+, the Mg2+ cation is the least competitive and its deficiency can be induced when both K and Ca are at high levels in the rooting medium, whether the rooting medium is a nutrient solution or a solid matrix. Ammonium (NH4+) is another cation that is highly competitive among the three major cations and its presence in the rooting medium can be a factor in creating Mg deficiency in a plant. There are plant species and varieties within species that are sensitive to Mg and deficiencies can occur when the plant is under moisture or temperature stress, even when there is sufficient Mg in the rooting medium or in that being supplied by a nutrient solution. Tomato is a plant that would be classified as Mg sensitive. Among field crop plants corn is also Mg sensitive, although plant breeding has reduced this sensitivity to some degree. The appearance of visual Mg deficiency symptoms in greenhouse tomatoes is not an uncommon occurrence when plants are setting and maturing fruit, as this function acts as a form of stress on the tomato plant's metabolism. Adding more Mg to the nutrient solution or rooting medium at this plant stage of growth will not correct the deficiency, though, as its origin occurred during the vegetative plant growth period, either due to insufficient Mg being supplied or as a result of an imbalance among the major cations that reduced Mg root absorption. Although insufficiency of Mg during vegetative growth may not result in visual leaf symptoms it can eventually be a factor resulting in lower than expected fruit yield. Based on my own hydroponic tomato growing experience, I have increased the Mg content in the Hoagland/Arnon nutrient solution formulation by 50 per cent, so that when the tomato plant enters its reproductive stage there is sufficient Mg in the plant to advert the potential for deficiency. I believe that many nutrient solution formulations are insufficient in Mg or that the other major cations in the formulation are at such concentration levels that Mg root absorption is impairedthe Steiner concept of elemental balance among the ions in a nutrient solution addresses the issue of cation balance as essential to ensure plant nutrient element sufficiency1. Most hydroponic nutrient solution formulations do not take into consideration the effect that Mg concentration and its ratio among the other major cations can have on the potential for creating a Mg insufficiency. What determines Mg sufficiency in a rooting medium and within the plant is a complex subject, with the occurrence of its insufficiency being the result of various factors as discussed in this article. Magnesium is an essential plant nutrient element that has attracted little attention, but its insufficiency can significantly affect plant growth and fruit yield. How do you avoid an insufficiency? You should ensure that there is sufficient Mg in the rooting mediumin proper balance among the other cation essential nutrient elementsand that those factors that can place undue stress on your plants be minimized.