Understanding Crop Response to Micronutrients
Alan Blaylock, Ph.D.
Dr. Alan Blaylock brings extensive North American and international experience in nutrient management to the agronomy team. University studies and service as a university extension soils specialist prepared him for a long career in the fertilizer industry. Having managed both domestic and global research and education programs, Dr. Blaylock has a wealth of experience in applying science-based nutrient management principles and products to solving practical questions. Dr. Blaylock earned Bachelor of Science and Master of Science degrees in agronomy and horticulture from Brigham Young University and a Ph.D. in soil science from Iowa State University. He has been in agriculture his entire life — from his childhood on an irrigated farm in eastern Oregon to teaching soil science at Iowa State University to his current role as an agronomist at Nutrien. These diverse experiences helped Dr. Blaylock develop the skills to excel in translating complex scientific principles into practical solutions. Although early in his university studies he explored computer science as a profession, deep family roots in agriculture brought him back to the people and values of his heritage. His career satisfaction comes from helping others improve the performance of nutrients and cropping systems.
There are many factors that affect crop response to micronutrients, and response to micronutrients is often less predictable than response to macronutirents, such as nitrogen (N), phosphorus (P), and potassium (K). Responses to micronutrients may be dramatic if the nutrient is deficient, but more often, responses are incremental yield increases or even only maturity or quality improvements. Micronutrient chemistry in the soil is complex and there are numerous interactions with other nutrients and environmental conditions. While predictability of micronutrient response may be less than some other nutrients, it can be improved by considering the interactions with some of the factors affecting crop nutrient response.
Determining micronutrient needs
Micronutrient need is evaluated first by soil testing. Appropriate soil testing provides an indicator of the probability of crop response to a given nutrient. Because of the complexity of micronutrient interactions with other production factors, the predictability of micronutrient response is often somewhat less than for other nutrients. Responses to micronutrients can be observed when soil test levels are adequate. Conversely, a low soil test does not guarantee crop response when other factors are limiting. Nevertheless, soil test information should not be ignored. Crop history and field scouting records help to pinpoint previous problem areas and gain some insight as to how the crop responded to treatments. Plant analysis provides a good evaluation of the micronutrient status of a plant when soil tests and field scouting are not conclusive. For best results, soil and plant analysis should be used together to detect shortages and to develop effective micronutrient management programs. Micronutrient response is also impacted by soil pH, environmental conditions, soil texture, and organic matter level.
Soil pH is a major determinant of micronutrient availability in the soil. Availability of boron (B), copper (Cu), iron (Fe), manganese (Mn), and zinc (Zn) decreases sharply as soil pH increases. Soil pH governs the reaction products of fertilizers applied to the soil and is a major reason most of what we apply as fertilizer becomes fixed or “tied up” as insoluble minerals in the soil. The chemistry of high pH soils makes them prone to deficiencies of most micronutrients. Over-liming can actually induce micronutrient deficiencies. Conversely, acid soils are less likely to be deficient in available micronutrient.
Soil Conditions and Root Growth
Adverse soil conditions such as cold temperatures, wet soils, poor drainage, compaction, root pruning, and disease all decrease rooting volume and therefore negatively affect nutrient availability. Some nutrients, such as zinc are well known for being less available under adverse conditions. Starter fertilizers are often applied to overcome some of these conditions. Addition of micronutrients in the starter fertilizer can produce additional response even when soil test levels are high and response is not expected.
Soil Organic Matter and Texture
Soil organic matter and texture extremes are often associated with micronutrient deficiency. Very high soil organic matter results in organic complexation of the micronutrient metals, especially copper and manganese. Low organic matter, especially when resulting from topsoil loss by soil erosion or leveling when a calcareous subsoil is present, often causes micronutrient deficiencies. Organic matter decomposition is an important soil source of these nutrients. Natural chelators that can make micronutrients more available are present at much lower levels when soil organic matter is low. Very sandy soils have low cation-exchange capacity and are easily leached. In very clayey soils, diffusion, the mechanism by which many of these nutrients move to roots, is slower. Although soil levels of the nutrient may be high, movement to the roots may be inadequate to supply the plant during periods of peak demand.
Timing and methods of application
Broadcast applications are often most convenient, but require higher rates for response and sometimes, such as with Fe on alkaline soils, are rendered ineffective by the soil conditions which originally created the need for treatment. Soils that are low in a particular nutrient may benefit from broadcast applications at higher rates to build available nutrient levels. Broadcast applications of B are usually more effective than for other nutrients because it is mobile in the soils and moves more easily to plant roots.
Band placement is usually superior on alkaline soils because of the propensity for micronutrients to be tied up with other soil minerals. The micronutrient metals, Cu, Fe, Mn, and Zn, are immobile in the soil and are supplied to plant roots primarily by diffusion. Band applications frequently produce the desired response at one-fourth to one-half the rate needed with a broadcast application. In a band application, soil pH can sometimes be manipulated as a tool for improving micronutrient availability. Several studies have documented better performance of micronutrients applied in a band with other acid or acid-forming fertilizers (Miner et al, 1986; Petrie and Jackson, 1984a; Petrie and Jackson, 1984b). A summary of Kentucky field trials evaluating methods of Zn application to corn (Table 1) showed band placement to be better than either broadcast or foliar methods.
Foliar sprays allow for the use of a minimum of product and rapid absorption and correction of deficiencies. Foliar applications should be made as soon as the deficiency is observed. By the time visual symptoms are observed, some yield potential may have been lost. Rescue foliar applications may be less effective because they are often made after deficiencies are observed and yield potential has been lost. For some situations, foliar applications may be the best method of correcting micronutrient deficiency. For example, Fe and Mn deficiencies on alkaline soils are difficult to correct with soil applications, but can be readily corrected with timely foliar treatments. Only small amounts of product are needed, but repeated applications may be necessary to maintain proper nutrient supply. Foliar applications should be made in consideration of plant nutrient demand and soil supplying capacity. Foliar feeding strategies should be accompanied by regular field scouting and knowledge of the soil, environment, and production system. Plant tissue analysis can be useful in diagnosing nutrient status before symptoms appear.
Seed coating fits best with Mo applied with the inoculation for legume crops. Seed coatings have been used with some success with some other micronutrients, but it may be difficult to supply sufficient amounts of some nutrients to provide the entire crop need.
Confident diagnosis of micronutrient needs requires more than a scan of laboratory results from a 0-6″ soil sample. Success increases dramatically when considering overall fertility management, management level of the producer, soil type and conditions, crop sensitivity, and past observations of crop response, quality, or deficiency symptoms. Responsible agronomists can give better recommendations and improve the probability of economic return if this additional information is used in making micronutrient recommendations.
- Frye, W. W.; H. F. Miller, L. W. Murdock, and D. E. Peaslee. 1978. Agronomy Notes. 110. https://uknowledge.uky.edu/pss_notes/110
- Miner, G.S., S. Traore, and M.R. Tucker. 1986. Corn response to starter fertilizer acidity and manganese materials varying in water solubility. Agron. J. 78:291-295.
- Petrie, S.E. and T.L. Jackson. 1984a. Effects of nitrogen fertilization on manganese concentration and yield of barley and oats. Soil Sci. Soc. Am. J. 48:319-322.
- Petrie, S.E. and T.L. Jackson. 1984b. Effects of fertilization on soil solution pH and manganese concentration. Soil Sci. Soc. Am. J. 48:315-318.