Jarrod O. Miller, Extension Educator, Somerset County, MD; jarrod@umd.edu
In many cropping systems micronutrients may be overlooked until a problem arises. These nutrients are needed in such small quantities that it often takes certain soil types (e.g. sandy) or conditions for deficiencies to occur. There are seven commonly discussed micronutrients for crops, known as boron (B), chloride (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo) and zinc (Zn). Other important micronutrients in forage systems is cobalt (Co) and selenium (Se). Of those two, Se is important for livestock, but not considered an essential plant nutrient.
Whether we are dealing with grain crops or forages, soil texture and pH can tell you a lot about micronutrient availability. Crops are more likely to show deficiencies in micronutrients (except Mo) when pH rises above 6.5. Sandy soils are naturally deficient in micronutrients due to parent material, increased drainage and minimal cation exchange capacity. In addition, both Mo and B are likely to leach from sandy soils due to their negative charge. Soils with high organic matter (peaty, swamp type soils) may also adsorb and limit micronutrient availability. At high pH (>7) in Maryland, Mn, Cu and Zn are the most likely to be an issue, even when soil levels may appear adequate.
Most forages shouldn’t have micronutrient issues if pH is managed properly, however, two of the micronutrients (B and Mo) can be a problem where legumes are planted. Research worldwide has observed that legumes (alfalfa, clover, soybeans) are more sensitive to B and Mo deficiencies than grasses. Both B and Mo are important in nodule formation and nitrogen fixation, so their lack will be noticeable in legume crops.
Boron tends to leach easily from sandy soils and is the only non-metal of the micronutrients. In alfalfa, B deficiency results in yellow/red young leaves and prevents flower formation. Alfalfa is more sensitive than clover to a lack of boron. Soil applications of 0.5 to 1.0 lb/acre of B on sandy (or soils low in B) can make up for deficiencies.
Molybdenum is the only micronutrient to become more available as pH rises. It readily adsorbs to iron oxides, so it can be a problem in acidic, weathered soils. Oxides are typically higher in clay textured soils, which show a better response to Mo availability with liming than sandy soils. Grasses will respond to Mo fertilization, but not as much as legumes. Alfalfa is more sensitive than both soybeans and clover to a lack of Mo. Due to the low concentration of Mo in most soils, tissue test may be a better indicator of plant deficiencies. Seed treatments for Mo are recommended (0.5oz/acre) or foliar applications of 2-3 oz/acre.
Tissue tests are also better indicators for Co and Se deficiencies in plants. While Co is essential to the nitrogenase enzyme for plant growth, Se has shown no essential plant function. Animals do require Se, and can receive it through mineral supplements as well as forages. Typically it is recommended to supplement animal diets, however soil fertilization has shown positive results. In Oregon, applications of 0.5-2 lb Se/acre increase the Se content of forage clippings as well as the blood levels of grazing animals. It was not recommended to both fertilize and supplement animal feeds though.
In summary, soil tests can reveal micronutrient deficiencies, but tissue tests can be more effective at determining actual bioavailability. In most cases maintaining a pH 6.0-6.5 will cover most micronutrient issues. Alfalfa, which prefers a pH close to 7 for nodulation, should be scouted more often on sandy soils if a higher pH is desired.
– Kelling, K.A. Soil and Applied Boron (A2522). University of Wisconsin.
– Kelling, K.A. Soil and Applied Molybdenum (A3555). University of Wisconsin
– Gupta, U.C. et al. Micronutrients in Grassland Production. Crops and Livestock Research Center. Agri Food Canada.
– Angima, S. 2007. Selenium Fertilization of Forages. Oregon State University.