Foliar Madness

Are soil or foliar micronutrients better?

Dan Davidson
By  Daniel Davidson , DTN Contributing Agronomist
Testing soil is one way to collect information about existing micronutrients. (DTN photo by Greg Horstmeier)

Micronutrients are getting more attention thanks to farmers doing more routine soil testing and plant analyses. Those tests show more micronutrient deficiencies even though farmers might not see foliar symptoms in the growing crop.

Applying micronutrients also is making more economic sense because today's higher yields remove more micronutrients from the soil and many of today's commercial fertilizers contain less residual micros.

The questions many farmers ask center on how to best determine a deficiency, and if found, whether to apply micronutrients on the seed, in the soil or to foliage. The best time, of course, is to apply before a deficiency occurs (seen or unseen).

There are eight nutrients that crop plants need in very small quantities; boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), nickel (Ni) and zinc (Zn). Soil and plant tests measure them in parts per million instead of a percent and constitute, in total, less than 1% of the dry weight of plant biomass. A soil application rate is typically less than 10 pounds per acre.

Most growers are already aware of the importance of adding zinc to corn and manganese to soybeans. Boron is now getting more attention as well.

DISCOVERING THE LEVEL

It is easy to get soil micronutrients levels, it's just an additional measurement during standard N, P and K soil tests.

However, micronutrient soil tests aren't reliable predictors of a crop response especially when test levels are low to medium. In those cases an actual crop response to micronutrients depends on the crop grown (corn responds to zinc, soybeans more to manganese) as well as soil characteristics that affect nutrient availability including pH, organic matter, soil texture, and soil phosphorus levels.

Plant tissue testing uncovers unseen deficiencies, nutrient imbalances and even toxicities. Tissue testing is easy and measures the concentrations of nutrients taken up to date and scores the values as deficient, sufficient or excess. Since crop deficiencies can exist without visible symptoms, tissue tests are a good check on whether soil resources and your fertility program are providing enough nutrients to the crop.

Combining soil and tissue tests can help answer the question of how and when to apply micronutrients.

Soil applied: Historically the most common method has been soil application. It's a natural next step when soil tests show overall deficiencies. Micronutrients can be blended with commercial fertilizer at a retail plant, manufactured with other nutrients (such as with Mosaic's MicroEssentials) or offered as a granule coating (such as the WolfTrax EvenCoat line). Micros can be added to either granular or fluid fertilizers, and applied either broadcast or banded (including popups.) Including micronutrients with fertilizers is convenient, enables uniform distribution and reduces application cost.

Seed-applied: Seed treatments are an attractive way to deliver small amounts of micronutrients. Any micronutrient seed treatment can improve stand establishment, speed up early growth and development and overcome short-term spring nutrient deficiencies. The limitation is that seed companies today are loading the seed with so many pesticides, inoculants and biologicals that adding the necessary amounts of micronutrients may not be practical.

Foliar applied: It's become more popular to apply some type of foliar spray, which might include micronutrients, growth regulators and carbon. Most of the foliar micronutrients are soluble and chelated inorganic salts and easily taken up across leaf surfaces. Foliar sprays are uniform, applied at lower rates than soil application and can be combined with pesticide passes. Crops respond immediately by taking up and banking these nutrients in their tissue rather than relying on roots to access them. There is a risk of some leaf burn and the plants are limited in how much they can absorb, so foliar applications can't overcome soil deficiencies of macronutrients.

Rely on soil and tissue tests to guide application to get a return on your investment.

Dan Davidson can be reached at AskDrDan@dtn.com

Follow Dan Davidson on Twitter @dandavidsondtn

(GH/SK)

Dan Davidson