Regis Voss, Agronomy, (515) 294-1923
Karen Bolluyt, Agriculture Information, (515) 294-3701
MICRONUTRIENTS PROBABLY ADEQUATE ON MOST IOWA SOILS
AMES, Iowa -- Iowa farmers are buying more micronutrients for their fields than they did a decade ago, a trend documented in reports from the Iowa Department of Agriculture and Land Stewardship. Many of these purchases probably are not necessary, according to Iowa State University specialists.
Among the micronutrients are iron, zinc, manganese, molybdenum, chloride, copper and boron.
"I can understand the upward trend in these purchases," said Regis Voss, soil fertility specialist at Iowa State University. "These crop nutrients are needed in very small quantities. In the time that has elapsed since the last research on micronutrients, crop yields have gone up steadily. It's not surprising that farmers start to wonder if micronutrients in their soils still are sufficient."
But Voss added that he would be surprised if new problems with deficiencies have developed. "Micronutrients in plants behave like on-off switches. If you don't have enough, you get clear symptoms of plant deficiencies. Once you have enough, more than enough doesn't increase yields," he explained.
Voss listed several reasons most Iowa cropland doesn't require application of micronutrients. These are consistent with information provided by soil specialists in the states bordering Iowa.
Iron, zinc and manganese availability has been sufficient on acidic soils, and most Iowa cropland is on the acidic side of the pH scale.
Molybdenum, which is essential for nitrogen fixation in legumes, has been sufficient if soil pH is above 6.0, and it is common practice in Iowa to use lime applications to keep acidic soils above a pH of 6.0.
Chloride has been ample throughout Iowa because of the many tons of potassium chloride (potash) applied in the state each year.
Copper availability is sufficient unless soils have a very high organic content. This problem has occurred in other states on organic soils used for specialty crop production.
Boron availability has been adequate except during dry years on alfalfa fields that are low in organic matter.
And the exceptions? For major crops, Voss said iron has been insufficient for soybeans grown on calcareous soils, but soil applications of iron usually are not effective. In those situations, agronomists recommend choosing soybean varieties that tolerate low iron availability and using foliar applications of iron when symptoms occur.
Zinc deficiencies have occurred in corn in Iowa, but only on calcareous soils.
Voss said it may be necessary to conduct periodic research on the adequacy of micronutrients in Iowa soils. "With increasing crop yields and new molecular biology tools to use in crop improvement, plant needs may change in ways they haven't before," Voss said.
One challenge in Iowa would be to find soils on which a micronutrient deficiency occurs with some consistency, so that new tests could be conducted. "Our soils apparently have enough micronutrients to keep that on-off switch in the on position. With the exception of zinc and iron, we haven't been able to study deficiencies in Iowa soils and environments. The only soil test we've been able to calibrate for Iowa soils is one for zinc for corn grown on our calcareous soils," Voss said.