Dr. Dan Talks Agronomy

When Zinc Adds Zip

Dan Davidson
By  Daniel Davidson , DTN Contributing Agronomist
The addition of zinc as a micronutrient benefits corn and soybeans more than wheat. (DTN file photo)

Many of my columns concentrate on inputs we need and how we are short-changing the crop. So here's a reprieve -- it appears adding zinc to wheat acreage isn't necessary under most circumstances.

You probably add zinc to your fertility plan if you plant corn and/or sorghum. Some growers even give soybeans a shot of the micronutrient. Zinc is the most common micronutrient applied to crops -- it can be added as zinc sulfate or oxide in a dry form. Other options include liquid chelated materials applied in-furrow, in a 2-by-2 band or over-the-top foliar.

Wheat is not as sensitive to zinc shortages as other grain crops. So it's easy to assume it will get by on residual zinc in the soil or what's left from fertilizer applied to previous crops. The question is: Does it?

Dave Mengel, Kansas State University soil fertility expert, told me he has yet to see a zinc deficiency in wheat in his state. "In wheat, sunflower and oats we don't recommend zinc and we don't even recommend doing a soil test for zinc," said Mengel. He does recommend soil testing for zinc levels in corn, sorghum and soybeans.

"It is likely that in most situations there is enough zinc either available in the soil or applied available to other crops to meet the routine needs of wheat at the yields being harvested," Mengel said.

"In Kansas our soil tests are often low for zinc and many soils test from 0.2 to 0.4 ppm and 1 ppm is our critical threshold," Mengel added. "But even when zinc levels are low, wheat still isn't sensitive. The lack of deficiency symptoms in wheat and absence of response to zinc applications strongly suggests that applying zinc on wheat won't be economic."

Mengel pointed out that growers regularly apply zinc fertilizer on their other crops like corn and some growers build their soil test levels up to 0.8 to 1 ppm. He emphasizes that it takes 1 pound of zinc sulfate to raise soil test levels 0.1 ppm. Applying 6 to 8 pounds over time to build soil test levels will provide enough zinc to meet the needs of all crops, he said.

After reaching those higher soil test levels, growers can easily maintain zinc soil test levels by applying maintenance amounts of zinc in starters, seed treatments or foliar application at a low cost. Mengel pointed out that it is common for growers to add 0.1 to 0.2 pound of zinc in starter, but emphasized those applications meet only the immediate needs of the corn crop without leaving any residual for following crops.

Of course soil conditions can vary. In a University of Minnesota Extension article, George Rehm and Michael Schmitt (http://bit.ly/…) listed soil conditions that influence zinc availability and uptake. Cold soils in the spring slow mineralization of zinc. If the previous crop was zinc-hungry like corn and soil test levels were already low, there could be a response. Lighter sandy and fine-textured soils hold little zinc because the cation exchange capacity is very low. In areas or erosion such as on hilltops and slopes where topsoil has eroded away, there will be more free lime (calcium carbonate) that can tie up zinc. Excessive build-up of phosphorus, usually related to manure applications, can create a phosphorus-induced zinc deficiency. Phosphate can tie up zinc, just as calcium can tie up phosphate.

Zinc is an important micronutrient in all crops, but rarely does wheat show a response. Concentrate on applying zinc on corn instead of investing in applying it on wheat. If corn is getting enough zinc, it is likely wheat will have enough. In light, sandy soils, eroded hilltops or soil extremely low in organic matter, you may consider applying 2 pounds per acre before planting wheat, either in the fall or spring. If there is not history of zinc deficiency, you might not see a response.

Dan Davidson can be reached at AskDrDan@dtn.com


Dan Davidson