While the industry and farmers alike push for higher yields, replacing essential micronutrients is often forgotten. 

As corn yields have increased over the past 40 years from 100 to 200 to sometimes 300 bushels per acre – in fact, a world record of 616 bushels per acre was set in a 2019 National Corn Growers Association contest entry from Virginia – farmers should consider the soil nutrients being expended to produce those ever-increasing yields.

While farmers generally pay attention to the nitrogen, phosphorus and potassium that these high-yielding crops need to flourish, they should also consider the micronutrients in the soil that are also being depleted as their fields produce more corn.

Micronutrients are essential for plant growth and play an important role in balanced crop nutrition. For example, iron and copper play a significant role in the production of chlorophyll in leaves and manganese activates several enzymes involved in photosynthesis. The primary micronutrients in the soil include boron, copper, iron, manganese, molybdenum, zinc, nickel and chloride.

In major crop production areas of North America, the micronutrients most often supplied by fertilization include zinc, manganese, boron and iron.

Farmers can detect micronutrient deficiencies by testing soils and plant tissues or sometimes by visual symptoms in their crops. Plant tissue analysis is often more reliable than soil testing for identifying micronutrient problems, and can help identify deficiencies before visual symptoms occur.

In a research paper on crop nutrition, Dr. Fred Below, a professor of crop physiology at the University of Illinois, said the high yields we see today have been accompanied in many places across the U.S. by a significant drop in soil nutrient levels, including the micronutrient zinc.

“This combination – higher yielding hybrids and decreasing soil fertility levels – suggests that producers have not sufficiently matched their maintenance fertilizer applications with nutrient uptake and removal by the corn,” Below said.

“Agronomic advancements have brought corn yields to new heights, but understanding how to maintain soil nutrient levels can go a long way in helping to sustain high yields.”

Dr. Daniel Kaiser, an Extension soil fertility specialist at the University of Minnesota, said zinc is the micronutrient that most farmers should be keeping an eye on as they test soils and crop tissues.

“Soil testing is a way to get a general idea of where you’re at,” Kaiser said. “With a single application you can often supply enough zinc for many years.”

Plant tissue analysis can sometimes be an effective way to determine if a plant is short of zinc or other micronutrients, notes Kaiser.

With plant tissue testing, nutrient status can be monitored throughout the growing season and a nutrient deficiency can be detected before signs of stress appear. This type of analysis takes a precise measurement of the plant’s nutritional profile at the time the sample was taken. When used in conjunction with a soil analysis which can identify other fertility constraints like soil pH, tissue testing can provide information on what nutrient levels are most limiting to growth and how best to correct them.

Kaiser suggested that growers submit multiple tissue samples from a field – in a good growing part of the field and a poorer growing area – so accurate comparisons can be made. Taking a single sample from a field doesn’t really give growers the information they need as to where the field needs help.

“If you have a problem with low micronutrients, I would suggest trying to correct the problem up front,” Kaiser said. “If starter fertilizer is an option, put a little in-furrow with a micronutrient package or put something in your variable rate application. It’s a good option just to correct the soil where it might be deficient.”

As farmers continue to look for ways to cut costs, Kaiser said he wouldn’t “throw (fertilizer) at it blindly” without taking the proper soil or tissue samples to make sure what is needed.

Dr. Jim Friedericks, director of Outreach and Education for AgSource Laboratories, said AgSource’s laboratories is seeing the amount of micronutrients removed from the soil increasing. With plant tissue and soil testing, “The potential is there to see micronutrient deficiencies in crops rather than later,” says Friedericks. “We do see responses to applied micronutrients where we see deficiencies in soil tests or tissue samples.”

Friedericks noted that over the last 40 years corn genetics have changed so much that growers are getting higher yields but not necessarily changing their fertilizer application rates. Farmers might not be taking into account the nutrients that are leaving the field with those great advancements in crop production.

Table 1 shows the nutrient removal rates of higher yielding crops like corn and soybeans.

When these high yields for corn or soybeans are maintained over several years the cumulative removal of nutrients like boron or zinc can add up to several pounds per acre. If this is not accounted for, crop yield can be impacted in subsequent years.

Besides the diagnostic plant tissue testing described by Dr. Kaiser, growers can use plant tissue analysis as a routine method of determining whether the plants are getting the nutrients they need. Some AgSource clients submit samples on a weekly or bi-weekly basis, with test results available within a day, Friedericks said.

“Growers can use that information to monitor nutrient levels and make nutrient application changes for maximum yields,” Friedericks said. The nutrients are often most efficiently applied through irrigation systems but can also be applied with a side-dressing or foliar application.

“It is essential to understand the needs of the crop, validate those needs by testing the soil and plant tissues, and then manage those crops and nutrient levels for continuous high yield production,” Friedericks said.


Written by Jim Massey for AgSource