Manage Iron to Meet Crop Needs

January 23, 2016 02:38 AM
Iron Deficiency

Once you confirm the diagnosis, here’s how to handle iron problems 

When it comes to iron, like most nutrients, problems arise when there’s too little and too much. An iron deficiency, for example, can cost you 3 bu. to 20 bu. per acre in soybeans and 5 bu. to 10 bu. per acre in corn.

The first step to managing iron is to diagnose the problem. Initially, you will likely spot low-yielding areas on yield maps, which show up year after year. The next growing season, visit those areas with a comprehensive scouting manual in hand. In corn, don’t confuse iron deficiency symptoms—yellowing between leaf veins—with other nutrient deficiencies. Magnesium deficiency, for example, shows up in lower leaves, but iron deficiency shows up in the new growth at the top of the plant.

       In soybeans and corn, iron             deficiency symptoms show          up in new leaves because iron   doesn’t move inside the plant.  Because deficiencies result     from soil environmental     conditions, symptoms can    come and go. 

“In soybeans, the giveaway for iron deficiency chlorosis is when an area turns yellow and stays that way for a week,” says Farm Journal Field Agronomist Ken Ferrie. “That means something is hurting chlorophyll production. Spotting the symptoms is easy because the yellowing is in new growth at the tops of plants.”

High-resolution aerial photos can help reveal the scope of the problem. You’ll probably want to follow them with normalized difference vegetation index (NDVI) or thermal images, which might reveal a deficiency covering more acres than what shows up in a normal photograph. With corn, NDVI and thermal images are especially valuable because it’s harder to spot corn deficiency symptoms—interveinal yellowing—from the air. (With soybeans, the entire leaf turns yellow.) 

Soil and tissue tests help with diagnosis but can be misleading. That’s because the availability of iron in the soil depends on soil temperature and moisture, so deficiencies might come and go throughout the season. 

“If the timing of soil or tissue tests or aerial images is wrong, and the soil environment has changed, we might miss seeing the deficiency,” Ferrie says. “Once we identify the location, we can return to the spot, look at the crop and pull soil and tissue samples. 

“If both tests read low in iron, we can be pretty certain there’s a deficiency,” Ferrie adds. “Even if they read high, we can’t rule it out. Because tissue tests are reliable for other nutrients, such as manganese, we can rule them out as the cause of the problem.” Be sure to pick leaves showing deficiency for tissue testing. 

Another way to identify iron deficiency is to mix a foliar iron solution in a spray bottle, apply it to yellowed leaves and see if they turn green.  

Although soil testing might not be reliable for diagnosing iron deficiency, it’s accurate for soil pH, a significant clue. “We often see iron deficiency when the soil pH reading is 7.2 or higher because iron is less available in alkaline conditions,” Ferrie says. 

So lowering soil pH might solve the problem. “In some soils, the pH is naturally high,” Ferrie says. “But high pH can result from mismanaging lime applications.” 

One cause of over-liming is failing to base rates on soil type or soil texture. “Base application rates on the buffer pH reading of your soil test, not the water pH reading,” Ferrie advises. 

“A water pH reading of 5.6 in clay loam soil may require 3 to 4 tons of lime per acre to neutralize. The same pH in light, sandy soil may only need 1,500 lb. per acre. If you apply a high rate of lime to the sandy portion of the field, it might cause iron deficiency,” he adds. “If soils vary in a field, collect soil samples by soil type or texture, rather than by grids.” 

Another cause of iron deficiency is incorporating too much manure with bedding. Soil microbes release carbon and bicarbonates as they decompose the bedding. If the soil gets saturated with water during that period, the bicarbonates might be trapped, causing an iron deficiency. The solution is easy: Apply manure in smaller amounts. 

“Especially with iron chlorosis in soybeans, pay attention to soil nitrate levels,” Ferrie says. “If nitrate carries over from the previous year because of drought that’s a one-time problem. But if soil nitrate levels are naturally high, you may need to revise your nitrogen program, especially in a corn/soybean rotation. Before sidedressing corn, test for soil nitrate to determine how much you need to apply.”

If excessive soil nitrate is contributing to iron deficiency chlorosis, plant a cover crop in the fall or a catch crop such as oats in the spring, and kill it with a herbicide when it’s 6" to 8". The plants take up nitrate from the soil, and when they decompose, microbes temporarily tie up some soil nitrogen.

The effect of iron deficiency chlorosis on soybean yield depends on when the problem occurs. “From our studies, we’ve learned if chlorosis shows up in the early to mid-vegetative stages, and the crop turns green again, the yield effect is minimal,” Ferrie says. “But if the plants stay green most of the season, and chlorosis occurs at R2 or R3, you may see significant yield loss.” 

Some soils, in some areas, are prone to iron deficiency. “In Illinois, some soils have a natural pH around 8,” Ferrie says. “It’s hard to grow high-yielding soybeans on those soils because yield is too inconsistent. It’s best to switch to a corn/wheat rotation or continuous corn.” 

If you have to grow corn and/or soybeans in soil with iron deficiency chlorosis issues, plant seed with good ratings for iron chlorosis tolerance. 

“In our test plots, selecting a corn hybrid or soybean variety that can handle iron deficiency chlorosis is the most efficient measure,” Ferrie says. 

If chlorosis tolerance ratings are not available for your area, ask your seed dealer to recommend hybrids and varieties that can handle high-pH soil, and plant test plots. Include your regular varieties, and run the strips all the way through the high-pH areas, so you can see how each one performs on both types of soil. Save as-applied maps at planting, so you’ll know where each variety was planted, and overlay them with your yield maps after harvest.

Usually, the hybrid or variety that yielded the best on high-pH soil doesn’t yield the best in the rest of the field, and vice versa. “In one trial in 2015, Hybrid A yielded 9 bu. per acre more in the high-pH pocket but 7 bu. per acre less elsewhere,” Ferrie says. 

Another way to manage chlorosis is to apply iron. “Remember iron deficiency chlorosis is not caused by a lack of iron in the soil but by the lack of available iron,” Ferrie says. “Broadcasting iron oxides or sulfates to soil is pretty ineffective. Foliar applications are more successful.

“Remember iron is not mobile in plants or a single leaf. So thorough coverage is essential. If the cause of the deficiency doesn’t change, new growth following the application will have chlorosis; so you may have to make several applications.” 

In-furrow seed treatments are an option, but it might be difficult to maintain season-long availability. 

In test plots, Ferrie and others are planting soybeans in wide rows and boosting the population to more than 200,000 plants per acre. “The more crowded plants are in the row, the more acids are given off around the roots, which creates a temporary acidic condition around the roots and makes iron more available,” he explains. 

“In our trials, we have seen less iron chlorosis in the high populations. But, so far, the increased yield hasn’t paid for the additional seed. Outside of high-pH areas, the higher populations had no effect on yield.”

The opposite problem of a deficiency is toxicity resulting from too much iron in the soil. “The solution is to correct soil acidity,” Ferrie says. “Base your lime application rate on a soil test to make sure the soil doesn’t go from one extreme to the other.” For tips on lime application, see “The Basics of Lime Management” at bottom left. 

By diagnosing iron problems and fixing the cause, you can keep iron issues from hijacking yields.

The Nutrient Navigator series focuses on efficient, environmentally sound management of 
nutrients. The goal is to provide practical knowledge that helps drive yields and profits higher.


Three Basics of Lime Management


Maintaining proper pH is vital to prevent iron toxicity problems. The first step is to apply the correct rate of lime, based on the buffer pH reading for each soil type or texture, says Farm Journal Field Agronomist Ken Ferrie. Second, match the rate to your tillage practices; and third, spread lime uniformly.    
“We used to apply 3 or 4 tons of lime per acre and incorporate it with tillage,” Ferrie says. “But applying that much on strip-till or no-till will drive surface pH too high. You could wind up with iron deficiency chlorosis early and iron toxicity later, as the roots grow downward. In no-till or strip-till, apply lower lime rates annually or every other year. In highly acidic soil, to fix problems faster, you may have to till in the lime and then go back to no-till or strip-till.”

Apply lime uniformly, like paint on a wall. “Never apply municipal lime with a manure spreader,” Ferrie notes. “You’ll find clumps the size of a softball and some areas with almost no lime. Tillage can’t incorporate the lime enough to overcome poor application.

“I’ve seen soybean fields where iron chlorosis shows up on individual plants after this kind of application. Where a softball-size clump of lime was incorporated, the soil pH might be 8.0—equivalent to a 30- or 40-ton lime application. You may have iron chlorosis and toxicity in plants 6" to 8" apart.”

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