Route 2: Fields That Never Catch Up

By the middle of July, you start to notice them—fields, like the one pictured above, with plants that are green on the top but yellow on the bottom. The plants are starving for nitrogen (N) and cannibalizing themselves, trying—but failing—to fill their ears. They are labeled as the fields that never caught up.

Of course, there are various reasons why a corn field can run short of N, as revealed by yellowing plants. But, the one shown above was a victim of carbon mismanagement, which caused N to be immobilized and locked away.

Look closely, and the first symptoms of N immobilization turn up as early as the V3 or V4 growth stage. The corn in such fields is sporadically light green to almost yellow, compared with normal fields nearby.

“If you don’t understand how carbon affects nitrogen availability, you are setting the stage for a field that grows slow after emergence and never catches up, unable to yield its potential—even if you do everything else exactly right,” says Farm Journal Field Agronomist Ken Ferrie.

The cycle. Here’s the story behind the field in the photo: Several years ago, the farmer grew high-yielding corn with lots of stover. The following year, he no-tilled soybeans into the residue. He harvested the soybeans and then no-tilled wheat.

After harvesting the wheat, the farmer left the straw in the field. Late in the fall, he chiseled the field, leveled it and applied 200 lb. per acre of N as anhydrous ammonia—figuring that would be enough for 180-bu. corn. He applied no other N.

“Incorporating all that crop residue into the top few inches of soil created a huge carbon food source for the soil microbes,” Ferrie explains. “The population exploded.

“In order to break down the carbon in the crop residue, microbes need nitrogen, and they get it from the soil. If there are enough microbes—as was the case here—there is no nitrogen left for the corn plants,” he says. “The anhydrous ammonia was less available in the fall to feed the microbes because it was in a band 6" to 8" deep. The incorporated crop residue and most of the microbes were in the top 5" to 6" where the oxygen is.”

The N taken up by the microbes was immobilized in a form unusable by the plants. “It will be available again—some this year and some as much as three years out—along with the nitrogen in the crop residue through the process we call mineralization,” Ferrie says. “Unfortunately that was too late to help the corn in the photo. So, the plants showed nitrogen deficiency early on and never came out of it.”

There are several options the grower could have pursued to change the field that never caught up to the field that kept right on truckin’.

“Following the corn crop, he could have chiseled the cornstalks,” Ferrie says. “Then the cornstalks would have started decomposing during the soybean year. Or, following the wheat crop, he could have baled the straw, leaving much less residue to decompose. Finally, he could have no-tilled this year’s corn crop into the wheat stubble and left the residue on the surface. Incorporating the residue into the zone where the microbes live accelerated decomposition and caused the population to explode.

“Of course, no-tilling corn into wheat residue becomes a bigger and bigger challenge the farther north you farm,” Ferrie quickly adds. “In fact, all these scenarios come with their own problems, which must be resolved.

“It’s a matter of managing the risk. The grower could have surface-applied some nitrogen to feed the microbes and keep them from depleting the soil. That would have left some nitrate available for the corn plants.

“If the grower wanted to make just one nitrogen application, broadcasting 200 lb. of N per acre as 28% solution [UAN], at the surface with the carbon, would have worked better,” Ferrie adds. “That’s because 75% of its nitrogen is in or will convert to the ammonium form, which is what the microbes use. The other 25% is in the nitrate form, which is what the plants need. Because UAN is 50% urea and subject to volatilization, it is wise to incorporate after application.”

There are a number of ways to meet a field’s early season N demands. But, with rare exceptions, you will never get ahead of demand using only a fall ammonia application and no surface application if there is a great deal of residue incorporated.

Understanding the carbon/nitrogen cycle is the real secret to successfully grow corn—especially continuous corn or other situations with large volumes of crop residue. (The academic world considers the carbon cycle and the nitrogen cycle two separate cycles. But, Ferrie prefers to think of them as one cycle because they are so interconnected.)

You’ll be a better carbon manager if you know a little bit about the soil microorganisms that make the cycle happen. “Think of soil microorganisms as livestock,” Ferrie says. “You can manage them just as you do regular livestock that live on the surface, rather than below it.” That means providing them with food, water and comfortable living conditions. But, managing soil microorganisms is not as simple as routinely throwing some N onto the soil surface in the spring.

Cast of characters. Carbon-decomposing microorganisms include fungi, bacteria and actinomycetes. They consume the carbon contained in crop residue (plants obtain carbon from carbon dioxide in the air through the process of photosynthesis). The microbes use N from the soil for an energy source to drive decomposition.

You can actually see some soil microorganisms—a handful of fungi look like long, white threads running through decaying plant material. Fungi are involved in all stages of decomposition. They can tolerate acid conditions better than the other two organisms but not drought or high temperatures.

Bacteria are single-cell organisms. They are sensitive to moisture and pH levels. There are more than 20,000 species of bacteria in a tablespoon of soil. Bacteria include nitrosomas and nitrobacter, which play a big role in the carbon/nitrogen cycle.

Actinomycetes are a little like fungi and a little like bacteria—unicellular but filamentous and branchlike. They release a volatile derivative that gives tilled soil its distinctive aroma. “The absence of that smell tells me that something’s wrong because no actinomycetes are present,” Ferrie explains. Actinomycetes break down resistant compounds, especially cellulose. They handle warm, dry conditions significantly better than fungi and bacteria, but pH is crucial.

Your goal is to keep all three types of microbes happy and comfortable.

Keep microbes happy. Like livestock, microbes need water, food (carbon) and oxygen. Their activity is affected by temperature and pH ranges. The type of soil determines water-holding capacity and oxygen capacity, so you can expect to have more microbes in black prairie soil than in sand.

“Climate is a huge factor,” Ferrie says. “For example, Minnesota warms up at the end of April, farmers harvest in October and soil microbial action has pretty well slowed down by the end of September. So, the decomposition window for residue is short, and carbon is sequestered in the soil.

“In Georgia, farmers plant by the first of March and harvest by the end of August. The soil is warm, and organisms are active for most of the year. There, decomposition proceeds faster than plant growth. That’s one of the reasons Southern soils are lower in organic matter—carbon in the soil decomposes faster than you can raise it. That’s why cover crops are needed in the South. And, that’s why no-tilling continuous corn is easier in the South.”

In other words, Southern growers usually don’t have to deal with considerable volumes of residue at planting time or with large volumes of microbes that tie up N—unless they incorporate a cover crop before planting.

Ratios count. The value of the carbon in the microbes’ “feed” determines how populations behave. This is reflected in the carbon/nitrogen ratio. “Plant residue like that from soybeans and alfalfa is made of simple carbon bonds, and they have a low carbon/nitrogen ratio,” Ferrie explains. “The lower the carbon/nitrogen ratio, the quicker the microbes break down the carbon chain and release, or mineralize, nutrients. Soybeans have a carbon/nitrogen ratio of 30:1. Hairy vetch is 10:1. Manure is 20:1 or less. On the flip side, corn is 60:1, and wheat straw is 80:1.”

Another way to explain it: Soybeans contain simple carbon bonds that break down easier. Soybeans produce less tonnage, and their leaves start to drop weeks before harvest and decompose during warmer weather. Wheat stubble and cornstalks contain more complex carbon bonds.

Fungi and bacteria break down the simplest carbon bonds. Actinomycetes handle the more complex bonds.

Consequently, if your corn crop follows soybeans, you likely won’t have as big of a problem with microbe populations building up and immobilizing N. But, the reverse is true if corn follows corn or wheat, especially if the residue is incorporated.

How you can help. Managing soil density by breaking up compaction leads to happy microbes by allowing water to percolate through the profile. Just like roots, microbes don’t like saturated or dried-out soil.

Managing pH is important. Acid soils are less productive because the soil microorganisms are less active. “Corn can grow with pH as low as 5.6 or 5.7, but microbes need between 6.0 to 7.5, and 6.3 and 7.0 is best,” Ferrie says. “We try to hold pH around 6.3 to 6.5. To do that, we apply smaller amounts of lime more often.”

The type of tillage influences microbe management. Tillage lets in air, and the oxygen helps microbes decompose residue faster. It also puts the residue right where the microbes are. (Strip-till and no-till practices sequester carbon because they keep residue away from microbes, and the tillage releases free carbon stored in the soil as carbon dioxide.)

How you handle residue is important. “If you’re in no-till or strip-till conditions and leaving a mulch of corn residue on the surface that’s different from chiseling in 200-bu. residue,” Ferrie explains. “When you incorporate residue, you place food where the microbes live, like the farmer at the beginning of the story. There is a huge population explosion with vast demands on nitrogen.”

Timing is a factor. “If you do tillage in September, when the soil is warm, it lets bacteria start decomposing the plants’ simple carbon bonds, causing a flareup in population,” Ferrie says. “That draws down the nitrogen in the soil. But, there is no crop growing at that time, so it’s not a problem.

“However, if you chisel in April, you increase the microbe population just before planting. The nitrogen gets immobilized, and it may not be mineralized in time for the crop to put it to use, so the plants will grow slow.”

It’s not hard to tell whether your microbes are happy and active. “Simple, easy-decomposing carbon bonds, such as those in leaf tissue and husks, should disappear fast,” Ferrie says. “If you still have leaf and husk tissue from last year’s crop remaining in August, something is wrong, probably involving soil bacteria. It is mainly the bacteria that decompose these bonds.

“If you have lots of big stalks that have not decomposed after two or three years, you have an actinomycete problem. Actinomycetes are extremely sensitive to pH. They must be inside the 6.0 to 7.5 range.”

Here’s still another way to look at what transpires in the soil: “Most of the nitrogen farmers apply is not used by the crop,” Ferrie says. “In reality, it is eaten by microbes, which, in turn, mineralize nitrogen (along with phosphorus, sulfur and other nutrients) from the soil, where there is a huge supply waiting to be used by the crop. If we take care of the soil system, the system feeds us.”

You can email Darrell Smith at dsmith@farmjournal.com.