The morning of April 3, David Dunn looked out his Portageville, Mo., office window as a hard rain pelted the glass. Not far away, wheat was getting another drenching.

“It's rained at least an inch in this last event,” said the Missouri Extension agronomist and supervisor of the Delta Center soil test lab. “The ditches are already full. Earlier rains dumped 4 inches at the station (in Portageville). Fifty miles north of here, they got 15 to 16 inches of rain.”

As flooding of wheat entered a second week in some areas of the Bootheel, Dunn has been fielding queries about fertilizer in the crop.

“Nitrogen is pretty mobile in the soil. It tends to move between forms. In the past, we used a good bit of ammonium nitrate — half ammonia/half nitrate — that contains the two main forms of N in the soil.”

The nitrate tends to be easily leached and also lost by denitrification (a process where bacteria in flooded, waterlogged conditions attack the nitrate — one atom of N and three of oxygen — grab onto the oxygen to use for respiration. That leaves the N free to move into the atmosphere). Nitrate is also water-soluble and will leach through the soil.

“I worry about water movement through the soil carrying nitrate off. Ammonia (NH₄), on the other hand, is very stable in the soil. Soil particles can hold it pretty well. Any N in the ammonia form, I'm not terribly worried about moving under heavy rainfall conditions.”

The problem with ammonia, however, is it will eventually convert to nitrates through natural processes. At that point, it's liable to leach.

Urea — ammonium carbonate — is a bit different than ammonium nitrate. It's very water-soluble and, when placed on the soil surface, is attacked by the urease enzyme. That liberates ammonia as a gas that is then lost through volatilization.

“So, farmers should make sure urea is below the soil surface. Once it's there, it can still leach rather easily for several days. But within a couple of days, it will convert to ammonium and won't be so liable to leach. Then, after 10 to 14 days, it progressively converts to nitrate. That's why we're worried about those that applied urea just prior to the arrival of rains.”

One thing Dunn and colleagues have found in the soil samples they've run is a lot depends on how the rains fall. In Portageville, “we had a gentle rain — which is a good friend for crops — for 12 hours prior to the heavy rains. That means any fertilizer on the soil surface was washed into the soil a little.

“Then, the ground was saturated and we had the heavy rains. And most of that heavy rain ran off without moving the N. So this area was okay.”

Farther north, though, fields experienced a deluge “right off the bat. That caused a lot of fertilizer to be washed away or leached through the soil profile.”

Soil texture

Soil texture plays a part in the process. “Sandy soils tend not to have such bad run-off. They have the ability to allow rainwater to flow down through them and that's how most of the water leaves the field.”

For every inch of rain, nitrates in the soil tend to move 6 inches downward. Four inches of rain means N can drop 2 feet.

“Wheat roots tend to reach 2 feet deep, maybe a little more. So nitrates will still be available to the plants.”

In fine-textured soils, water movement is much more random. It tends to move in cracks in the soil. Nitrogen near those cracks is quickly carried through the soil.

“If I had those types of soil, I'd go out and collect soil samples from the top 2 feet and see the nitrate and ammonia levels. I'd want to know how much N is left.”

Should farmers sample when the soil is still wet? “I'd do it as soon as possible. What we've found is you need about 150 pounds to 160 pounds of N — as nitrate and ammonia — in the top 2 feet of soil at jointing. Wheat basically just sits around until jointing. At that point, it rapidly grows, so it needs the N available. And once it's at jointing, it will lose yield by not having enough N.”

Dunn worries about farmers further south in the Delta. “A lot of the wheat we've looked at from Portageville north is just now beginning jointing. But south from here, wheat is more mature and if the N isn't out there, yield has probably been lost.”

The sampling Dunn suggests has been done on the station. Urea was applied about four days prior to the first major rain. When those soils were later tested, “we found most of that N was still there. But we had that easy rain that allowed it seep into the soil profile. Then, when the heavy rain arrived, the soil was already saturated and the water ran off and didn't cause leaching. The soils here aren't terribly coarse — mostly silt loams.”

In sandier soils further north, “we found an appreciable amount of N had been lost. Those fields tended to have lost about half the N in the top 2 feet of soil.”

A situation that researchers were worried about involved fields top-dressed a month ago. In those situations, “all the N had time to convert to the ammonia form, which is readily leached.

“We're telling people that probably another 30 to 40 pounds of N wouldn't be a bad idea.

“However, once past the first joint stage, it probably isn't worth doing — the N needs to be there prior.”

Asked about tissue samples, Dunn isn't enthusiastic. “We don't think tissue samples are of much value currently. If tissue contains sufficient N but doesn't have N reserves in the soil to draw from, the yields will be limited.

“A lot of people are calling because their wheat is yellowing up. They want to know if they have adequate N. I can just look at the crop usually and tell it's inadequate.”

Another thing Dunn is addressing: what happens if water stands on a field for more than three days?

“What we figure is after more than three days, problems other than N will appear. What's happening in some areas is the fields are draining off and yellowed wheat survives, much to our surprise.

“I suppose it was cool enough and the crop was at a stage where it was able to take the hit.

“But, typically, we're finding that if water stayed on for more than three days, denitrification has become a significant issue. That's something farmers need to consider.”