What Happens to a Flooded Field?

Every flood is different, ag expert says.

Millions of acres of farmland have been flooded this year, resulting in millions of acres of prevented plantings (including one field of mine). 

I started wondering about some questions I’ve had for many years. Do you gain or lose soil on a flooded field? If new soil is deposited there, is the composition of it better or worse than what you already had?  And, do you lose soil fertility, or perhaps gain it, from a flood?

To get the answers to these questions, I sat down with Howard Heavner to chat. He has a doctorate in ag systems and education from Southern Illinois University, and he’s currently an instructor there. 

Heavner also happens to be the mayor of Valmeyer, Illinois, which was the town flooded so badly in 1993 that they moved the whole village 2 miles uphill, to a spot 350 feet higher. Consequently, he knows a few things about flooding.

The main point Heavner made clear to me was that floods, on balance, are bad – the negatives are going to outweigh the positives nearly every time. He said, “Every flood is different; one may deposit soil and the other may scour it out.” 

Heavner said that rapid flash flooding, especially near a stream, washes your topsoil away, but it may also deposit some sand in its place. Also, when a levy breaks, there can be several feet of sand deposited on the other side of the levy. Sand is not really something you want to add, according to Heavner, because it’s not good at retaining moisture or fertilizer. 

It’s kind of a double whammy. You lose a crop and some topsoil this year. Then next year, your field is even more susceptible to moisture and fertility loss. Therefore, physical removal of the sand may be in order.  

Heavner says that silt and clay hang on to fertilizer a lot better. A slow moving flood, over the course of weeks or months, that floods a low basin may actually add a little silt and clay. This is good for fertility in the long run, but you’re going to lose a year of production whenever it happens. So, it’s not a good trade-off financially.  (Another small benefit you might receive is thousands of dead fish left behind – natural fertilizer.)

He also talked about something called flooded soil syndrome. Apparently, when a field spends a growing season fallow, it loses beneficial fungi, which causes the following year to be less productive. The fungi in question are called arbuscular mycorrhizae. They grow on and in plant roots and aid in the uptake of phosphorus. The good news is that you can fix the flood-induced fungi die-off by planting certain cover crops after the flood, such as winter rye or winter wheat.

Heavner reminded me, “Don’t forget about the problem with debris.” You end up with driftwood, rocks, and other things that need to be removed. He also said, “You have serious health risks with floodwater.” It may contain sewage, industrial waste, LP tanks, petroleum products, or insecticides. Obviously, these are things to be careful with when you are the guy doing the flood cleanup.

The last topic we discussed was the effect of flooding on fertilizer. Heavner said that nitrogen fertilizer moves readily in the soil, more so than potassium does. Therefore, you tend to lose nitrogen more quickly in a flood. 

As a result, you are probably better off planting beans after a flood than nitrogen-hungry corn. It’s also a good idea to do soil testing after a flood to see what you have left. Unfortunately, post-flood soil composition can change dramatically, and productivity can be permanently reduced.

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