"The flooding in the Delta this past year was truly devastating," said Jac Varco at the 2019 MSU Row Crop Short Course in December.
Varco, a Mississippi State University professor and Triplett Endowed Chair in Agronomy, spoke on soil fertility and plant nutrition problems after flooding.
"Unfortunately, flood events are expected to increase," he said. "They have been increasing through the years, and they don't just occur in Mississippi or the Delta areas. It is becoming more common with changes in weather patterns and overall climatic conditions. For example, instead of getting 1 inch of rain, we're getting 3 inches per event."
Flooding is not new to Delta farmers.
"For rice, we grow it under flooded conditions, but the rice plant has aerenchyma cells that bring oxygen down to the roots, so rice can survive under those conditions. Floodwater is aerobic or variably oxygenated as it is in contact with the air above it, but there are active microorganisms in the water which either principally consume or produce oxygen.
"Depending on where the floodwaters came from, you can have varying degrees of sediment and organic matter in the floodwater, which can influence oxygen levels. It's not as simple as some people might think," Varco said.
Soil fertility and soil chemistry can be tricky.
"For example, if an emerged crop such as corn gets flooded, photosynthesis is reduced as roots lack sufficient oxygen. Respiration rates increase while the crop tries to adapt by growing specialized cells to try to get oxygen to the roots, and adventitious rooting increases. For crops like corn, that is not good. Eventually, flooded conditions are going to kill it," he said.
"Many folks assume that floodwaters wash away all the nutrients. Some floodwaters bring in nutrients, such as in the form of sediments and organic matter, and directly in nitrogen forms. But yes, in a permeable soil, you can lose nitrates, sulfates, and other nutrients from leaching and other processes."
Flood duration on acid soils creates interesting soil chemical changes. For acid soils, the longer the flood, the more alkaline they become up to neutrality (pH 7.0). Alkaline soils, however, increase in acidity and the pH drifts the opposite directions towards neutrality.
"For acid soils, phosphorus availability has a tendency to increase when flooded because of an increase in pH, and iron becomes more soluble and begins to release some phosphorus," Varco said.
When the soil becomes flooded, oxygen starts to be consumed.
"With time and decreasing oxygen status of the soil, sulfate starts to be converted into hydrogen sulfide, which has a rotten egg smell," he said. "In our wet winters, manganese availability, especially in acid soils, can increase to the point of being toxic to crops such as soybean. Another consequence of flooded conditions is managing for high manganese in your soils."
Losing nitrogen under flooded conditions is a problem.
"The pore space of the soil goes from 50 percent or less filled with water to near 100 percent saturated, which is about half of the total soil volume," Varco said. "When half of the pore space or a quarter of the soil volume is full of water, the soil is near field capacity. The larger aeration pores are empty, and oxygen and carbon dioxide can readily diffuse in and out of the soil.
"However, with flooding even the larger pores become saturated, reducing aeration or movement of oxygen into the soil and carbon dioxide out of the soil. With the onset of reducing conditions microbial nitrification declines, while denitrification increases, and gaseous losses of nitrogen begin to occur. At this point, available nitrogen either derived from the soil or fertilizer additions begins to be lost. While this is viewed as a negative in agricultural soils, in a wetland it is good because it's cleaning up the water before it gets into the river. In farming, losing fertilizer equates to economic losses as well as negative environmental consequences."
Flooding effects: Soil biology
Consider the word microbiome.
"We have a microbiome in our gut, but we also have a microbiome in the soil," Varco said. "Fungi don't survive under waterlogged conditions. Some of the bacteria can switch to anaerobic enzyme systems, and they promote anaerobic processes. However, these bacteria require a carbon source to feed on and reduce the nitrogen to gaseous forms."
One of the things that happens in an extended flood is a loss of beneficial fungi.
"One particular important beneficial soil fungi is known as vesicular-arbuscular mycorrhizae (VAM)," Varco said. "These fungi can infect roots, producing vesicles from which hyphae grow and extend out of the roots into the soil. Essentially, they increase root surface area as they extend out into the soil. Consequently, they help to absorb water, phosphorus, zinc, and other nutrients which are transported back to the roots.
"Under extended flooding conditions, VAM populations decline. The next time a corn crop is grown, yields can be negatively influenced due to the lack of their presence. This is what's called fallow soil syndrome, and corn is very susceptible."
VAM populations do recover with time.
"One management strategy going into the fall would have been to plant a cover crop. If planting a cover crop was feasible, a legume species would have been a good choice because they are known to attract VAM fungi as well as other beneficial organisms in the soil," he said.
Following a flood, it is recommended to plant crops the following year that are less dependent on VAM, such as soybean.
"In soil tests, phosphorus usually shows higher availability after a flood," Varco said. "It can be misleading as without the help of the VAM fungi, a corn plant doesn't have as much access to the phosphorus."
A sign to look for to determine whether extreme anaerobic conditions exist is the presence of gray to bluish soil colors indicating a reduction in iron has occurred.
"You find this a lot in wetland soils, but this can happen in soils which are in production that become saturated and have adequate available carbon to influence microbial growth resulting in a lack of oxygen," he said.
Managing soil after flood
Following an extended flood, you might see crusted soil conditions.
"So how do we manage it? You should monitor soil moisture levels when the flood recedes," Varco said. "Check the oxidative status. If it's still gray, you still have a problem. Be patient and avoid tillage while the ground is too wet. Also, don't drive a tractor on soil that is too wet below the surface, or you may cause a lot of damage to soil properties such as structure."
Be sure to do a soil test as close as possible to when you're going to plant a crop.
"You want to allow soil chemical conditions to recover," he said. "If you take a soil test right after the flood, you're going to be testing your soil as a direct result of the flood.
"You can use a starter fertilizer high in phosphorus especially on cotton, corn, and soybean, and I would recommend in-season tissue testing to monitor plant nutrients, especially for nitrogen. Also, consider whether zinc fertilizer should be added."
Plant a cover crop in the fall to begin to restore the physical and biological conditions over the winter.
"Wheat would have been an acceptable choice," Varco said. "Legumes help build rhizobia and mycorrhizal populations. Do avoid brassicas as they are not a host for VAM fungi, so planting them is not going to help get the VAM fungi back."