Soil Management After the Flood in Mississippi

Significant acreage in Mississippi will flood in the current event offering enormous challenges to families and homes. Much of the cropland in the affected region was already planted for the 2011 cropping season.  As the waters recede, a new landscape for in both land and nutrient management will emerge. Much of our modern experience in post-flood agronomic soil fertility was obtained after the 1993 flood in the upper portion of the Mississippi River Basin. That event impacted significant acreages of row- cropland, with very interesting soil fertility related issues apparent in the latter half of 1993 and the subsequent crop year.

The soil chemistry that controls the bioavailability of most plant nutrients is very dynamic as soils transition from a ‘normal’ state to inundation for several weeks, and then back to drier conditions.  The conventional or popular wisdom is that soils are more fertile immediately after a flood because upstream nutrients have come to rest in downstream fields. If this were true, the affected and very fertile south Delta soils likely would be a net loser of nutrients.  What is perceived to be better fertility from sediment on the surface, is more likely a flush of nitrogen (N) being mineralized from organic residues in the pre-flood soil (the N becomes a plant available form after being released from soil organic matter) as soil microbes, which depend on oxygen from air, become more active as the soil dries.  This microbial activity will also increase as the flooded soil warms following water removal.

Others will offer suggestions for post-flooding cropping options. This discussion will focus on managing both the physical and fertility soil components.

Safety

It is important to remember flood water carries materials such as pesticides, petroleum products, and other substances swept up in its downstream progress. The water may contain both dead and live animals (source of bacteria as well as the physical presence), uprooted trees, and other solid detritus. Follow appropriate precautions when exposed to the water and when working in the landscape after the water subsides. The Mississippi Department of Environmental Quality and other state and federal agencies can provide specific guidance on these issues.

Debris Removal

In reclaiming flood-affected agricultural soils, the first task is removing large debris left behind. Safety is to be stressed in this process. Any relevant local, state, or federal guidelines for disposal of particular debris types should be followed. As above, consult Mississippi Department of Environmental Quality or other state and federal agency resources for the proper guidelines for various types of debris.

Soil Compaction

It is desirable to aid surface drainage with water furrows, but many will be tempted to work in fields as soon as the water recedes. Heavy equipment traffic on wet soils leads to compaction layers 10 to 20 inches below the surface and, therefore, should be avoided. These dense layers restrict the rooting zone available to subsequent crops, thus limiting nutrient availability and plant available water.  In addition, soil compaction lessens water infiltration capacity in subsequent rainfall events, and results in more runoff from the field. Therefore, allow soils to dry when possible before addressing physical issues, or restrict equipment traffic to specified zones in the fields until they are dry.

Deposition of solids or erosion

There will be deposits of silt and sand on the surface of soils, especially in areas where water was slow moving; erosion also will occur. Assess fields for needed tillage, re-forming beds, or in extreme cases land-leveling to incorporate the sedimentation. While it may be tempting to plant another cash crop quickly, as noted above, working wet soils will cause more problems from the flood and can lead to additional financial loss.

Flooded soil syndrome

Phosphorus (P) deficiencies were prevalent in many crops planted just after the 1993 flood subsided, and in the early part of 1994 on soils that had tested adequate for P after the flood. Among other factors, the deficiency problems were shown to be associated with diminished activity of  beneficial fungi essential in P nutrition and P uptake by  almost all important agricultural plants (see Wetteraur and Killorn, Journal of Production Agriculture 9:39). These vesicular-arbuscular-mycorrhizal (VAM) fungi are termed obligate symbionts; they require a host plant to complete their life cycle. They attach to the roots of the host, can be up to 100 times the length of the root.  They  have been shown to play roles in the plant acquisition of N, P, zinc (Zn), and many other essential elements. (see Ellis, Journal of Production Agriculture 11:200). Plant nutrition with P and Zn is particularly associated with VAM activity; corn seems to be the crop most affected by Flooded Soil Syndrome.

Paired comparison of corn plants grown in adjacent flooded and non-flooded fields by Ellis found VAM were diminished and P deficiencies worse in the previously flooded soils than the soils not flooded. Over the course of the growing season, VAM development increased where flooding had occurred previously to the same level found in the unflooded soil.  However, the early season P deficiency led to lower shoot weights and grain yields. Starter fertilizer at 25 lb P/acre applied in a band at planting resulted in stunted, purple, and P deficient plants; starter fertilizer in a band at 80 lb P/acre had no visual or analyzed P deficiencies. Broadcast fertilizer at 80 lb P/acre did not significantly increase P uptake. While not explicitly discussed above, VAM are also particularly critical in zinc (Zn) uptake; thus Zn nutrition may be lacking for corn following the flood.  Soybeans grown in the same situations were less affected.

This study implies that flooding soils decreases VAM spores; VAM colonization increases with time as plants grow in the affected soils. Phosphorus fertilizers in high testing soils may be an option if: a) corn is the crop; b) the fertilizer is placed as a starter, not broadcast on the surface; and c) at a relatively high rate. Cover crops may be an option after the flood subsides if little probability exists for an economically viable tap-rooted summer crop such as soybean to be planted and harvested in a timely manner.

A logical question is whether to allow weeds to grow instead of a cover crop. This is a risky alternative given the current challenges of herbicide resistant weed species, and the large number of seed produced by these species.

Nutrient management

The first step is to determine whether previously planted crops can be rescued. Advice is available elsewhere on those criteria. If the crop is salvageable, the considerations below should be evaluated.  Alternatively, the decision to replant will depend on the local situation, capital and seed availability, the time frame, and other considerations.

With some fields forecast to be inundated for weeks, it is unlikely that N fertilizers applied prior to the flood remain in the soil except in some special situations. Nitrogen in the nitrate form is considered lost at the rate of about 5% per day of saturation or flood. There may be available N in fields fertilized with urea just before layby, but this will have to be considered in the site-specific situation.  With the mobility of N in the soil and relatively rapid transformations of different N forms, N is by far the most affected nutrient.

As noted above, P fertility management may be an issue in fields flooded for some time despite having a high soil test value for P. Given the management considerations, the best current year practice may be to plant a cover crop, and to consider using starter fertilizers when planting fiber rooted crops such as corn in 2012.

Flood related potassium (K) issues are not often seen. However, in low CEC sandy soils there has likely been leaching of K during the flood. In addition to possible K leaching in sandier soils, there may be sulfur (S) leaching loss in those soils.

Soil testing

Soil testing remains the foundation of nutrient management despite the complexity in P fertility management following soil saturation. Because the flood has offered a new farming landscape, these fields should be tested before the next growing season even if they were tested going into 2011. The sampling should be done after the soils have dried.