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Corn+Soybean Digest

Too Much Of A Good Thing

Many years, farmers wait for water to move crop development along. But in 2008, corn and soybean farmers had too much of a good thing. Whether you farm river bottoms that were flooded early on, or you were just victim to standing water, soil specialists suggest a postharvest soil evaluation is in order. Acres unplanted in 2008 are especially at risk.

“Floods change the whole make-up of the soil,” says Mahdi Al-Kaisi, Extension soil management specialist at Iowa State University. “Potential economic and environmental consequences of leaving these soils unattended exist. When planning for the next season's crop, farmers need to consider the possible long-term damage to soil and water quality in areas of significant flooding.”

Al-Kaisi says several biological changes take place when soil is saturated for an extended period of time. Growing plants traditionally help build up the microbial community in the root zone, which is essential to nutrient cycling. But flooded soils left unplanted do not. Soil vascular-arbuscular mycorrhizae (VAM) fungi colonization rates decline.

“VAM fungi are colonized around the root systems of crops in a mutually beneficial relationship. Fungi benefit from host-plant roots and crops benefit from the increased nutrient uptake zone,” he explains.

“The absence of this root system essential to maintaining the microbial community can affect the next season's unplanted areas. Chemical and physical changes occur as well in unplanted areas, including changes in oxidation and soil structure, soil stability and pH levels,” says Al-Kaisi.

HE RECOMMENDS FARMERS affected by an overabundance of water in 2008 begin the evaluation process with soil tests for nutrient status. Whether fields were planted or unplanted, soil samples allow farmers to assess major nutrients. In the case of flooded soils, phosphorus (P) is especially important to monitor. After harvest is also a good time to document any unusual observations with crop production.

“The same basic rules apply regardless of the crop planted or region of the country. You need to get a sense of changes, and know what applications of major and micronutrients might be needed where there is a lot of stress on the microbial community,” he says.

Farmers who were able to crop a flooded area in 2008 should review any yield differences picked up with a yield monitor. Al-Kaisi says, for example, pH changes in the soil may affect soybean response and could create iron deficiencies for 2009.

Weeds are another concern heading into spring. University of Wisconsin Extension specialists encourage farmers to be alert for new weed problems the year after a flood.

Some weeds may germinate after an assessment of weeds was made during the flood year, while others may remain dormant until the next season. Changes in soil texture, pH and organic matter content can also influence herbicide performance and crop safety. Farmers should base herbicide selection and rates on current soil characteristics.

“If you farm near the river, your soil has probably changed,” says Al-Kaisi. “You likely have sand and debris that need to be removed or spread evenly over your soils, depending on the severity of the situation. This will likely affect your existing soil texture. Mix it in the top 6 in., and cultivate or chisel it to create more uniformity.”

North Dakota State University (NDSU) experts say sand deposits on top of silty or clay-type soils deeper than 4 in. may decrease potential crop yields. On the other hand, NDSU specialists also note the opposite can occur. If sediment came from fertile fields of upstream neighbors, the fertility status of your fields may be unchanged or higher than before the flood. To evaluate the situation, take 6-8-in.-deep soil samples in at least 15 locations/field representing 20 acres or less each.

“Farmers need to consider longer-term changes to help prevent future problems,” says Al-Kaisi. “Extensive tillage can compound problems with soil compaction, which reduces water penetration and increases potential soil erosion. Tillage is often the worst thing you can do to destroy the soil structure. Evaluate field conditions and consider the need for tile drains or grass waterways to help remove excess water.”

Al-Kaisi notes adding or improving field tiles can promote better drainage and create aeration. Grass waterways will reduce water velocity, improve soil structure, increase infiltration and reduce surface runoff to help protect soil productivity. In addition, he asks farmers to consider leaving crop residue unchopped in fields, so it does not wash into fencelines with a high-intensity rain.

“While this advice could be applied across the Midwest, farmers must remember that different soils are affected in different ways. Different damage is seen between soils that drain well and soils where standing water was a problem,” he says. “Understand your soil and put a good system in place.”

LESSONS FROM 1993-1994

Soil testing, yield evaluation and long-term drainage improvements should be considerations for every farmer reclaiming soil productivity. But in some instances even after such deliberation, little or no intervention may be best.

The floods of 1993 wreaked havoc on farms along the Mississippi River in western Illinois. Bob Hoeft, University of Illinois soil fertility professor and Extension specialist, says corn planted near Quincy, IL, in 1994 was purple in the early spring due to the lack of mycorrhizae on the roots. Researchers broadcast phosphorus (P) on the corn in small plots, and noted no difference in time to green up whether or not P was applied. In all cases, he says the corn greened up within a few weeks after emergence.

“In this part of the world, we do not have as many P issues as those to the north,” he says. “We did not take yield measurements, but there appeared to be little, if any, yield loss at year's end.”

Hoeft says researchers were able to determine where old livestock operations had been. The corn was greener, potentially due to organic matter or undestroyed mycorrhizae.

“The spring of 1994 was not a stressful one, so another year you might see a different reaction,” he says. “Bottom line: If you are growing crops in an affected area in 2009, the crop may look bad initially, but you may not see any serious impact on yields.”

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