Soil test your wet fields.

How much nitrogen is left in your soil after this year’s flooding? It’s anyone’s guess. A soil test is the only way to find out

Many farmers who endured wet conditions through the 2010 growing season are left with nutrient-poor soil, says Dr. Jeff Schoenau, a soil scientist with the University of Saskatchewan. “The 2010 growing season was really unusual. Some areas had three to four times their average rainfall. Such atypical conditions can do a pretty good job of removing nutrients, especially nitrogen, from the soil profile.”

Dr. Don Flaten, a soil scientist with the University of Manitoba agrees: “After unusual weather, there tends to be wide variation in nutrients available the following year. Under wet conditions a large portion of plant-available N can be lost through leaching and denitrification (bacterial conversion to nitrogen gas). Growers are often very disappointed to find soil test results that show very low residual levels of N after a wet year.”

In fact, research in 2002 at Indiana’s Purdue University found that nitrate-N loss from warm, waterlogged soil can exceed five per cent per day.

With this in mind, the best way to get an accurate read on N levels is through soil testing. Schoenau expects a big rush for testing next spring, and advises farmers to get moving on this chore as soon as possible.

Schoenau also expects nutrient levels will be highly variable across fields, so you may need more than one soil sample per field. Landscape testing, or partitioning a field into like soil types, 2010 yields, or dry versus saturated soil areas will help you put together a more precise fertility plan.

Following a year such as 2010, residual nutrient levels cannot be predicted and Flaten believes field-by-field soil testing is essential. He suggests producers even consider soil testing based on the topography of a field or by separate testing of areas of the field that were farmed this past year and areas that were flooded. “Flooded out land may be quite different than land that produced a crop.”

However, John Heard, crop nutrition specialist with Manitoba Agriculture says, “Rainfall and flooding does not always result in nutrient losses. You cannot presume there will be a nitrogen deficiency because of flooding. In some flooded fields, soil tests have shown N has actually increased. You must test to know!”

As an example, Heard reports that in 2000, heavy July rains in the Starbuck area of southern Manitoba flooded a large acreage of crop land. Soil testing found areas of wheat fields that produced a crop in 2000 had 16 lbs/acre of residual N remaining. But flooded areas that were not harvested had 80 lbs/per acre of residual N.

On the other hand, Heard points to a Manitoba fertilizer demonstration site this past summer that clearly showed how an N deficiency can appear following heavy rains. “Initially the heavy Red River clay soil tested with 116 lb nitrate-N in the top 24 inches, primarily carryover from an alfalfa crop removed two years ago. Some additional N was applied as hog manure and various rates of N as urea were applied in a Nitrogen Ramp Calibration Strip (see www.farmforum.ca/nitrogensensing),” says Heard.

“After nine inches of rain in May and June on this heavy clay soil, wheat displayed severe N deficiency — even where as manure or urea had been applied. It appeared that supplemental N would be needed to meet needs of the wheat crop.”

So where did the N go and which form was lost? “Probably a large portion of the existing nitrate-N in the soil was lost because it is vulnerable to denitrification under waterlogged conditions,” Heard explains. “This microbial process is accelerated at warm temperatures. And while manure and urea N would initially exist in the ammonium form and be less vulnerable to loss, it would in turn be converted to the nitrate form.”

Heard did not measure the N losses in the soil through lab testing. Instead, he simply let the wheat crop be the detector. N Ramp Calibration Strips were used to gauge how much N was lost and how much was needed to make up the difference. “It appeared that some 60-80 lb N/ac would be needed to meet the needs of the crop even though we started with a substantial amount of nitrate-N.”

Heard adds that in a humid environment it is very important to soil test to a two-foot depth rather than just testing the top six inches.

Farmers need to keep in mind that rainfall is just one factor that affects the loss of N. The type of fertilizer used; date of application; soil temperatures at application and during the growing season; length of time land is flooded; amount of crop residue present; and the crop (if any) harvested from the field can all have a bearing on the amount of residual N. Soil tests are the only way to determine the extent of N lost during the past year, and the amount of residual N available for 2011. FF