May 24, 2011

Wet springs seem to be the new normal. When it comes to nitrogen management, continual wet weather can increase leaching and maybe denitrification losses. We had a wet spring in 2010, and the Crop Watch article from last year covers the main factors that need to be considered when assessing whether the nitrogen applied may be lost:

Briefly, if more than 4 inches of rainfall infiltrated into a field after nitrogen application, additional nitrogen may be needed to maintain yield potential.

If water has been standing in a field for more than three days, this 2008 CropWatch article would be worth reviewing.

Soil Moisture Resources Can Aid in Assessment

The High Plains Regional Climate Center website has two data sources that may be useful in assessing the soil moisture situation.

1. Weekly Nebraska Soil Moisture Report. This site offers maps that are updated weekly and show statewide precipitation, the Soil Moisture Index (SMI), and the soil moisture surplus/deficit from a point of 50% soil moisture.
   
   These maps are based on automatic weather station data and will be most useful if you live near a reporting site. SMI values are based on the range of the historical daily soil moisture readings at a given station. The index is scaled from -5 (driest recorded) to +5 (wettest recorded). Any SMI above 3 suggests the soil moisture content is in the top 20% of values experienced at this site. A value over 5 indicates that the site has exceeded its wettest recorded measurement and the SMI will be rescaled to reflect this new upper limit.
    
2. The site also has historical soil moisture (for the top 2 ft) for each AWDN reporting station. These stations are located on grass so they may understate soil moisture relative to what it would be under crops. It includes historical data for the last 365 days. Data on the site can be displayed in several ways. When you select “Root Zone” depth, the graph shows the daily soil moisture as the historical range of values, historical mean, and the current value. If you choose "Discrete depths,” the graph shows the soil moisture at 10, 25, 50, and 100 cm (5 cm = 2 inches).

Combining precipitation and soil moisture data should indicate the potential for leaching and denitrification. Leaching is likely in wetter-than-normal soils that received significant rainfall. Denitrification is most likely to occur in low lying areas, where water ponds and stays wetter longer.

With the cooler spring, plant development this year has been delayed, allowing time to assess the situation and, if needed, plan for how to apply additional nitrogen. In the 2010 article cited above, the section Steps to Take suggests that if nitrogen status is questionable, producers should create a reference area to compare nitrogen status of the corn and, when possible, fertigate as needed.

In rainfed corn this option is limited by corn height. Where questions remain, soil sampling may provide useful information; however, in order to have confidence in the sampling, it is best to sample the depth of the rootzone, which may be 5 ft. A shallow sampling to 2 ft may miss the nitrogen below it and show a deficient, when in fact, nitrogen is available lower in the root zone.

Charles Shapiro, Extension Soils Specialist
Haskell Ag Lab, Concord
Allen Dutcher
Extension State Climatologist, Lincoln