Where Is The Soil N?

Anthony Bly
Soils Field Specialist

With the high rainfall amounts received in southeast South Dakota during the first half of June, questions about soil N in corn fields are justified. First of all, the N cycle is very complex! Nitrogen occurs in many forms and is influenced by soil microbiology. The majority of plant available N in the soil is nitrate-N (NO3-N) and therefore used to measure availability to plants. Nitrate-N is mobile in soil water because of its negative charge and is also vulnerable to denitrification. Water moving through the soil profile will leach or move it to lower soil depths. Saturated soil conditions provide the set-up for denitrification.

We know that large precipitation events occurred. Some areas in Lincoln County received up to 15 inches with some private reports being even higher. The rainfall intensity was extremely high as some records show rates of 6.1, 4.2, and 3.1 inches per hour in Sioux Falls, 3.9 inches per hour at Baltic, 4.3 inches per hour at Parkston, and 2.4 inches per hour at Beresford. The un-answerable question is how much water actually went into the soil and what was run-off? This fact will greatly influence the extent of nitrate-N movement in the soil profile.

After extreme rainfall events, we recommend soil sampling as deep as possible and analyzing the samples for nitrate-N. Sample a few fields, or locations in fields to get an impression of where the N might be. The degree or extent of nitrate-N leaching will be hard to determine. However, if there are higher amounts of N in the top two feet compared to deeper depths, leaching was probably low. If there are lower amounts then leaching probably occurred. The question is can some of the N in the 2 to 4 foot soil layer be extracted by corn roots? If the growing season remains wet, plant roots might not go deep enough and some water from lower depths could be drawn upward with heavy crop water uptake and therefore provide N to the corn plants.

A soil sample survey was taken at the end of last week to help determine the status of soil nitrate-N in the high rainfall areas. Four sites in Minnehaha and three in Lincoln County were sampled as deep as possible for nitrate-N analysis. Samples were obtained from one foot increments to the four foot depth except for two sites in Lincoln County where only three foot samples could be obtained. Areas sampled were not flooded or saturated, and represented corn that had good yield potential. The objective of this sampling project was to show the amount of soil nitrate-N in the upper root zone (0 – 2 feet) and estimate if leaching had occurred, irrespective of the amount of N that was originally applied by the grower. Nitrate-N ranged from 82 to 423 lbs/a (Table 1). Percent of nitrate-N in the 0-2 ft soil layer compared with the 0-4 ft total were 61 to 74%. Lincoln County site 2 may not have received a planned side-dress application and that could explain the lower 0-2 ft. nitrate-N. Past experience has shown when soil nitrate-N leaching has not occurred that 2-4 ft. background nitrate-N is 30-40 lbs/a. Comparing the 2-4 ft nitrate-N values to these typical background levels, these samples show that 30-50 lbs/a nitrate-N could have leached from the 0-2 ft layer into the 2-4 ft. layer. Except for the Lincoln County 2 site, all sites should have adequate N for good corn grain yields, given that the remaining growing season doesn’t result in continued above average precipitation.

Table 1: Soil nitrate-N survey after June 2014 heavy rains in SE South Dakota.

* % nitrate-N contained in 0-2 ft compared to 0-4 ft total.

Source : SDSU