Estimating nitrogen losses

Another wet spring hit Iowa this year. From mid-April to mid-May northwestern Iowa had about 90 percent of normal precipitation, whereas areas of northeastern Iowa received more than 200 percent of normal. Heavy spring rains, recharged subsoils, a high percentage of fall-applied nitrogen (N), and a warm fall have many producers wondering if this is a scenario for repeating last year's N losses. The ponded areas have mostly dried up, but the question is, "Will the effects of that water appear as N deficiency later in the corn growing season?"One difference so far this year is that the excess rain occurred early in the spring. If heavy rains return, June will be a critical period to watch for N losses.

Nitrogen processing in soil

If applied N (fertilizer or manure) or N formed from organic-matter mineralization (the conversion from organic to inorganic N) would stay in the ammonium (NH4+) form then losses would not occur because NH4+ does not leach or denitrify (denitrification is the microbial conversion of nitrate to nitrogen gases when soils are saturated). Unfortunately that isn't the way it works. The NH4+ is converted to nitrate (NO3-) via nitrification. Nitrate is the form that can be moved out of the soil profile by leaching or that can be lost by denitrification. The conversion of NH4+ to NO3- and the conversion of NO3- to N gases are both microbial processes. Hence, potential N loss is highly dependent upon factors that influence each--for nitrification, mainly soil temperature, for denitrification, soil temperature and soil moisture status. If fertilizer N is applied in the NO3- form then that N is immediately subject to loss.

Estimating N loss and additional N need

Greater losses can occur when soils enter the spring season with recharged subsoil moisture, when more N is in the NO3- form, and when soils are warm if they become saturated. Deciding if losses are substantial enough to warrant supplemental application must therefore take into consideration the following factors: (1) amount of nitrate present, which is affected by time of N application, form of N applied, rate applied, and use of a nitrification inhibitor; (2) when and the length of time soils are saturated; (3) leaching; and (4) loss of yield potential from water damage. Water movement into soil, leaching, and denitrification are not uniform across the landscape. Thus, the potential for N losses also is variable. For example, if rains are sudden and heavy, then runoff occurs, and not all of the water soaks into the soil. Instead, rain in excess of infiltration moves to the lower landscape where it may form ponds in fields or spill over stream banks into floodplains. This variability adds to the difficulty in decisions about the amount of loss or if additional N should be applied.

An important consideration is the conversion to NO3-. In Iowa, much N was applied as ammonia last fall. Analysis of soil sampled from ammonia bands (150 lb N/acre) applied on October 30 and November 12, 1998, to a Nicollet soil near Ames indicated about 30 percent of the applied N remained as NH4+ on April 19, 1999. For application on March 26, 1999, about 70 percent were still NH4+ on April 19. Running the computer simulation model "Fate of Anhydrous Ammonia in Iowa Soils," developed at ISU by R. J. Killorn and S. E. Taylor, indicates that in a warmer-than-normal scenario, and with ammonia applied either October 1 or November 1, all of the NH4+ would be converted to NO3- by May 1 (with use of the nitrification inhibitor N-Serve, estimated NH4+ remaining on May 1 would be about 35 percent for October 1 application and 50 percent for November 1 application). With an April 1 application and average spring temperatures, by May 1 approximately 50 percent would still be NH4+ and by June 1 only 10 percent would remain as NH4+ (with N-Serve, 70 percent remaining as ammonium on May 1).

This information indicates that fall- and early-spring N applications are and have been at risk. If an N form that has more rapid nitrification than anhydrous ammonia was applied (e.g., urea or ammonium sulfate) or contains part of the N in the NO3- form (e.g., ammonium nitrate or UAN solution), then conversion would be faster and more N would be present as NO3- and subject to loss. Conversely, if an NH4+-containing fertilizer (anhydrous ammonia, urea, or ammonium sulfate) was applied shortly before a wet period, then loss would be negligible because little nitrification to NO3- would have occurred (nitrification does not occur in saturated soils and will not resume until soils dry and become aerobic).

Conversion to NO3- does not equal loss; it just means the N is susceptible to loss. Losses occur only with excess leaching (a predominant problem on sandy soils) or with saturated soils (a predominant problem on heavier-textured soils).

Research conducted in Illinois (reported in the 1993 Integrated Crop Management Conference Proceedings, pp. 75-89, and a 1993 article in the Journal of Production Agriculture) indicated approximately 4 to 5 percent losses of nitrate-N by denitrification per day that soils were saturated. In these studies, an all-NO3- fertilizer was applied when corn was in the V1 to V3 growth stage. Soils were brought to field capacity moisture content and then an excess 4 inches of water (above ambient rainfall) was applied evenly over a 3-day period, which maintained saturated soils for 3 to 4 days on the heavier-textured soils, or an excess of 6 inches of water was applied over an 8-day period (with saturated soils for an additional 3 to 4 days). The excess water application resulted in loss of 60 to 70 lb N/acre on silt loam and clay loam soils--due to denitrification loss. On a sandy soil, virtually all NO3- was moved out of the root zone by leaching. On the heavier-textured soils addition of 50 lb N/acre after the excess water was applied was sufficient to increase corn yields to approximately the same level where no excess moisture was applied. This was not the case on the sandy soil as more N was lost through leaching. When 8 inches of precipitation plus irrigation was received in May and June on the sandy soil, yield was reduced by 20 percent.

Following the excess rain in the spring of 1995, four fields with a full rate of fall-applied N in Boone County, Iowa, had additional N sidedress injected with anhydrous ammonia. Results showed an average yield increase of 15 bu/acre from addition of 50 to 75 lb N/acre (A.M. Blackmer, 1996 Integrated Crop Management Conference Proceedings, pp. 55-59). Significant (40 bu/acre) yield responses to additional N were again measured in similar studies conducted in 1998.

Using the late spring soil nitrate test

A procedure to measure additional N need is to use the late spring soil nitrate test. One-foot soil samples are collected when corn plants are 6 to 12 inches in height. ISU Extension publication Pm 1714, Nitrogen Fertilizer Recommendations for Corn in Iowa provides information on use and interpretation of this test. Where fertilizer or manure is band injected, collecting at least three sets of eight cores from assigned positions relative to corn rows will help minimize sampling errors. Do not sample if soils are still saturated. Thoroughly mix all cores and send about 2 cups of soil to the soil test laboratory for analysis.

For fields where less than full rates of N were preplant applied, a suggestion is to lower the critical concentration from 25 ppm down to 20 to 22 ppm when rainfall from April 1 to time of sampling is more than 20 percent above normal. This would include much of Iowa this spring. When full rates of N were applied preplant (fall or early spring) as anhydrous ammonia, or with manured soils, the suggested critical concentration is 15 ppm if May rainfall exceeded 5 inches (use Table 3 of Pm 1714 to interpret test results). In those fields, if tests are between 16-20 ppm you may want to consider a small N application. These adjustments take into account nitrate that may have moved below the 1-foot soil sample, but remains within the effective rooting depth. In situations where manure or full rates of N were applied preplant, a suggestion would be to limit additional N application to 90 lb N/acre, even if the test result is 10 ppm or less.

Taking chlorophyll meter readings

The corn plant expresses N deficiency through reduced leaf greenness, which can be measured with a chlorophyll meter. To effectively know if deficiencies are occurring, readings need to be compared with adequately fertilized reference areas (a relative value of 95 percent or less is commonly suggested as indicating an N deficiency). This comparison eliminates bias due to different growing conditions, soils, hybrids, or factors affecting leaf color other than N deficiency. Chlorophyll meter readings (values relative to the reference area) will not indicate an amount of N loss or additional need, but will give an indication of the severity of deficiency (i.e., the lower the relative value, the greater the N deficiency). Chlorophyll readings should be helpful in confirming suspected N loss situations and need for supplemental N. The later into the growing season these readings are taken, the more they should be able to indicate small N deficiencies and the more they will relate to total crop N need.

Assessing the corn crop

Before a decision is made to apply supplemental N, one should consider the potential productivity remaining after waters have receded. Has the stand been damaged, will the plants recover, is the area being planted late or replanted, is the yield potential reduced because of conditions other than N loss? It is possible that the combination of N remaining in the soil, plus N mineralized during the rest of the growing season, will supply adequate N. If corn approaching about the V6 to V8 stage is showing N deficiency symptoms (due to N losses and not just due to waterlogged soils restricting root activity and growth), but otherwise has a good stand and appears to be growing well, then application of additional N could be beneficial. It is important to be as certain as possible that additional N is needed. If extra N is applied, but is not required, then excess N could remain in the soil at the end of the season.

How to apply needed additional N?

When conventional application equipment can be moved through the field (i.e., the soils are dry enough and the corn is short enough), then injection of anhydrous ammonia or UAN solutions would top the list of best options. Next would come dribble UAN between corn rows, then broadcast urea. Broadcast UAN solution should be avoided as it will burn corn foliage, especially of large corn. If injection or conventional broadcast application is not possible, then UAN could be applied with high-clearance equipment by using drop nozzles that direct the solution onto the ground, or urea could be aerially applied.

How late can I apply and expect the N to be of benefit?

In N-deficient situations, yield responses that return income greater than the costs for application and fertilizer materials have been observed for N applied up to and slightly beyond the tassel stage (although this is more difficult at the current corn prices). The magnitude of yield increase is dependent upon the severity of N deficiency and the ability of the crop to recover and respond to applied N (the growth and yield potential left after water damage and early-season N deficiency). Any surface application is dependent upon rainfall to move applied N into the root zone, otherwise it will not benefit the corn crop.

This article originally appeared on pages 100-103 of the IC-482(14) -- June 14, 1999 issue.

Updated 06/13/1999 - 1:00pm