Rain, water everywhere. How much nitrogen has been lost? Did the rain infiltrate, runoff, or get lost to the atmosphere through denitrification? That is the question that should be on every producer’s mind after the last two week's of rain. Good information on various loss mechanisms is available in a previous CropWatch article,  Assessing Nitrogen Loss Due to Saturated Soils.

We cannot provide estimates of nitrogen loss with any certainty — there are too many variables. Given that corn is growing fast, the time for sidedressing is short and where fertigation is not available, the time window for rescue treatments may be ending shortly. The general strategy should be to determine how much nitrogen may have been lost, and then calculate a replacement strategy.

How to Estimate Loss

Drainage/Leaching

Conditions will be unique, and so one has to ask several questions before deciding on a course of action:

1. How much nitrogen was already applied and when? How much more was scheduled?
2. What is the soil water holding capacity and soil moisture deficit at the time of the rainfall?
3. How much of the rain infiltrated and how much ran off?
4. How long has the soil been saturated?
5. Is it possible to add nitrogen at this time?

The following section provides further information related to these questions, which may only be answered with rough estimates.

Nitrogen that is in the ammonium (NH₄) form will not leach as easily nor will it denitrify, so nitrogen applied just before the rains, if in the ammonium form may not have moved as far as if the N was fall-applied. Loss of nitrogen will be proportional to amount applied, so if all the planned nitrogen was already applied, it is likely that more N might be needed. Nitrogen yet to be applied should still be applied unless there was major crop damage.

Soil texture and moisture deficit affect the amount of absorbed rainfall percolating below the root zone. In sandy soils the water-holding capacity might be 1 inch per foot, while in silt loams it may be 2 inches per foot. If available soil water was 50% depleted to a 2-foot depth before the rain, there was capacity to hold 1 to 2 inches of water, depending on soil texture. Simply put, the soil profile has to “fill up” before there is much downward movement of soil water.

The amount of water that infiltrated into the soil is the amount of rainfall minus the water lost in runoff which varies across the field. Infiltration rates for Nebraska soils range from 0.25 inch per hour to 2 inches per hour. Those with an infiltration rate of 2 inches per hour probably held all the water, while those with a rate of 0.25 inch per hour did not. Data for individual fields can be found at http://websoilsurvey.nrcs.usda.gov/app/

Based on the soil water deficit and infiltrated number, you can calculate possible drainage:

Internal soil drainage = Water holding potential - Infiltrated rainfall

For example, with 6 inches of infiltrated rain, and a water-holding potential of 3 inches, 3 inches of water could drain below the root zone. In a soil with a water holding capacity of 2 inches per foot, the 3 inches of drainage potential would move water 1.5 feet (3 inches/2 inches/foot). Nitrogen in the soil water will not move all at once. Infiltrating water will dilute nitrate and will continue to dilute and move it as the profile fills with water and drains, so it is difficult to predict how much N leached and where it is now. Nitrogen is not evenly distributed with water as it moves; rather, it slowly disperses in the soil water. In these cases the nitrogen may not be lost, but rather exists deeper in the root zone so plant roots could grow into the enriched nitrogen zone in the future.

You could soil sample the profile and determine how much nitrogen is in the profile and judge from that. However, it is difficult to sample in wet soils, it takes time to get the results, and you cannot predict future events. Nitrogen that has not nitrified will not be reported in a normal soil nitrogen test which only analyzes for nitrate.

If you use these calculations and decide there was significant water drainage, and the nitrogen was diluted to the extent that some of the nitrogen will be below the bottom of the root zone, you should probably revise your nitrogen plan. This nitrogen can be added with whatever other nitrogen is planned.

Denitrification

How much nitrate is lost due to denitrification depends on how long the soils are saturated and the temperature during that time. Estimates when soil temperatures are 55-60^oF are in the 2% per day range, when soil temperatures are greater than 65^oF, losses can be up to 5% per day of saturation.

Steps to Take

Sidedressing extra nitrogen now to replenish lost nitrogen should be done as soon as it is possible to get in the field. The rate to apply is almost impossible to determine with confidence. A sidedress application of 40-50 lbs nitrogen per acre may be a good risk reducing strategy. The nitrogen could help the corn reach nitrogen that has moved deep in the profile, or replace nitrogen that has moved beyond the reach of the roots.

If you can apply nitrogen with an irrigation system or a high clearance rig, another strategy can be used. You can apply nitrogen to strips or areas of the field now to create a reference strip to compare the rest of the field to, and then re-apply if nitrogen deficiency starts to show up. For example, on one field a sidedress N treatment of 75 lb was planned before the rainfall, and now the rate has been increased to 115 lb to compensate for calculated losses. In that field apply a strip, or several strips with 165 lb of nitrogen. Over the next few weeks monitor the reference strip and observe the leaf color of the plants. If the non-reference area starts to look nitrogen deficient, apply another 30 lb of nitrogen. You can use a chlorophyll meter to get a more accurate assessment. Since nitrogen translocates easily in the plant, look at the lower leaves for differences between the reference and bulk area of the field.

Soils Extension, Agronomy and Horticulture
Charles Shapiro, Richard Ferguson, Charles Wortmann, Gary Hergert, and Tim Shaver