Agronomy Talk

Agronomy Talk: IMPACT OF SATURATED SOILS ON NITROGEN MANAGEMENT

Published on Wednesday, June 30, 2021

After planting, the effects of saturated or flooded fields can have a severe and potentially long-lasting impact on corn. The degree of the effect depends on the length of saturated conditions and the growth stage of the crop. Following corn emergence, saturated soils inhibit root growth and nutrient uptake due to a lack of oxygen in the soil. In higher organic matter soils, nitrate-nitrogen (N) rapidly converts to N gases and is lost to the atmosphere. As a result, under persistent saturation, corn roots stop functioning and plants turn yellow as photosynthesis slows.

Nitrogen Management in Saturated Soils

Denitrification is the leading cause of N loss in saturated soils. Denitrification occurs when the anaerobic bacteria in the soil convert nitrate-N into a gas form like nitric oxide, nitrous oxide, or dinitrogen. These gaseous forms of N are then lost to the atmosphere. The amount of loss depends on the amount of nitrate-nitrogen present before saturation and the soil temperature. Warmer soils have a much greater potential for N loss. These losses need to be replaced to ensure there is an adequate supply to meet the crop’s needs. The tables below provide a guide in determining the amount of N to replace due to loss.

Nitrate component and nitrate conversion rate are listed by N fertilizer source and time following application in Table 1. 

Nitrogen fertilizer sources (Table 1), other than anhydrous ammonia, would be 100% nitrate 2.5 weeks after application. Based on Table 2, if temperatures are below 60°F, soil denitrification could reach 2% N loss per day and at temperatures above 75°F as much as 20% N loss per day.

An alternative method in determining post-plant N needs is the pre-sidedress soil nitrate test (PSNT). Soil samples are taken to a 12 in. depth and tested to determine the amount of nitrate-N available to the corn crop. Research has determined nitrate levels above 21 ppm are adequate to supply crop needs for the season. A challenge for this test is sampling fields with banded nitrogen or manure. This creates a real challenge for taking representative samples. 

Late-Season Scouting

The effects of saturated soils may be evident all season long as plants exhibit less stress, disease, or drought tolerance. Early soil saturation is often implicated in late-season root lodging and crown and stalk rots. Later, plants have less tolerance to stress and root lodging due to their smaller, shallower roots. As plants reach maturity, inspect roots for signs of disease by digging and pay close attention to lower stalks for signs of stalk rot or weakness threatening standability. 

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Author: Craig Kilby

Categories: Agronomy, Agronomy Talk

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