Wheat Ahead of Schedule Changes N Plans

Lloyd Murdock, Extension Soils Management, University of Kentucky

The winter has been mild and the wheat has continued to grow and for the fields that were planted a bit early there is more than normal growth. Some of these fields are showing some N deficiency but the plants have enough tillers. So some of the questions are: Do I need to apply N a bit early to take care of the yellowing beginning to show?

One does not want to apply N early on these fields and cause them to accelerate their development. This would increase the changes of freeze damage down the road. I think early N would be a mistake under almost any conditions this season. We still have plenty of time to develop some tillers if they are needed. Generally, wheat in these fields have plenty of tillers and plants were a little N deficient. This is right where the crop should be to slow down the development a bit and protect it from some of the very cold temperatures that might be yet to come. At normal green up I would recommend a minimum N to protect the wheat crop from any accelerated development beyond the normal time and put most of the N on at Feekes 6. This recommendation would be especially needed for no-till wheat.

Estimating N Losses in Wet Soils

Greg Schwab and Lloyd Murdock

Wet soils cause nitrogen losses. In cases where high intensity rain results in high runoff, leaching losses will probably be low. The primary nitrogen loss mechanism in saturated soils is denitrification, which occurs when soil nitrate nitrogen (NO₃-N) is converted to nitrogen gas by soil bacteria. Two to three days of soil saturation is required for bacteria to begin the denitrification process.

Well-drained upland soils that have been wet from a series of rains probably have not experienced much denitrification. Soils in lower landscape positions that stay saturated longer will likely lose more N. Losses can be calculated by estimating 3 to 4 percent loss of fertilizer NO₃-N for each day of saturation. Use the Table below to determine how much fertilizer NO₃-N was in the soil.

EXAMPLE: Determining the Amount of N Loss
A farmer applied 175 lb nitrogen (N)/A as urea to corn grown on poorly drained soil. Three weeks after application the field became saturated for seven days. How much N was lost?

Step 1. Determine the amount of applied N that was in the nitrate (NO3‐N) form.
According to the table, 50% of the urea will be in the NO3‐N form three weeks after application. 175 lb N x 50% = 88 lb N.

Step 2. Determine the amount of N lost.
Remember that two days are needed for the bacteria to begin the denitrification process. Therefore, denitrification occurred for five days (seven days total saturation minus two days to start the process). With 4% lost each day for five days, 20% would have been lost. 88 lb N x 20% = 18 lb N lost and 157  lb N remaining. The N loss calculated in this example is not as high as most people would assume. A soil N test can verify this estimation.

Nitrogen Soil Test
An additional tool for determining NO₃‐N in the soil after flooding is a NO₃‐N test. The soil sample should be taken down to 12 inches deep, and several samples should be taken in each field of both the low and higher ground. The samples should be mixed well and a subsample sent for nitrate analysis.

If the nitrate‐N is less than 11 ppm, there is a low amount of plant‐available N in the soil. Therefore, there is a good chance corn will respond to a sidedress application of N ranging from 100 to 150 lbs N/acre.

If the nitrate‐N is between 11 and 25 ppm, there is a greater amount of plant‐available N in the soil, indicating corn may or may not respond to sidedress N. The recommended sidedress N application at this soil test level is 0 to 100 lbs N/acre. If the soil test nitrate‐N is close to 11 ppm, then higher sidedress N rates would be used. Lower rates would be used as nitrate‐N approaches 25 ppm. The test is least accurate in this range, so the test results can only be used as a broad guide.

If soil test nitrate‐N is greater than 25 ppm, there is adequate plant‐available N in the soil, which indicates corn will probably not respond to sidedress N application.

Nitrogen Broadcast Prior to Rain
Farmers sometimes broadcast fertilizer nitrogen on a field within 24 hours of a heavy rain. In most cases, very little nitrogen is lost to runoff, especially if the field was under no‐till soil management. The nitrogen fertilizer begins to dissolve almost immediately after being applied to the soil surface and will dissolve completely in a short period of time. As rain begins, the first water that falls moves into the soil, taking most of the fertilizer nitrogen with it. Once in the soil, most of the fertilizer nitrogen is protected from runoff. The only exception is a very intense rain soon after application that also erodes topsoil from sloping areas. Even in this situation, the loss would probably be less than one third of the fertilizer applied.

Corn, Late Planting and Management

Photo: Possible Mn defiency in corn, which is most likely due to weather and not lack of Mn in the soil.

Believe it or not, corn has emerged on 96% of the acres in Kentucky which is right in line with the five-year average, according to the Kentucky Crop and Weather Report. While emergence is on track, planting was not, and management decisions need to be adjusted.

Preplant nitrogen fertilizer and preplant herbicides may have been lost to the heavy rainfalls. Farmers may need to sidedress additional N and apply postemegernce herbicides. Because of the late planting, farmers will have a smaller window to make these postemergence applications.

Corn planted April 1 will likely reach V6 growth stage in 35 days, while the same hybrid planted May 15 will reach V6 in about 22 days. Corn planted even later will reach V6 earlier.

Many postemergence herbicide labels have restrictions for V6 corn. Corn will need about 20 to 40 lbs of N/acre through about V6. If additional N has not been applied by V6, then yield losses can be expected.

Determining how much N fertilizer or herbicide was lost from the heavy rains is not easy. A calculation for N fertilizer lost is available in the Corn and Soybean Newsletter from April. Assessment of herbicide losses are more of a case-by-case assessment.

In addition, the rapid growth and wet weather can result in some transient nutrient deficiency symptoms...symptoms where weather has more to do with nutrient deficiency that availability of the nutrient. Normally in these cases, the best cure is sunshine and warm weather. Often farmers will spray a foliar fertilizer over the top and a couple days later the crop turns greener, but it may not yield any more. We always suggest to leave a test strip, and take both the treated and non-treated areas to yield.

As always, consult your county agents for more information.