Todd Vagts
Iowa State University Extension
Field Crops Specialist
1240 D. Heires Avenue Office: 712-792-2364
Carroll, IA 51401 Cell: 712-249-6025
Email: vagts@iastate.edu Fax: 712-792-2366

Crop Development and Update
The earliest planted corn in the more southern counties should be at or near the V6 stage (475 DD50’s). The growing point (in the V6 corn) is now above ground and is more susceptible to environmental influences. As mentioned in the previous newsletter, the V4 stage is important as this is the time in which the ear is initiated in the growing point and the number of kernel rows is determined. Any physiological (environmental or cultural induced) stress at this time may influence the potential size of the corn ear.

Daily GDD Accumulation: We’ve seen a drastic change in daily temperatures over the last three weeks (Figure 3), this corresponds to large variances in daily accumulation of GDD’s. Most importantly the lack of GDD accumulation from May 20 through May 27 where very few to no degree-days were accumulated each day. This seven day period (where the corn was in the two to five leaf stage and the soybeans were either in the ground to V1) was stressful to both crops, keep this time period (and the leaves that would have been exposed) in mind as you scout crops and look at crop injury and general health.

Soil Fertility and Management
If you plan to use the late spring nitrate test (LSNT) to determine the current N status of your corn fields, now is the time to take those soil samples. Proper sampling procedures and interpretation of the results is essential to make the LSNT effective on your farming operation.

• When to sample: Soil sampling should occur when corn is from 6 to 12 inches tall
• Sample area: Samples should be collected within several test areas that are 1 to 10 acres in size
• Depth of sampling: Samples should be taken to a depth of 12 inches.
• Number of cores per samples: Samples should be comprised of 16 to 24 cores.
• Handling and shipping: Refrigerate or air-dry the sample if it will not be at the lab within two days.
• Soil Analysis: The LSNT is based on concentrations of nitrate-N in the soil sample. Most soil testing labs can perform this analysis.

Often times the most difficult procedure with the LSNT is in interpreting the test results. Interpretations vary with cropping and manure history as well as with weather conditions prior to and after sampling. Table 1 should be used when corn is preceded by alfalfa and/or has a manure history in two of the previous four years.

When corn follows soybeans or corn, the critical N concentration is typically set at 25 pmm. With the situation across most areas this spring where excessive rainfall has occurred, the critical concentration should be lowered to 20 ppm. Excessive soil moisture generally moves nitrate deeper into the soil profile, often times below the sampling zone. The sample results may not accurately predict the total nitrate in the rooting zone.

Estimate the additional N needs by subtracting the soil-test nitrate-N concentration from the critical concentration, either 20 or 25. Multiply the difference by 8, the result is the required N to apply to the field. A factor of 8 is used because studies have shown that it usually takes 8 lb of N/acre to increase soil-test nitrate-N by 1 pmm.

Using the LSNT can be a very useful tool to help predict current nitrogen status in the soil. Yet results may be variable and personal discretion should be used when results appear to be far from what would be expected. Increased use and field history will increase the reliability of the LSNT on individual fields.