No. 18 Article 8/July 25, 2008

Do You Need More Nitrogen?

The wet conditions for most of the spring this year caused much concern about the need to apply additional nitrogen (N). Most of the early symptoms of yellowing and small corn plants were related to wet conditions and cloudy days and not to a lack of N. As the frequency of sunny and dryer days increased, most of those symptoms disappeared, and now the corn crop looks deep green in most places at or near tasseling. This is in part because the root system was finally able to grow and take up nutrients. Additionally, more recent soil conditions have been such that large amounts of N are being mineralized for the crop and will compensate for some of the N that was likely loss due to excessive wetness early on. Last week I examined the root systems of corn plants at various stages of development, and different from the common belief that high soil water content causes roots to grow closer to the soil surface, I saw a lot of downward growth. This growth will be important in tapping into additional N and other nutrients, especially if the soil surfaces start to dry out under limited rainfall.

Despite the generally good fertility status of the corn crop now, for some people with corn not reaching reproductive stages yet, the question remains of whether there will be sufficient N as the corn plant starts rapid growth. Typically corn will accumulate 30% of its N by V12, 60% by silking (R1), and 80% by milk stage (R3). If N was previously applied for this crop and the corn plants are deep green, there should be little concern of lack of N later in the season. If the weather turns dry during critical times of early reproductive stages, coming very soon in many fields, the concern should be more related to the number of kernels that will form than to N availability. It is also important to remember that due to the poor early-season conditions, some yield potential was lost, even if corn was planted early. This means that the crop will not need as much N as in recent growing seasons.

Application of additional N may be considered in fields where corn is at or before tasseling and is looking N-deficient. Most research has shown that application of N after tasseling does not enhance yield unless the crop is severely deficient. If additional N is necessary, high-clearance equipment is likely to be needed. Make sure to check for availability of such equipment before making plans. If N solutions are used, care must be exercised to prevent leaf burning, which can occur when foliage is sprayed. It is recommended that you set up hoses long enough to reach the soil surface and attach a weight to help the hose stay in the middle of the rows as the fertilizer is dribbled in a band on the soil surface. Aerial application of dry or liquid N is another alternative, but rates should be low (less than 10 lb N/acre for liquids) to avoid excessive foliage damage. Also, to reduce the chance of leaf burning, the application should be done when the leaves are dry. The potential for damage using a granular fertilizer increases as the plants get larger and granules have a greater probability of falling into the corn whorl.

Keep in mind that these late-season applications are more effective when rainfall occurs shortly after application, so the N can be moved down to where the roots are growing. Finally, given the factors previously mentioned for this growing season, if additional N is deemed needed at this time, it is unlikely to expect a yield increase with rates above 50 to 70 lb N/acre. --Fabián G. Fernández

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