ISSUE 7 June 25, 2009
YELLOW SOYBEAN SEASON
With the recent wet weather and many soybeans in the region reaching the first trifoliate state, yellow soybeans have returned. The most common reason that soybeans in the region turn yellow is from iron deficiency chlorosis (IDC) and nitrogen deficiency. To find out which one is causing the yellow problem, look at the leaves. If the mid-rib is greener than the area between the veins, the problem is IDC. If the veins are also yellow, the culprit may be nitrogen deficiency. This early in the season, N deficiency is seldom a problem. The condition may persist if the soil remains saturated with water and nodules are late developing. First or second year soybeans in a field is particularly worrisome, because it suggests that the field has not been adequately nodulated. If soybeans grow to the 4th true leaf and N deficiency yellowing persists, a top-dress N application may be warranted. These statements are purely precautionary, since N deficiency to this degree in soybean is seldom a problem.
If a field exhibits IDC symptoms, this is a perfect time to take an aerial photo of the field, or call in a satellite image. If you ever wondered where the calcareous portions of your fields were, now is the time to take the shot.
Iron deficiency chlorosis is primarily caused, and cannot arise, without carbonates in the topsoil or shallow subsoil. A host of other additional soil, plant and cultural factors serve to make the problem worse. In addition to calcareous soils, salinity, cool weather, moist to wet conditions, soil nitrate, and spring compaction or lack of compaction (depending on soil moisture conditions), variety, and herbicide application can all make IDC symptoms more severe. This would also be a good time to evaluate any seed-placed iron products that might have been applied this spring.
CORN SEED FERTILIZER INJURY
In evaluating corn stands, it is clear that some fields suffered from fertilizer injury at seeding. In wet fields, some growers wanting to deep-place fertilizer had to move the fertilizer placement towards the surface. Fertilizer placement with high urea-N rates too close to the seed can and probably have resulted in corn germination injury. In addition, anhydrous ammonia applications were for the same reason more shallow than normal. The closer anhydrous ammonia is to the seed, the greater chance of seed death. In the future, application of anhydrous ammonia at an angle to the projected seed-row direction is advised, since there is no truly safe waiting time between anhydrous ammonia application and seeding. Not even the fall before. Also, if deep-banded fertilizer applications move nearer the seed due to wet conditions, rates need to be correspondently lower, and urea may need to be taken out of the blends.
TOP-DRESSING AND WIND
Any liquid N applications to increase yield on small grains need to be applied with stream-bars. Just having stream-bars on an applicator, however, is no guarantee that the application of N will actually be streamed. Wind plays a role in determining whether stream-applied N stays in the stream. Streaming N in windy conditions breaks up the stream and the application can turn into a leaf-burning broadcast application. Usually wind less than 10 mph does not disrupt the stream. However, some awareness by the applicator would be wise so that the effect of applicator speed in addition to wind is considered. Having someone watch to make sure that the stream is hitting the ground as a stream would be a good plan.
If conditions are too windy and the stream breaks up into a broadcast application, major leaf burning will result. In most studies in North Dakota with 10 gallon/acre of 28% N, leaf burn did not reduce yield. A recent study on winter wheat in Oklahoma confirms our observations. However, higher rates than 10 gallon per acre may cause some crop damage if leaves are burned in a broadcast application.
NDSU Extension Soil Specialist