ISSUE 8 June 24, 2004
Crops can yellow due to a number of factors. The major ones this year are the continuing cool/cold weather, exceptionally wet or dry soils, and nitrogen movement or loss in areas with high rainfall prior to or following seeding. It is important to distinguish these symptoms between things we canít do anything about and things we can. If soil testing shows that soil nitrate levels are adequate, but the field is still yellow, then a warm-up will solve the problem. If soil testing shows that soil nitrate is very low in the surface two feet of soil, the field should receive supplemental nitrogen applications. If soil testing shows that soil nitrate is very low in the surface foot, but there is sufficient nitrogen in the next foot, the decision to apply supplemental N is harder to make. Crops that are reaching a critical stage of growth (5 leaf, small grains; close to bolting, canola) should probably receive a supplemental application of N. Warm-season crops, including corn and sunflower, will be able to reach the deeper N before their critical time.
In the northern counties of the state, reports are coming in about mottled leaves in young canola plants despite the application of sulfur earlier in the season. If the decision is made to apply supplemental N to these fields, it would probably be a good idea to include soluble sulfur source in the application. Streamer bars would be a good idea if liquids are used. Once canola bolts, the yield advantage of supplemental N and S is quickly reduced.
YELLOW TOPS ON FLAX
Similar to soybeans, but not nearly as severe, flax can sometimes show iron deficiency in high carbonate soils, especially under wet, cool conditions. The condition would be more severe if higher soluble salts were present. This chlorosis will appear on upper leaves with interveinal chlorosis. Usually these plants survive if soils dry out and warm up.
Another condition that might also appear as a companion to iron chlorosis is called "chlorotic dieback". This is the symptom for zinc deficiency. The top whorl of flax yellows and becomes necrotic. The growing tip dies. Often, the plants recover as they continue to grow, but axillary buds take over and the plants later have a "candelabra" appearance. A soil test for zinc and plant analysis might help diagnose the problem. The condition is likely to be very spotty within a field, with low organic matter, coarser textured soils high in carbonates and low in available zinc being the prime areas to find it. A foliar spray of zinc chlelate or another soluble zinc source might alleviate the problem.
Candelabra effect on surviving plants.
SUPPLEMENTAL N FOR MALTING BARLEY?
Like wheat, once malting barley reaches jointing stage, yield increases due to supplemental N application is greatly reduced. What would most likely happen is the protein level would increase and make it much less likely to achieve malting grade. Therefore, unless there is a severe N deficiency in young barley plants, no supplemental N is recommended.
NDSU Extension Soil Specialist