ISSUE 8 June 21, 2001
With the recent relatively dry weather, side-dress nitrogen applications are and should be proceeding rapidly. Corn, sunflowers and other needy row-crops can be side-dressed until the crop is too tall to move the applicator through without breaking over stalks.
Small grains more than 5-leaf in maturity will not benefit appreciably from the application of N in terms of yield. Application after this time may green the plants up, increase kernel size somewhat, and will translate mostly into additional protein. A more common protein enhancement time has been after wheat flowering at the watery-ripe stage of kernel development. Application of about 30 lb N (about 10 gallons of 28%) cut to half strength with water to reduce burn has increased protein levels up to 1% in tests recently in North Dakota. This application should be made if possible under cool conditions, cloudy is better than sunny, and mornings are better than noon.
The cool, wet weather, or soil conditions and variety selection has again come together to bring us the soybean chlorosis season. In an additional bonus, these same conditions have already resulted in chlorosis on flax and dry beans as well. Chlorosis is mostly caused by the inability of plants to take up iron. It can be seen in nearly any crop under the right circumstances. Soybeans, however, are our most susceptible crop. Soybeans do not become chlorotic until the first trifoliate leaf emerges and physiologically iron becomes immobile in the plant. Chlorosis will not appear on soils with pH less than 7. At higher pH, some amount of bicarbonate ( a product of soil carbonate minerals and water) is present. We measure carbonate content through a relative measurement called the Calcium Carbonate Equivalent (CCE) which can be measured in various ways in the laboratory. The higher the CCE, the more likely chlorosis will appear. Soybeans take up iron through a two step process. First, the soybean exudes acids which acidify the root zone. Secondly, the soybean exudes a reducing agent (something that transfers electrons from one thing to another, resulting in a lower oxidation state for that other substance), which transforms FeIII (the oxidized form of iron which is EXTREMELY insoluble) to FeII, which is extremely soluble. The kicker is that the reducing substance only works in an acid environment. Enter bicarbonate. Bicarbonate neutralizes acidity. So the more bicarbonate, the harder the soybean needs to work to replentish the acidity. Additional stresses of any kind harm the soybeans ability to overcome bicarbonate. So something like soil salt levels (EC), increases the chlorosis through its stress on the plants.
Iron amendments have not been very helpful. In fact, a recent study at NDSU discourages additions of iron amendments to herbicide applications because in some cases it heats up the phytotoxic affects of the herbicide on the crop and in some cases antagonizes the effect of the herbicide on the weeds.
Certain herbicides can be more active on soybeans under chlorosis. A recent study just completed showed that when salt levels are high (above about 0.8 mmoh/cm on coarse textured soils and over 1 mmoh/cm on heavier textured soils), it might be important to avoid harsh contact herbicides or herbicides with residual and systemic activity. The injury potential for all herbicides increased with the degree of chlorosis and the intensity of factors which caused the chlorosis.
Dr. Dave Franzen
Extension Soil Specialist