Dave Franzen, North Dakota State University Extension soil specialist, answers three of your soil fertility questions:

Q: How long can NBPT-treated urea remain on the soil surface without loss?
A: If urea is surface applied, and a proper rate of NBPT was included in the application, urease activity and resulting ammonia volatilization is nearly zero for about 10 days. After 10 days, ammonia volatilization slowly increases, with subsequent volatilization being roughly exponential. If the soil is perfectly dry, ammonia volatilization is slow. A little rain (1/10 inch for example) would result in more loss, as ammonia partitions into the water as dissolved ammonia and is whisked away from the field with evaporation. Ammonia volatilization rate is low in soils with pH less than 7, and much greater in soils with pH greater than 7. Higher surface residue content also increases ammonia volatilization, because soil urease concentration is much greater in residue than in bare soil. It takes about 1/2 inch of rainfall in one event to move urea down into the soil enough to adequately protect the urea.

Q: Does nitrogen increase drought stress?
A: I have had several experiences in replicated trials where this adage was put to the test. In 2008 at Williston, N.D., rates of N were 0-160 pounds per acre. The average yield in that dry, hot environment was 13 bushels per acre, and yield did not vary between the check plot and the higher N-rate treatments. The protein, however, was under 10% for the check plot, whereas the higher N rates were well above the 14% protein market threshold. If one thinks only of the effect of vegetative growth and N rate, it might seem reasonable to think that the plant is using energy and water early in the season compared to lower N rate growth. However, in small grains, tillering is directly related to root mass. As tillering increases, so does root growth, so the plants are better able to take advantage of any moisture in the soil from a greater volume of soil. Therefore, it makes sense that higher N rates would not result in a greater effect of drought on yield. If there is a drought, yield will suffer, but N rate will not decrease yield if applied at recommended rates.

Q: When is the best time to fertilize alfalfa?
A: The ideal time to fertilize alfalfa is after the first cutting. Following soil test results, if the phosphorus level is 11 parts per million or below, application of 100 pounds 11-52-0 per acre would pay premiums of greater hay yield in the future. In addition, alfalfa hay harvest takes off about 50 pounds K20 (potash) per ton. If the previous year hay crop was four ton, and one ton was removed at first harvest, then the grower should consider the application of 5 x 50 pounds K2O, or 250 pounds K2O, or a little over 400 pounds 0-0-60 per acre to replace its loss IF the K (phosphate) soil test is less than 200 ppm. If the soil test K was more than 200 ppm, then thank your field for the favor and K replacement is not necessary to sustain good yields.

Source: North Dakota Crop and Pest Report

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