ISSUE 16    August 18, 2005


As the season of 2005 rolls on, harvest brings thoughts about next year. Fertilizer prices are expected to remain high, and if the $10 futures natural gas is any indication, N prices might be even higher than this last season. Therefore, some of my thoughts:

Soil testing has been accepted in this area since the late 1970ís. We hang a lot of weight on the nitrate soil test when recommendations for N are made. Itís not perfect. Weaknesses include a guess on next yearís mineralization and also a guess as to how much N will mineralize between sampling and next spring. But if consideration is made about whether we need to be aggressive on rates or conservative on rates depending on where we are in the state (northeast generally more mineralization than the rest of the state, east more than west, north more than south) or the crops to be raised (wheat/corn aggressive; sugarbeets, malting barley, canola, sunflowers conservative), then itís not a bad first step to forming a good recommendation.

The debate about when to begin soil testing in the fall will continue. There are pluses and minuses to soil testing early or waiting. The pluses include a much better, easier to obtain 0-6 inch soil core for P, K, OM, and most micronutrients. Also, volunteer grains take up nitrate from the soil, and soil sampling even after these might be fall incorporated result in an underestimate of potential N due to their late spring decomposition. The minuses include more uncertainty regarding how much soil nitrate values might change between August and October or November. The answer is sometimes it will decrease (tie-up by straw decomposition) sometimes it will increase (mineralization in the soil with slow straw decomposition) and sometimes it will stay the same (losses and gains are equal).

This year will add its own twist by having not only 70 bushels of straw returned to the field, which would normally result in more N tie-up, but also in some areas there will be a large amount of scabby grain thrown out of the combine which would be expected to have some mineralization potential and perhaps offset the extra straw tie-up.

Regardless of what might happen, fluctuations after small grains are relatively small. It is more important to get the soil testing done than worry about whether it is done early or late. I wouldnít know how to guess how much residual N is in a field unless it was sampled.

Our precision sampling research continues to show that zone sampling effectively identifies important patterns of residual nitrate in fields. As I summarize a recent 3-state project, topography remains a primary means of delineating these zones across the region. In the west, soil EC is a good second layer. In the east, yield maps, particularly yield frequency maps, satellite imagery, aerial photography and soil EC can also be good secondary layers. More than one data layer helps to stabilize the patterns between years and usually results in better-defined zones.

With the exception of some fields of dry beans and soybeans in fields with problems, annual legumes and alfalfa need no supplemental N. The acres that are put in these crops mean lots of N input savings. A curious sidelight in soybeans occurred this season. Some observations were made on fields split with N fertilizer meant for wheat, but a field put into soybeans this spring. The side with fertilizer N was more chlorotic (Iron Deficiency Chlorosis) than the side without. Dr. Goos has observed this in tests he has conducted in the greenhouse, but this is the first time I have had anyone mention it in the field. The lesson is that perhaps on fields susceptible for IDC, application of high rates of N (greater than 50 lb N) may not be an acceptable plan.

Legumes supply N to the subsequent crop. Our recent publications state that about 40 lb N/acre should be expected following any of our annual legume crops harvested for grain. In the west after dry falls and springs, some have backed off on these credits to maybe 20 lb/acre. This is a reasonable adjustment. However, if the west continues to have moisture this fall, winter and spring, the book adjustments are very reasonable. In the east, they are probably conservative nearly every year.

Sugarbeet tops supply N to the next crop through decomposition. The satellite images and patterns of green to yellow in the field will help growers reduce N levels to the next crop. However, unlike past years, the yellow areas of the field may be more related to disease than residual N. Therefore, it will be important to sample these various zones more diligently than in the past to determine what the levels really are under the yellow beet tops. Usually yellow beet tops mean no more than 20 lb N in the top 4 feet. I would anticipate more than that this fall.

There is time between now and N application time to explore ways to better apply N. For no-tillers, this means figuring out a way to put down anhydrous of other products under the soil surface by means of a coulter or other residue management tool. For growers on sands, splitting application, with the splits going beneath the soil surface is very important. For those applying N in the fall, donít apply N too early, especially urea. Make sure that anhydrous applications close the trench adequately.

Dr. Dave Franzen
NDSU Extension Soil Specialist

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