NDSU Crop and Pest Report

ISSUE 15   August 19, 2004


The best tool for helping determine N fertilizer rates is soil testing. Soil testing may begin whenever grain is removed from the field. There is a historical tendency to wait until mid-October to begin soil testing. That tendency was fueled until recently by a "fall nitrate soil test adjustment" if the sampling was conducted before September 15. However, consideration of data from North Dakota and Minnesota has resulted in eliminating the fall nitrate soil test adjustment. The adjustment tried to tell people more than we understood about the trend of soil nitrate levels following grain harvest.

A fairly large number of sites have been periodically sampled throughout years and seasons at the same locations during the last thirty years. At some sites, nitrate levels following harvest are steady, and no increase or decrease is recorded. At others, the test levels decrease over time, while others increase. Sites with relatively high residual soil nitrate levels seem to have the greatest variability over time and the greatest differences between sampling dates. However, these levels are often high enough that a recommendation of zero N would be appropriate whatever time period was chosen during the study.

The notion that nitrate levels settle down after September 15 is not valid. Sites were just as susceptible to temporal variability after that date as before. Actually, sites were most stable during the August-early September period, compared to later in the fall, perhaps due to a tendency for drier weather than later in the fall. Differences between fall and spring soil sampling showed similar trends. Although on average the spring test was slightly higher than fall, there were sites where soil levels decreased from fall to spring.

Regardless of when the soil sampling is taken, it needs to be recognized as a scientific starting point for N fertilizer recommendations. Fields that I have sampled in North Dakota have averaged 20 lb/a residual N some years, and 80 lb/a N in the same field in other years. Without soil sampling each field each year, large errors in application can result. Even though a fall or a spring nitrate soil test can sometimes seem like a moving target, the trend in each field and base nitrate level defined by soil testing at any date is much better than a casual guess.

I recommend an early fall soil sample if physically possible for the following reasons:

1. Taking a reproducible 0-6 inch depth soil core is possible before fall tillage, and nearly impossible following fall tillage. This is important if phosphate, potassium, soil pH, and zinc are important to analyze.

2. Volunteer grains sap soil of residual N, but in the spring they rot and some of the N is mineralized. Later sampling when volunteer grain was present at the time or earlier results in an underestimation of available N levels. We have no previous crop credit adjustments for volunteer grain at this time, although we know that it has an impact on N levels in some years.

3. Late fall sampling is no guarantee of residual nitrate stability. Nitrate levels following a late October sampling could go up, down or sideways, just as easily as an August/September sampling.

4. A late fall sampling is often a post mortem. Fall N is often applied before fields are sampled or samples are analyzed. I would rather know what I should do, as compared to what I should have done.

The most important point to make is that more soil sampling needs to be done. There is a very low percentage of growers who use the power of soil testing to improve N application efficiency in North Dakota and the region, with the exception of sugarbeet growers. With the cost of fertilizer N in the 22 to 28 cents/lb range for the foreseeable future, soil testing is the best tool available to improve N rate efficiency and make operating dollars go farther.



In North Dakota, little ink has been spent talking about potassium (K). Small grain demand is relatively low, especially considering the tendency for soils to be high in available K. However, now that soybeans and corn are commonly grown in the eastern half of the state, I am beginning to hear reports of K deficiencies. Corn and soybean crops remove far more K than small grains. Also, a trend to more no-till and reduced till systems tends to make K more unavailable for some reason. There is serious discussion regarding lowering the critical soil test level for certain crops, especially corn and soybeans, due to a large number of deficiency symptoms and crop responses reported in South Dakota and Minnesota recently. For now, including K testing in the plans for corn and soybean acres seems to be a prudent addition to the list of soil testing nutrients. Also, a simple zone testing for K would help isolate potential problem soils before they become a problem, as compared with a simple composite sample.



The new revision of Fertilizing Flax is now available through the NDSU publication distribution center. Highlights include dropping P recommendations for flax, and limiting the total N available to flax to 80 lb N/acre. Discussion of the role of mycorrhizae in flax nutrition is presented. The circular is available on the web at:




Along with gas prices, fertilizer prices will remain high. Anhydrous ammonia prices may again be in the $350-$400 range this fall, with urea prices above 25 cents/lb N. Phosphate prices also remain high. A big surprise this fall is an increase in potash pricing. For the last several years, prices have been flat at about $150/ton to the farm, but this year prices may be $30-$50 higher, as export demand overseas for Canadian potash has greatly increased compared to previous years. Budgeting an additional 20% on fertilizer inputs for this coming year would be wise.



At the recent 7th International Conference on Precision Agriculture in Minneapolis, it was notable that many presenters discussed zone soil testing and hardly anyone talked about the merits of grid sampling. Zone soil testing has been shown to be a practical method of dividing a field into areas of relatively uniform available soil nutrients. A number of us have also seen that using several "layers" of information that can be used to delineate the zones is generally superior to a single method. Layers that we have used successfully include multiple-year yield maps (yield frequency maps), soil EC sensor readings, aerial photography, Landsat 7 satellite NDVI imagery, and topography. Several consultants now have the technology to accomplish these tasks. Zone sampling can give a grower more confidence in soil test levels and help distribute fertilizers more efficiently within a field, resulting in less lodging and better quality.

Dr. Dave Franzen
NDSU Extension Soil Specialist

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