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Collection of soil samples from depths below 6 inches has been a suggested management practice in southwestern, western, and northwestern Minnesota for more than twenty years. Analysis of these samples for nitrate-nitrogen (NO₃-N) has allowed for the fine tuning of current nitrogen recommendations. Nearly everyone who works with nitrogen and nitrogen fertilizer recommendations realizes that measurement of NO₃-N in soils is not absolutely perfect. Yet, use of this test where appropriate, can have a substantial effect on nitrogen fertilizer recommendations.

For many years, monitoring of NO₃-N in soybean fields that precede an intended corn crop led to the conclusion that soil NO₃-N has been relatively constant at 30 to 40 lb. N per acre for these production situations. However, nitrogen is very transient in soils and quantities measured are highly affected by environmental conditions. So, there should be no expectations that measured NO₃-N following soybeans will remain constant over time. Some information collected in the fall of 2002 is ample evidence for the variability of NO₃-N in soils.

Some of the information summarized by Agvise Laboratories is provided in the following table.

In all cases, wheat was the previous crop. In general, the percentage of samples in each category appears to be related to the weather for the past spring and summer. The growing season in south central ND could be characterized as dry whereas there was ample, and in many cases, above normal moisture in northern and northwestern Minnesota.

Past experience has shown that soil nitrates increase where there is a drought that severely restricts crop growth. Even though there was ample moisture in northwest and northern Minnesota, a substantial number of samples had levels of NO₃-N in excess of 40 lb. per acre. The results of this summary indicate that substantial amounts of nitrate-nitrogen were produced as a result of mineralization of soil organic matter.

Another summary of the analysis of soil samples for nitrate-nitrogen is provided by Centrol of Cottonwood, Minnesota. Samples to 24 inches were taken from fields where corn would follow soybeans as well as fields where the sugarbeet crop would follow corn. That summary is provided in the tables that follow.

For the planting system where corn will follow soybeans, 50% of the samples showed amounts of NO₃-N in excess of 40 lb. per acre. This is not consistent with what we would expect.

Substantial amounts of NO₃-N were found where corn is to follow the sugarbeet crop. Recognize that the sampling depth was 42 inches rather than 24 inches. Nevertheless, several samples showed substantial amount of NO₃-N.

High rates of fertilizer N are not usually applied to the sugarbeet crop and no nitrogen fertilizer is applied to soybeans. The source of the NO₃-N can be questioned. The only logical explanation is the mineralization of soil organic matter. Most crop producers are reporting excellent yields for the 2002 growing season. Thinking back to past years, measurements of NO₃-N have shown relatively high levels of NO₃-N following a good crop. This was true even though the high-yielding crop would have removed substantial amounts of N from the soil system. These observations over the years lead to the conclusion that environmental conditions, which produce good yields, are also favorable for above average rates of mineralization of soil organic matter. Thus, the higher than expected amounts of residual NO₃-N.

With two summaries available at this time and shown in previous tables, this would appear to be a good fall to collect soil samples for measurement of residual or carryover NO₃-N in southwestern, western and northwestern Minnesota. For intended crops other than sugarbeets, guidelines recommend collection of soil from 0 to 6 and 6 to 24 inches. If the sugarbeet crop is planned, guidelines call for collection of soil from depths of 0 to 6, 6 to 24 and 24 to 48 inches. For this crop the calculation of residual NO₃-N is based on analysis of soil from each depth.

Considering the information available at this point in the fall season, it would be a good practice to get a measure of NO₃-N. If values are higher than expected, rates of nitrogen fertilizer can be reduced thereby lowering costs without reducing yield.