No. 25 Article 3/December 7, 2012

First Results from the Illinois Soil Nitrogen Monitoring Project

In October we announced a new project, funded by the Council for Best Management Practices, designed to sample soil this fall to see how much nitrogen is left after 2012's dry growing season and, in many areas, low corn yields. This is a report of what we've found so far.

There are two main reasons for doing this project. One is to see how much N is in the soil now, to help us know how much N could potentially be lost over the winter and early spring. November was relatively dry in Illinois, and so far there have been very few reports of tile lines running. Now that soils have cooled down, the nitrate-N currently present in fields will mostly likely stay in the soil unless tile lines run, in which case we can expect some of it to exit in tile drainage water.

The second main reason to look at soil N in the fall is to gauge the potential for some of it to be available to next year's crop, especially if you come back with corn in the same fields. It's typical for some N loss to take place if precipitation is normal from fall to early spring, but if it remains dry again like last winter, some of the N in soil now should be available for next year's crop. Knowing how much is there in the spring requires taking soil samples again in the spring, preferably close to planting.

We had a good response to our call for volunteers to take samples for this project, with about 130 sites sampled to date in the U of I portion. Cooperators were asked to take 0-to-12-inch and 12-to-24-inch samples at a single site. Samples along with some site information, including how much N had been applied in 2012 and what the yield was, were sent to a lab to be tested for both nitrate and ammonium. Both cooperators and we received the data from the lab.

Nearly all of the samples for which we have received data were taken following corn in 2012, and many were from fields where 2012 corn yields were low due to dry weather. Because we had not done such a survey in the past, we did not know how much N to expect to find, though previous sampling work had typically shown nitrate-N levels of less than 10 ppm.

Samples received were somewhat concentrated in the central part of the state, but with a good distribution there from east to west (Figure 1). Nitrate-N concentrations in the top foot ranged from near 0 to 89 ppm among the 120-plus sites reported in Figure 1. The 89 ppm was from a field in the northern part of the state; yield reported from that field was 175 bu/acre. As the only value above 50 ppm, we can consider it an "outlier."


Figure 1. Soil nitrate-N values from Illinois fields sampled following harvest of the 2012 corn crop.

Table 1 summarizes average N rates used, corn yields, and the nitrate-N data from the top foot of soil by region. The division of Illinois into regions was arbitrary, done using latitude of sampling sites but roughly following Illinois Route 116 and Illinois Route 16 as dividing lines. The average N rates used and corn yields produced in sampled fields reflect the generally poor growing conditions over much of Illinois in 2012. Some of the fields in southern Illinois yielded little or nothing, while several fields in central and northern Illinois yielded from 180 to 200 bu/acre.

Table 1. Summary statistics from 121 soil samples received in the fall of 2012 from Illinois fields. Nitrate-N data are from the 0-to-12-inch sample.

Measurement

Illinois region

North

Central

South

Number of samples

33

62

26

Average nitrate-N (ppm)

26

16

18

  Range (ppm)

11 to 89

5 to 55

6 to 36

  Standard deviation

16

11

9

Average N rate used (lb/acre)

202

185

180

Average 2012 corn yield (bu/acre)

125

105

20

Not all of the samples reported in Table 1 had samples from the second foot of soil, but average nitrate-N in the 12- to 24-inch samples were 16, 12, and 18 ppm in northern, central, and southern Illinois, respectively. In southern Illinois these values are as high as those in the top foot, and in central Illinois they are not much less than in the top foot. The weighted average amount of nitrate-N in the top 2 feet of all fields sampled was 136 lb N per acre. While we know the sampled fields probably don't represent accurately all fields in Illinois, 12 million acres of corn in 2012 with 136 lb N in the top 2 feet would mean some 800,000 tons of N in the form of nitrate (that can be easily leached, denitrified, or used by plants) sitting in Illinois fields now.

As we might expect when water limits yields, the amount of N used in a field and corn yield in that field did not match up very well (Figure 2). A few fields with very high N rates produced low yields, and while few fields with low N rates produced good yields, it is clear that the amount of fertilizer N available to the crop had little influence.


Figure 2. N rates used and corn yields of Illinois fields sampled for soil N in the fall of 2012.

To compare applied and leftover amounts of N, we multiplied ppm of nitrate-N in the top foot by 4 to get lb N/acre. The relationship between amount of N applied and the amount we found in the fall was, however, quite poor (Figure 3). We also found that low yields were not associated with high amounts of leftover N among the fields sampled this fall (Figure 4).


Figure 3. N rates used and amount of nitrate-N found in the top foot of soil in Illinois fields sampled in the fall of 2012.


Figure 4. Corn yields and amount of nitrate-N found in the top foot of soil in Illinois fields sampled in the fall of 2012.

Finally, we estimated N removal in the grain (using 0.75 lb N/bushel) and subtracted that from the amount of N applied as fertilizer to see if this would help predict the amount of N we found in the soil. As Figure 5 shows, this didn't help much. How can it be that the combination of low yields due to dry conditions and high rates of N fertilizer don't seem to add up to the amount of soil N we are finding after the season?


Figure 5. Amount of nitrate-N in the top foot of soil vs. the amount of N applied as fertilizer minus removal in the grain (0.75 lb N/bushel) in Illinois fields sampled in the fall of 2012.

We think this simply shows how complex the N interactions are in the soil. In a year like 2012, there is little N loss, N uptake ends early as the crop stops taking up water, and late summer and fall rainfall can produce new flushes of mineralized N long after crop uptake stops. Thus it's possible that soil moisture was the main factor determining both yield and the amount of N in the soil and that these two things were more or less independent of one another in the tough year that was 2012.

We hope that many of the volunteers who sent in samples this fall will be willing to sample in the same places in spring to see how much N remains. The spring measurement will of course be extremely valuable to determine N availability for the crop and how much additional N may or may not be needed. Where corn in 2013 will follow corn in 2012, nitrate-N present at planting should be available to the 2013 crop, unless high-loss conditions prevail after planting.--Emerson Nafziger, Fabián Fernández, and Dennis Bowman

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