USING PRE-PLANT NITRATE TEST TO REDUCE N INPUTS ON CORN:  11-YEAR HISTORY ON THE WICST PLOTS

Janet Hedtcke, Josh Posner, Jon Baldock, Dwight Mueller

INTRODUCTION

Accurately predicting the nitrogen needs of corn can improve farmers’ profits and reduce nitrogen losses to the environment.  In Wisconsin, it is a common practice to apply nitrogen for corn just prior to planting. The Preplant Nitrate Test (PPNT) was developed by Bundy et al (1995) to assess residual or “carryover” nitrate-N from the previous year so that current N-additions can be corrected for this readily available nitrogen in the rooting zone.  Nitrates can be carried over winter if crop yields are below expectations and unused nitrogen remains in the soil or if the growing season and winter precipitation is low, resulting in less than normal leaching.  Generally, medium or fine textured soils retain nitrates in the offseason better than sandy soils. Unfortunately due to the inconvenience of soil sampling in April, just prior to corn planting, not many growers use the PPNT.  According the UW-Soils and Plant Analysis Lab, they only receive 100-200 samples from less than 25 clients for PPNT each spring (John Parsen, pers comm., 2003).  In this paper we report on the value of the PPNT in reducing fertilizer N costs and losses to the environment over the past decade.

METHODS

As part of best management practices, the Wisconsin Integrated Cropping Systems Trials (WICST) (in Arlington and Elkhorn, WI) have routinely used the PPNT for the continuous corn system since their initiation.  Sampling was done according to protocol A3512 ‘Wisconsin’s Preplant Soil Nitrate Test’ (Bundy et al., 1995) except that 3 paired cores (in-row and between-row) were composited from each 0.7a plot.  Samples were taken from 0-12 inches and 12-24 inches each year between April 10 and April 20^th. At the Elkhorn site, PPNT tests have been done from 1990-1998.  At Arlington, PPNT tests have been taken from 1993-2003.  More recently (since 1998), the soil in the no-till corn-soybean system at the Arlington site has also been tested prior to corn planting.  Four replicates in each system were sampled each year.   Soils at both WICST sites are silt loams with organic matter levels above 3%. 

RESULTS

The standard recommendation for corn following corn without correcting for residual nitrates (the PPNT) at both Arlington and Lakeland is 160 lb N/a and is 120 lb N/a for corn following soybean (40 lb credit for soybeans) on silt-loam soils.  The data shown in Figs 1a and 1b are the mean of the 4 replicates in corn after corn with the range shown as a vertical line each year. By taking PPNT each spring on the WICST plots, we have been able to reduce our N rates on corn 25-50% in 14 of 19 site-years (Figs 1a and 1b). In only 2 of those 19 sites years was the PPNT recommendation at the standard 160 lb N/a. Similarly, in corn after soybeans, in 5 out of 6 years, the PPNT recommendation was less than or equal to 80 lbs N/a (Fig. 2).  Both 1993 and 1998 showed little residual N at both sites and both systems.  This was likely due to the wet fall/spring in those years.

DISCUSSION

The PPNT recommendations would result in an average N savings of 54 and 58 lbs N/a per year for continuous corn at Arlington and Elkhorn, respectively, and an N saving of 53 lb N/a in corn following soybeans.  At $0.15/lb N, that is a savings of $8.10/a at Arlington and $8.70/a at Elkhorn in corn after corn.  In years with high N prices (such as $0.22/lb N in 2003) the saving could be as high as $12.76/a.  The variability of residual nitrate from plot to plot under the same cropping system has been small except for two years at each site.  Thus, the N savings have been relatively consistent, too. Even in years with more variability such as 1997 or 2003 at Arlington, the maximum value was still significantly lower (120 lb N/a in corn after corn) than the standard recommendation (Fig. 1a).  Nevertheless, the variability in the PPNT from plot to plot in a few years, which simulates field-to-field variation, underscores the importance of taking PPNT every field each year.  Our results on the potential N savings agree with the results of Bundy et al. (1995).  They found that 25% of the sites sampled saved between $5-10acre and 40% saved more than $10/a using the PPNT.

It is often difficult to fully realize the potential N savings that would result from following the PPNT exactly.  Sometimes the difference from plot-to-plot (field-to-field) is not large enough to change calibrations and other times the PPNT recommendation is less the minimum setting of the application equipment.  These problems are particularly important when applying anhydrous ammonia as we did in the WICST plots.  But amazingly enough, actual applied N rates were quite close to the PPNT recommended rates on the WICST plots at both sites.    

Although using the PPNT has allowed us to reduce N-additions in the corn and corn soybean rotations, there is room for further improvement in tailoring N rates and timing to uptake.  One measure of how much of a mismatch there is between N applications and N use in corn is the level of N left in the soil after corn harvest.  The fall nitrates in these two systems are well above those in the restored prairie plots, which represent the natural synchrony of N release and use.  For the 2000 to 2002 years at Arlington, the continuous corn and corn after soybeans averaged 86 and 70 lbs nitrate-N/a in the top 2 feet compared to 24 lbs nitrate-N/a for the restored prairie.  Thus, even though the WICST corn plots received less N than the standard recommendation, substantial N was left in the fall and subject to loss.  In these 3 years, which have been dryer than normal, most of the fall nitrate has been retained in these systems overwinter.  However, we would expect significant leaching losses in wetter years.  Thus, the PPNT helps reduce such losses, but work still remains to be done to better synchronize N-additions with corn N uptake.

CONCLUSION

Since many conventional corn producers apply N prior to planting without doing the PPNT, it is a safe bet that in most years, especially following years with average or below rainfall, that N fertilizer is being over applied.  The PPNT data from the WICST plots shows that most years we can reduce the N inputs by 25 to 50% for corn following corn or corn following soybeans on the silt loam soils of southern Wisconsin.  Comparison of the N left after corn harvest to natural systems suggests that further improvements in accounting for all N sources and better synchronization between N applications and N use are needed.

REFERENCES

Bundy, L.G., S.J. Sturgul, and R.W. Schmidt.  1995.   #A3512 - Wisconsin’s Preplant Soil Nitrate Test. UW-Extension, NPM. R-5-95-3M.

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