Impact of Clover Incorporation on Ammonium, Nitrate, and ISNT-N over Time; 4-Year Summary

Quirine Ketterings1, Greg Godwin1, Charles L. Mohler2, Brian Caldwell3, Karl Czymmek1
1Department of Animal Science, and 2Department of Crop and Soil Sciences, Cornell University, 3Department of Horticulture, Cornell University

Red clover undersown into a small grain crop is commonly used as an N source for corn in organic grain production systems. How much N should be credited to the clover green manure is unclear. In this study we addressed the following issues: (1) ammonium and nitrate dynamics over time following clover plowdown; (2) release peak for nitrate related to the above ground biomass in the clover; and (3) clover plowdown influence on the results of the Illinois Soil Nitrogen Test (ISNT), a predictor of soil N supply potential. Results from earlier years were reported in Godwin et al (2009) and Ketterings et al (2011). Here we report the 4-year summary.

We monitored ISNT-N, ammonium-N and nitrate-N levels on a weekly basis in corn crops in one management system within the Cornell Organic Grain Cropping Systems Experiment at the Musgrave Research Farm near Aurora, New York. Beginning in 2005, this experiment has compared five management systems with differing fertility and tillage regimens and two entry points into a soybean-spelt/red clover-corn rotation ( For the project discussed here we sampled the Low Input Organic System (System 2) during years when corn was grown following plowdown of a 1-yr old clover cover crop.

Actual fertility amendments and their date of application are shown in Table 1. Plots were randomly split into two rotation entry points, so that one half of each plot was a year behind in the crop rotation sequence. The plots that were sampled for N dynamics were part of Entry Point A in 2007 and 2010 and Entry Point B in 2008 and 2011.

Prior to plowing, we collected samples of above-ground clover biomass. Below-ground biomass was also sampled in 2011. The initial soil sampling round (0-8 inch depth; 12 cores per 120’ x 40’ plot) occurred prior to plowdown of the clover. The next sampling round occurred at plowdown and was followed by eight sampling rounds at weekly intervals thereafter (seven in 2011). Corn was planted in late May or early June (Table 2). On New York organic grain farms corn is generally planted in late May with the goal of planting into warm soils to achieve optimal germination without the need for seed protectants, and to allow for sufficient clover growth to support the N needs of the corn crop.

Soil samples were analyzed for ISNT-N in the Cornell Nutrient Management Spear Program (NMSP) laboratory using the enclosed griddle modification of Klapwyk and Ketterings (2005). Soil samples were also analyzed for 2 N KCl extraction of exchangeable nitrate+nitrite and ammonium as described in Mulvaney (1996). The weather patterns showed two extreme rainfall events during the sampling periods: a 2 inch plus rainfall event in week 4 after planting in 2010 and another in week 5 in 2011 (Table 3).

Results and Discussion
Clover above-ground dry biomass was 1.6, 2.4, 1.5 and 1.7 ton/acre in 2007, 2008, 2010 and 2011, respectively. In 2011 the below ground cover crop biomass was 0.6 ton/acre, about 25% of the total (above and below ground) biomass of the clover cover just prior to plowdown. The year 2007 was a drought year with low corn yield (87 bu/acre), whereas 2008 and 2010 were excellent growing years (165 bu/acre in 2008 and 160 bu/acre in 2010). Despite challenging growing condition (wet spring and fall, dry mid-summer) yield averaged 150 bu/acre in 2011.

Soil nitrate-N levels increased following clover incorporation (Figure 1). The height of the nitrate-N peak following plow-down was consistent with clover biomass over the four years: just over 60 ppm in 2007, almost 90 ppm in 2008, and about 30 ppm in 2010 and 2011 (Figure 1). Peaks in nitrate-N were measured in week 3 in 2011 and 5 or 6 in all other years. The timing of nitrate-N release from clover was well-aligned with the period of highest corn N needs in 2007 and 2008. In 2010 and 2011, the heavy rainfall in week 4 (2010) and week 5 (2011) may have leached nitrate-N. Nitrate leaching may be the cause of the relatively low nitrate release peaks in 2010 and 2011.

Soil samples were analyzed for ISNT-N as an indicator of soil N supply potential through mineralization of organic matter. Previous work has shown that the test is accurate as a predictor of soil N supply potential for corn but that soil samples should not be taken within 5 weeks after manure addition or sod turnover; these amendments create a temporary increase in ammonium-N and hence also in ISNT-N. The question remained whether incorporation of a cover crop would result in a similar restriction in timing of sampling for ISNT-N. The results of the four years of testing showed that clover incorporation did not result in an accumulation of ammonium-N and hence it is also not surprising that the ISNT-N levels remained stable over time (coefficients of variation across sampling dates were only 4.3, 2.2, 3.9, and 2.6% for 2007, 2008, 2010, and 2011, respectively). These results suggest that for the clover-based system, timing of ISNT sampling is not restricted (i.e. sampling can occur before or after clover incorporation). The comparisons of ISNT-N in 2007 and 2010 (entry point A) and 2008 and 2011 (entry point B) suggest a slow decline in ISNT-N over time under current management and yield levels (Figure 2) but additional research (more data points in time) is needed to evaluate trends.

Averaged across plots, the pre-sidedress nitrate test (PSNT) results were 20, 29, 16, and 17 ppm where clover had been plowed down in 2007, 2008, 2010, and 2011, respectively. There was a strong correlation between clover above-ground N pool prior to plow down and PSNT in this study (PSNT (ppm) = 4.9 + 0.14 * Npool (lb N/acre); R2 = 0.94). Additional data points are needed before conclusions can be drawn about the use of above ground N pool as a predictor of PSNT-N and the impact of weather on the predictions.

The weekly sampling and the PSNT results of the clover systems suggest that the clover supplied a considerable amount of N. Application of 1900 lb/acre of 4-5-2 poultry manure compost in addition to the plowed down clover in the same experiment showed no yield increase in 2007-2010 (Caldwell et al., 2011) or 2011 (data not yet published), suggesting that in each of the four years, the nitrate-N released from clover decomposition was sufficient to meet the needs of the corn, despite the <21 ppm PSNTs in 2007, 2010, and 2011.

Summary and Conclusions
Clover incorporation greatly increased the amount of available N for the following corn crop. Decomposition of the clover resulted in nitrate peaks 5-6 weeks after incorporation, well-aligned with N needs of the corn and showing that clover plowdown is an excellent choice for providing N to corn in organic and conventional production systems. Clover decomposition did not result in ammonium-N accumulation. This study needs to be duplicated at other locations but results to date indicate that a clover cover crop can supply sufficient N. although actual N supply will vary depending on the biomass produced and mineralization conditions. The study also showed that ISNT sampling for assessment of soil N supply is not restricted in time where clover is a main source of N fertility.


  1. Caldwell, B, C.L. Mohler, Q.M. Ketterings and A. DiTommaso (2011). Yield and profitability during and after transition in the Cornell organic grain cropping systems experiment. What’s Cropping Up? 21(2): 7-11.
  2. Godwin, G., Q.M. Ketterings. C.L. Mohler, B. Caldwell, and K.J. Czymmek (2009). Impact of clover incorporation and ammonium nitrate sidedressing on ammonium, nitrate, and Illinois Soil Nitrogen Test dynamics over time. What’s Cropping Up? 19(3): 12-15.
  3. Ketterings, Q.M., G. Godwin, C.L. Mohler, B. Caldwell, and K.J. Czymmek (2011). Impact of clover incorporation and ammonium nitrate sidedressing on Illinois Soil Nitrogen Test dynamics over time 3-year summary What’s Cropping Up? 21(2): 1-4.
  4. Klapwyk, J.H., and Q.M. Ketterings (2005). Reducing laboratory variability of the Illinois soil N test with enclosed griddles. Soil Sci. Soc Am. J. 69: 1129-1134.
  5. Mulvaney, R.L. (1996). Nitrogen-Inorganic Forms. In Methods of soil analysis. Part-3- Chemical Methods. SSSA, Inc., ASA, Inc. Madison, WI. P. 1123-1184.

This work was supported by the USDA Organic Research and Extension Initiative, the New York Farm Viability Institute, and funds from the Cornell Experiment Station. We thank Kreher’s Poultry Farms for donating compost. For questions about these results contact Quirine M. Ketterings at 607-255-3061 or, and/or visit the Cornell Nutrient Management Spear Program website at:

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