Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 28, 1998
Publication Date: N/A
Interpretive Summary: Soil and plant analyses can provide farmers, crop consultants, and other land managers with information needed to improve N management and decrease the potential for non-point contamination of surface and ground water resources. We used preplant and pre-sidedress soil tests, and end-of-season cornstalk nitrate tests to evaluate the long-term effects of continuous corn production on four watersheds in western Iowa. Based on this research, we offer three recommendations. First, when developing management plans for large fields or small watersheds, crop yield goals should be computed using a weighted average for the various soil map units that are located within the management area. With this approach, our measured grain yields averaged 95% of the predicted values. Second, we recommend that stalk nitrate tests be used to detect whether or not excess N has been applied to the crop. Our data shows that this test was sensitive to differences in seasonal rainfall and responsive to decreasing fertilization rates. Finally, we suggest that for deep loess soils with a long-term history of continuous corn production, future fertilizer N rates should be adjusted using data from the upper 0- to 60 cm and a critical value of 16 ppm. For each ppm nitrate below this value, 9 kg/ha of fertilizer N should be applied. Using this fertilizer N management approach should reduce the potential for leaching of nitrate into groundwater resources without reducing crop yield or grower profitability.
Technical Abstract: Preplant and pre-sidedress soil nitrate tests and the end-of-season cornstalk nitrate test were used to quantify long-term effects of continuous corn production on plant available N within four western Iowa watersheds. Soil and plant samples were collected from top-, middle-, and bottom-slope positions in each watershed between 1990 and 1995. The soil-tests showed that long-term N fertilizer rates, which averaged approximately 188 kg/ha and were based on a 9.4 Mg/ha yield goal times 0.02 kg grain/kg N, resulted in a gradual accumulation of residual N and created a very uniform nitrate concentration to a depth of at least 90 cm. Cornstalk nitrate tests suggested plant available N was excessive in all four watersheds during a relatively dry year (1990). From 1992 through 1995, 28% of the cornstalk analyses indicated that excessive N was available to the crop. Cornstalk samples collected from the most severely eroded sites had the lowest nitrate concentrations, presumably because more shallow rooting limited N uptake. Pre-sidedress soil analyses indicated fertilizer N rates could have been reduced from an average of 146 kg/ha to between 50 and 66 kg/ha/yr. We conclude that using soil and plant analyses can reduce the potential for nitrate contamination of groundwater resources and that such reductions will also improve profitability to the grower.