LEACHING OF AGRICULTURAL CHEMICALS TO TILE DRAINS ON WICST TRIAL

Mike Hanke^[1] , David Perry¹, Sam Kung¹, Jim Stute ^[2] , Josh Posner ^[3] , and Gary Bubenzer¹

INTRODUCTION

Agriculture has a large impact on the water resources of the nation.  In a recent national survey of 17% of rivers in the US, it was found that that one-third were “impaired”.  Agricultural sediments and chemicals were the primary cause in 60% of these cases (USDA, 1997).  Looking specifically at nitrogen, it is estimated that nearly 40% of the nitrogen in the Mississippi River that causes hypoxia in the Gulf of Mexico (oxygen depletion that kills shell fish), comes from the Upper Basin, which represents only 15% of the entire watershed (Alexander et al.,1995).  The goal in this leaching study was two fold: 1) to understand the potential for leaching of agricultural chemicals on a productive silt-loam soil; and 2) to see if improved farming practices would reduce nitrate leaching to tile drains.

METHODS

At the Walworth County Farm, tiles drains were installed on the somewhat poorly drained (Griswold and Pella silt-loams) soils in the 1970’s.  In 1997, the tiles on the satellite blocks of the WICST trial were modified and man-holes dug and instrumented allowing independent monitoring of the individual tiles under different farming systems.

•1999 tracer study.  An irrigation tent was built and used to apply water to the plots over the tile drain.  Once flow had begun in the tile, a chemical not common in agricultural systems (like bromide) was added so that its movement could be easily traced through the soil and out of the tile drain.

•2001 nitrate leaching study.  Monitoring of flow rates and nitrate loading in the drains under the no-till corn soybean rotation and “chem-lite” corn-soybean-wheat/red clover system was conducted. 

RESULTS

Tracer study:  Figure 1 is a graph of the breakthrough times for tracer chemicals when added under different irrigation rates.  In both cases irrigation had begun several days earlier and the tiles were already flowing.  In the first case under a heavy rainfall scenario (blue line on fig. 1) of 0.12 in/hour (2.9 in/24 hr) the tracer was in the drains within 16 minutes, and 10% of the product was in the tiles in 18 hours.  Under a mild rainfall scenario (red line on fig. 1)of 0.035 in/hr (0.85 in/24 hr), the tracer was in the drains within 4 days, and it took nearly 8 days for 10% of the tracer to leach through the profile to the tiles.

Nitrate Leaching study: Figure 2 is a histogram that shows the amount of nitrate that leached to the tiles during each month of tile flow in 2001.  Two things are obvious in this figure: 1) most of the leaching took place in the spring (especially in June which received 6.28 inches of rainfall) and that more nitrate leached from the no-till corn and soybean plots (113 lbs/a) than the chem-lite corn- soybean-wheat/red clover rotation (66 lb/a).  
 

DISCUSSION

Heavy spring rains that coincide with the application of agricultural chemicals are not an exception in Wisconsin.  The final three “bars” in Figure 3 indicate that 25% of the time, total rainfall in June will be 6 inches or greater.  Under these conditions, the tiles will most likely flow and chemicals can move out of agricultural fields and into streams and lakes.

For the tracer to reach the tiles in 16 minutes under the heavy rainfall conditions (2.9 in/day), most of that water was moving by preferential flow to the tile lines and bypassing the tortuous soil matrix.  Apparently these heavy agricultural soils have channels that can carry water (and chemicals) to the tiles very quickly once the whole profile is wet.  Under the lower rainfall conditions (0.85 in/day) the water and tracer traveled by much smaller channels and took a good deal longer to arrive at the tiles.

Nitrogen losses were higher than expected, especially considering that wet weather prevented the application of side dressed N to the corn in either rotation.  The N leached to the tiles was from residual nitrogen added the previous year, soybean stover and green manure decomposition in the spring, and annual mineralization of N from the soil organic matter.  In addition to higher soil N-levels in the corn-soybean rotation, more water leached through this system than in the "chem-lite" rotation.  Probably due to the rapidly growing wheat phase in the chem-lite rotation, only 32% of the incoming spring rains (April-June) leached through the profile, while in the slowly emerging no-till corn-soybean rotation, 42% of the rainfall leached through the soil to the tiles.  It appears that in wet years, even without adding nitrogen fertilizer, significant amounts of N can be lost from the field.

CONCLUSIONS

Due to preferential flow patterns and generally high nitrogen levels in fields, significant amounts of agricultural inputs can be lost from cropping systems on silt loam soils to tile drains during a “wet” spring. Nitrogen losses under the “chem-lite” system were, however, substantially lower than the no-till corn-soybean rotation.

REFERENCES

Alexander, R.B., R.A. Smith, and G.E. Schwarz.  1995.  The regional transport of pint and nonpoint-source nitrogen to the Guld of Mexico.  Proceedings of the Gulf of Mexico Program.  Dec. 5-6, 1995.  New Orleans. LA

USDA (Natural Resources Conservation Service). 1997. America’s private land: A geography of hope. U.S. Gov. Print. Office, Washington D.C.