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Tile Water Monitoring 2018, Part 2: Crop and nitrogen rates

Article cover photo
The ISA water monitoring program includes collecting water samples from tile outlets across the state. (Photo: Joseph L. Murphy/Iowa Soybean Association)

By Anthony Seeman, ISA environmental research coordinator

This is the second installment of a four-part series on the Iowa Soybean Association’s tile water monitoring program and aggregated results from 2018. Read the first article: 2018 Results by Landform.

The Iowa Soybean Association’s (ISA) tile water monitoring program provides information each year to individual farmers about water quality on their farms. The water monitoring program began in 1999 through Agriculture’s Clean Water Alliance (ACWA) working with ISA staff who conduct the sample collection. It has expanded to the reputable program known today across the state and beyond.

In 2018, the ISA research team collected more than 4,000 samples from Iowa streams, drainage tiles and edge-of-field practice locations for analysis in ISA’s certified water lab. The research team also looks at the results in aggregate to glean additional information such as performance of practices and possible trends.

Nitrate concentration results

When focusing only on tile drainage from corn and soybean fields, three systems are considered:

  • soybeans with previous year corn (53 sites),
  • corn with previous year soybeans (63 sites), and
  • corn with previous year corn (27 sites).

Average nitrate concentrations in the tile water from the three rotations are shown in Figure 1. The box plots show a range of results.

Figure 1: Average nitrate concentration in tile water from three agriculture rotations. CC = corn following corn, CS = corn with previous year soybeans, and SC = soybeans with previous year corn.

The fields of corn following corn had the highest average nitrate concentration at 19.8 milligrams per liter (mg/L). Soybean fields with previous year corn had the lowest average concentration at 11.2 mg/L. Corn fields with previous year soybeans were slightly higher at 12.6 mg/L, although statistically these two groups were not different.

Since nitrogen fertilizer typically is not applied to soybeans, the similar nitrate concentrations in the tile water often comes as a surprise to farmers.

A closer look at corn nitrogen application rates reveals even more complexity in the cropping system. One would think the more nitrogen applied to corn, the higher nitrate levels in the tile water would be. This is somewhat true, but the relationship between the two is not strong.

Figure 2 shows three years of nitrogen application rates to corn compared to the average nitrate concentration in the tile water. A smoothing technique is used to plot the lines that follow the data. Their gentle slope indicates there would be little reduction in nitrate concentration from small changes in the typical fertilizer application rate range (150-200 lbs/acre).

Figure 2: Nitrogen application rates versus average tile drainage nitrate-N concentrations for three consecutive years ending in 2018.

The results indicate the complexity of managing cropping systems, soils and drainage for improved water quality. While the crop itself and the amount of applied nitrogen each play a role in nitrate loss, the solutions aren't as simple as just adjusting application rates.

Meaningful management solutions to reduce nutrient loss attempt to treat water and cropping system additions when the crop is most vulnerable. The addition of cover crops and edge-of-field structures, such as bioreactors or streambank buffers, will help to reduce nutrients in the water before entering rivers and streams. 

The ISA tile water monitoring program does not sample from a controlled experimental design, therefore results presented in this article are observational and additional scientific study may be warranted. Findings from the monitoring program generally align with the Iowa Nutrient Reduction Strategy and other published research. 

The next article in the series will provide a summary of results from cover crop usage. 

Contact Anthony Seeman at

For media inquiries, please contact Katie Johnson, ISA Public Relations Manager at or Aaron Putze, ISA Communications Director at

For permission to republish articles or to request high-res photos contact Aaron Putze at Iowa Soybean Association | 1255 SW Prairie Trail Pkwy | Ankeny | IA | 50023 | US

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