Consumers and retailers demand crops raised using innovative and regenerative agricultural practices

By Guolong Liang, outreach specialist, UW-Madison Extension Ag Water Quality Program

Nitrate (NO3–), a form of Nitrogen (N), is a key nutrient input that fuels current agriculture. It is directly available for plant uptake, but nitrate can also be leached through the soil into groundwater.

Nitrate leaching occurs more easily in Central Wisconsin because of coarsetextured soils and types of crops commonly grown in the area, which generally receive large amounts of nitrogen.

Soil nitrate can easily be lost to groundwater, particularly after heavy rainfalls. In Wisconsin, most residents rely on groundwater as their primary water supply.

High nitrate concentrations in drinking water can present increased health risks. Therefore, it is important to find ways to utilize N sources in agriculture efficiently and protect water quality and public health.

Above: Seek creative ways to apply nitrogen (N) fertilizer closer and more concentrated in the hills to increase N uptake in potatoes while reducing leaching in the furrows. Photo courtesy of Guolong Liang, UW-Madison Extension Ag Water Quality Program

Inset photo: Guolong Liang, outreach specialist, UW-Madison Ag Water Quality Program, gave his presentation “Estimating Nitrogen Leaching: A Step to Improve Water Quality and Nitrogen Use Efficiency” at the 2023 Grower Education Conference & Industry Show, Stevens Point, Wisconsin.

The Central Sands is one of the most nitrate-susceptible regions in Wisconsin. In Portage County, the most intensive vegetable production area in the Central Sands, 24% of private wells show nitrate levels exceeding the drinking water standard of 10 milligrams/liter.

Nitrogen Application

One of the dominant N inputs in an agroecosystem is commercial nitrogen fertilizer. In the Central Sands, the seasonal N recommendation for potatoes (Russet Burbank) ranges from 145 to 250 lbs. per acre depending on yield goals.

However, high N input on sandy soils poses risks for nitrate leaching in vegetable production fields.

Significant reductions of nitrogen leaching are key to reducing nitrate levels in groundwater. Nitrogen management practices are an important part of these reductions. What are some opportunities to reduce overall impacts on water quality? This article provides strategies to decrease N leaching in potato production systems and highlights some of the current University of Wisconsin (UW) research efforts to further

1. Apply nitrogen use efficiency strategies to meet potato needs and prevent N over-application;

Right Time, and Right Placement.

Plant and experiment with cover crops to minimize fall and spring N leaching.

Not all N fertilizer is taken up by potato plants during the growing season. Shrestha et al. (2010) pointed out that the risk of nitrate leaching in a potato field is the highest late in the growing season after kill. After vine kill, potato tubers’ ability to uptake N decreases as it reaches full maturity.

2. Plant and experiment with cover crops to minimize fall/spring nitrate leaching; and

Right Rate: To determine the correct N rate, test and account for all N sources, which may include irrigation water and credits from previous crops.

According to Figure 1, in August and September, the potato crop uses very little N from the soil, meaning unused nitrogen combined with precipitation leads to the peak nitrogen leaching risk period.

3. Diversify crop rotation to reduce overall N input.

Improved nitrogen use efficiency either means growing the same amount with less nitrogen or growing more with the same amount of

Right Time: Of total season N application rates, apply 25–30% at emergence, 50–60% at midtuberization, and 10–25% at tuberization plus three weeks.

Establish a cover crop as soon as possible after potato harvesting to realize more growing degree for the cover crop and capture leftover N during the fall and the following spring.

An additional benefit of establishing cover crops in the off seasons is enhanced soil health and reduced wind erosion, with living biomass contributing to these benefits for more of the year.

The basic considerations to deliver nitrogen efficiently boil down to the “4 R’s:” Right Source, Right Rate,

Right Placement: Seek creative ways to apply N fertilizer closer and more concentrated in the hills to increase N uptake in potatoes while reducing N

Researchers are experimenting with inter-seeding cover crops between potato rows within the growing season to capture unused soil N.

The goals are to manage the competition between the inter -seeded crop and potatoes, and to find right species or mix of species that minimally competes with or pot entially even enhances potato

Grow Potatoes to Reduce Nitrate Leaching. . . continued from pg. 25 leaching in the furrows.

Plant and experiment with cover crops to minimize fall and spring N leaching.

Not all N fertilizer is taken up by potato plants during the growing season. Shrestha et al. (2010) pointed out that the risk of nitrate leaching in a potato field is the highest late in the growing season after vine kill. After vine kill, potato tubers’ ability to uptake N decreases as it reaches full maturity.

According to Figure 1, in August and September, the potato crop uses very little N from the soil, meaning unused nitrogen combined with precipitation leads to the peak nitrogen leaching risk period.

Establish a cover crop as soon as possible after potato harvesting to realize more growing degree days for the cover crop and capture leftover N during the fall and the following spring.

An additional benefit of establishing cover crops in the off seasons is enhanced soil health and reduced wind erosion, with living biomass contributing to these benefits for more of the year.

Researchers are experimenting with inter-seeding cover crops between potato rows within the growing season to capture unused soil N.

Improved nitrogen use efficiency either means growing the same amount with less nitrogen or growing more with the same amount of nitrogen.

The goals are to manage the competition between the interseeded crop and potatoes, and to find the right species or mix of species that minimally competes with or potentially even enhances potato health and quality.

Crop Rotation

Diversify and extend crop rotations to reduce overall N leaching. Although most potato growers in the Central Sands implement a three-year crop rotation (two crops between) involving potatoes, over 20% of potatoes in the area return within two years of previous planting. (Heineman & Kucharik, 2022)

Common rotational crops currently used are corn, sweet corn, and snap beans. To reduce overall N input, choose rotational crops with lower N requirements, such as alfalfa, peas, or beans.

On top of choosing low-N rotational crops, extend the years between each potato crop. Consider a site prep year where a biofumigant such as mustard or millet is grown. Benefits include reduced soilborne pathogen loads and increased soil carbon.

With consumer and retailer demand for crops that are grown using innovative and regenerative agricultural practices, strategies like these need to be explored further. These strategies decrease weed and disease pressure and add carbon, in addition to other benefits that extend throughout the rotation.

If benefits to water quality can be quantified and reductions in nitrate leaching are shown to be significant enough, the potential for growers to be rewarded for these practices by retailers and consumers is going to be much greater.

UW researchers are working on the management of nitrogen in the Central Sands agroecosystem to refine these recommendations and seek new strategies.

• Dr. Matt Ruark from the UWMadison Department of Soil Science is conducting on-farm trials monitoring crop N uptake under different N rates.

Grow Potatoes to Reduce Nitrate Leaching. . . continued from pg. 27

• Dr. Yi Wang, UW Department of Horticulture, is now developing a remote sensing-based tool to monitor site-specific variation of potato N status to support in-season nutrient application decisions with the help of precision agriculture.

• Dr. Jed Colquhoun from the Department of Horticulture is also investigating innovative ways to incorporate cover crops into the potato agroecosystem.

• At UW-Madison, Dr. Chris Kucharik and Ph.D. candidate Tracy Campbell from the Department of Agronomy are quantifying variability of nitrate in irrigation water applied to potato fields in the Central Sands.

• Dr. Jingyi Huang, Department of Soil Science, is developing low-cost nitrate sensors with potential to gather in-season fluctuations in nitrate levels at the root zone and groundwater.

• At UW-Stevens Point, Kevin Masarik from the Center for Watershed Science and Education is measuring nitrate leaching under potato rotations in the Central Sands and developing a Nitrogen Budget Calculator for field-specific nitrate leaching estimation.

Reduction of nitrogen leaching requires refining the current nitrogen management. Research in the Central Sands is important to understand nitrogen leaching dynamics and to refine existing and find new nitrogen reduction strategies.

References

Shrestha, Raj K., Leslie R. Cooperband, and Ann E. MacGuidwin. “Strategies to Reduce