Tracking nitrate in farm fields

Excess nutrients from fertiliser can eventually reach groundwater. Predicting how nitrate will flow from the surface into groundwater can help farmers balance fertilising their crops with protecting the water they and others rely on, according to Thomas Harter, a member of the Soil Science Society of America.

Types of soil, crops and fertilisers affect the way nitrate moves through soil. During a study, designed by Harter and his team, across a single 140-acre California almond orchard 20 deep soil samples were gathered. Wells were installed to measure groundwater.

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One of 20 monitoring wells across the almond orchard test site, allowing researchers to monitor the levels of nitrate reaching groundwater from the soil above. – Photo: Thomas Harter

Nitrogen levels varied widely

They found that a complex quilt of different soils lies underneath the almond orchard. These different soil types made it very hard to predict how nitrogen from one part of the farm would affect another area. Nitrogen levels varied widely across the farm, even though it was managed consistently by a single grower.

“We did not expect the significant variability in nitrate concentrations between monitoring wells, given the relatively uniform management across the site,” says Harter.

A single farm can harbor big differences

Older studies often measured groundwater across entire regions. Those researchers usually assumed that different crops and growing practices controlled groundwater nitrate. But Harter’s team has now seen that even a single farm can harbor big differences. Nearby patches of soil and water can look very different.

Some factors could predict the amount of nitrate in groundwater. The total amount of nitrogen and water entering the field affected the nitrate in groundwater.

Scientists saw that the nitrogen in the sandy soils predicted how much ended up in wells. That’s because of how water moves through different types of soil. Sandy soils let nitrogen and water move freely compared to sticky clay soils.

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A row of flowering almond trees in the Sacramento Valley. Research shows nitrogen levels vary widely across the farm, even though it was managed consistently by a single grower. – Photo: Thomas Harter

New methods needed

But because of how much nitrogen varied across the fields, according to the scientistst new methods are needed to monitor and evaluate this source of potential pollution.

“We must develop efficient alternative tools that can be validated at field sites like ours but then used at thousands of individual fields and farms to assess their contributions to groundwater,” Harter says.

New irrigation and nitrogen management practices

Going forward, the scientists also hope to see how new irrigation and nitrogen management practices might improve groundwater quality at this farm. This work will help design and test solutions to improve groundwater quality.

“This work provides the foundation for better design and interpretation of field studies that are the foundation of improving agricultural practices and ultimately groundwater quality,” says Harter. “Over the long-term, this will lead to better solutions for protecting well water quality as well as stream water quality in agricultural regions.”

Thomas Harter’s research was recently published in Vadose Zone Journal, a publication of the Soil Science Society of America.