As part of the 2024 NOFA-VT and the Maine Organic Farmers & Gardeners Association (MOFGA) Climate Forums, Becky Maden discussed nutrient management within the context of a changing climate. Maden is a vegetable nutrient management specialist for University of Vermont Extension.

Maden cited a survey which stated that 68% of Vermont’s vegetable farms list climate change and extreme weather – such as frequent heavy rains and drought intensity and duration – as their top concern. According to Maden, these extreme events require a different approach to nutrient management.

Increased rainfall and storm intensity has negative consequences for soil health including compaction, the creation of a crusty layer on the soil surface and erosion which comes with its own myriad of impacts: loss of topsoil, seeds and fertilizer and nutrient runoff.

On the flip side, heat and drought pose their own set of challenges including reduced yield and quality, greater irrigation demands and altered cycles of water and nutrient demands.

“Nitrogen is probably the most relevant nutrient to this conversation from an agronomic perspective,” Maden said. Nitrogen is critical to nearly all aspects of plant growth, but it’s difficult to manage, especially after a rain event.

For organic farmers, sources of nitrogen, such as compost and cover crop residues, are in their organic forms, and the nutrients are not available to plants until they’re broken down into their inorganic forms – ammonium and nitrate. Microbes break down these organic substances through a process called mineralization.

“Ammonium is positively charged, and it’s fairly stable in the soil. It will attach to soil particles, and it’s not quite as prone to loss, whereas nitrate is negatively charged and has no clinging ability to the soil. So, especially in a sandy soil, nitrate is incredibly slippery and prone to loss with any rainfall,” Maden said. Nitrate poses an environmental risk to wells and all waterways.

Nitrate also gets lost in a process called denitrification. This occurs when there is a lack of oxygen in the soil, and the microbes “steal” oxygen from the nitrate, turning it into N2O gas.

The goal of the vegetable farmer is to match the amount of nitrate available to plants with the proper stage of growth. This is particularly challenging after a heavy rainfall. According to Maden, plants may need up to twice as much nitrogen in a wet year as in a dry year, and growers need to consider not only how much to apply but when it will be usable in its plant available forms.

Clay and sandy soils handle overly wet and drought conditions differently – and that affects their nitrate-holding capacity as well. Photo courtesy of Becky Maden

One tool that can be used to monitor nitrate is the pre-sidedress soil nitrate test (PSNT). It’s a soil test that measures the amount of nitrate available in the soil at the time the test is taken, which is ideally done just prior to peak crop uptake. The PSNT only provides a snapshot at how much nitrate is available; it’s not predictive of the organic nitrogen that is likely to continue mineralizing as time continues nor what the crop has already accessed.

Nevertheless, Maden thinks the PSNT is a useful tool, especially after a heavy rain. She also recommends using the test at the end of the growing season to show what residual nitrate is present.

“I think it’s very important both from an economic and environmental perspective. What are you leaving in the soil? What’s vulnerable to loss? And what are you wasting?” she said.

Maden thinks tissue and sap testing hold promise for helping to manage nitrate. These tests can reveal what crops have actually taken up and provides data about nutrient uptake. The downsides to tissue and sap testing are that they’re more expensive and have a longer turnaround time at the lab.

Cover crops are another tool growers can use to manage nitrate. “Keeping soils covered is one of the best things we can be doing. Specifically, with regards to leaching, cover crops are essential,” Maden said. “I think of cover crops as mopping up whatever’s left at the end of a growing season or as a placeholder at certain times of the year when there would otherwise be bare soil. Cover crops can soak up what is there and hold it until you need it.”

Growers should also consider irrigation practices within the context of nutrient management.  Maden said high tunnels tend to be overwatered, resulting in nitrate leaching, whereas field crops tend to be underwatered. Nitrate sticks around in dry soil but soil moisture is critical to crop uptake. This is where tools like tensiometers and water sensors can be used to get a better sense of how much irrigation is needed.

Sidedressing after a rain event is another strategy to consider. After a heavy rain, Maden recommended paying attention to the stage of crop growth. She said, “Is it still really in a phase where it needs nitrogen to put on growth or does it just need a teeny bit to keep going?”

Conventional vegetable growers have more sidedressing options than organic growers. For organic growers, sodium nitrate (15-0-2) is the most affordable option applied at a rate of 10 to 30 lbs./acre. It can be used in a granular form or dissolved and applied through a fertigation system. It should not, however, be used as a foliar spray as it could burn the crops’ leaves.

Maden noted that sandy soils are more vulnerable to nitrate loss than clay soils. On sandy soil, she said that you can expect that most of the nitrate will leach away, but there may still be organic nitrogen continuing to mineralize into plant available forms. It will likely take days to mineralize though.

She said, “The seven to 10 days after the rain is when you’ll be hit with that nitrate deficiency, and then it should come back up. It is really important to remember, if you front-loaded your nutrients or expect some nitrogen from a previous stand of legume cover crop, it shouldn’t be all doom after a heavy rain. It should just be a temporary loss.”

by Sonja Heyck-Merlin