In North American potato agriculture, chemical fumigants such as metam sodium and chloropicrin are often used to control soil-borne diseases. However, this approach to disease suppression may be a double-edged sword.
On the one hand, fumigation is highly effective at controlling soil-borne diseases like common scab and potato early dying, at least for one growing season. With fewer soil-borne pathogens, potato plants may develop healthier, more robust root systems, making them better at capturing soil nutrients and decreasing nitrogen input requirements. Alternatively, soil-borne disease suppression may increase nitrogen input requirements by increasing the maximum yield the crop is capable of achieving.
On the other hand, soil fumigants have effects on the soil microbial community beyond their effects on pathogens. For example, fumigants have been found to reduce rates of soil nitrogen cycling and microbial respiration. They may also decrease populations of some disease-suppressing microbes, increasing the degree to which the grower must rely on fumigation to control pathogens in the future. In short, chemical soil fumigation may have a negative effect on soil health.
Our Approach to This Question
We used the word “may” a lot in the last two paragraphs. To date, there has been very little research on whether fumigation changes the optimal nitrogen rate in any crop. It is also unknown how long it takes for the microbial community to recover from a fumigation treatment, or if it ever does.
We set out to address these questions in a field study on Russet Burbank potatoes conducted in 2016 and 2017. In each year, we had study plots fumigated in the fall before potatoes were planted with metam sodium or chloropicrin. A non-fumigated control was also included. In the following year, each plot was divided into five subplots, each receiving nitrogen at a different rate. All subplots received 40 pounds of nitrogen per acre as DAP (18-46-0) at planting, and each received either 0, 80, 140, 200, or 260 pounds per acre as ESN (44-0-0) at emergence, for total nitrogen rates of 40, 120, 180, 240, or 300 pounds per acre.
We measured crop responses to these fumigation and nitrogen rate treatments in terms of tuber set, yield, size and quality, symptoms of potato early dying, the amount of nitrogen taken up per pound applied, and the increase in yield per pound of nitrogen applied, among other things. We measured soil microbial responses in terms of Verticillium propagule density, respiration rates, and soil nitrate and ammonium concentrations.
Fumigation Suppresses Disease, Improves Yield
As expected, fumigants successfully served their primary purpose of controlling soil-borne pathogens. They both decreased populations of viable Verticillium dahliae propagules in the soil and the severity of foliar symptoms of potato early dying. Chloropicrin also decreased the prevalence of common scab in tubers.
Using these fumigants to control soil-borne diseases had positive effects on the crop beyond disease control. Total and marketable yield and tuber set were higher in plots treated with either fumigant than they were in the non-fumigated control plots at all of the nitrogen rates tested. This effect on yield did not meaningfully change the agronomic optimum nitrogen rate at which yield was maximized.
The amount of nitrogen the crop took up per acre and per pound of nitrogen applied were both higher in plots fumigated with chloropicrin or metam sodium than in non-fumigated control plots. The use of either fumigant also increased the amount of yield produced per pound of nitrogen applied. All of these differences reflect the differences between treatments in tuber yield (biomass) described in the previous paragraph. They were not due to differences in tuber nitrogen concentration or the amount of nitrogen present in the vines just before vine kill.
Not a Magic Bullet
While chloropicrin and metam sodium both provided benefits in terms of disease control, yield, and nitrogen use efficiency, their effect on microbial activity outside of pathogens was generally negative. Based on CO2 emissions from the soil, which are strongly related to microbial activity, both fumigants suppressed overall microbial activity before planting and at midseason. The effect was no longer detectable after harvest. In the pre-planting sample, metam sodium’s effect was especially strong.
Plots treated with either fumigant had higher soil ammonium concentrations than the non-fumigated control plots at planting and midseason. This indicates that soil microbes did not convert ammonium to nitrate efficiently under fumigation; this effect was especially strong when chloropicrin was the fumigant. So, in addition to suppressing microbial activity generally, fumigation slowed the rate of nitrogen cycling in the soil.
What Actually Works?
Our results show that, while soil fumigation with metam sodium or chloropicrin is beneficial to the potato crop in the short term, a side effect is a decrease in soil microbial activity and altered nitrogen cycling. How this affects soil health is uncertain, and it will take a longer-term study to find out—one looking more closely at soil health and the microbial community.
It is also important to consider other avenues for controlling soil-borne pathogens, especially if chemical fumigants are found to be detrimental to soil health in the long term. Are there other viable options for growers to control soil-borne pathogens? Could fumigation be used less frequently or in combination with inoculation of beneficial microbes? Maintaining soil health is essential to the long-term survival of a farming operation, but strategies to promote soil health must not be so costly that they compromise the farm’s viability in the short term.
To address these issues, we are collaborating with researchers across the country in a multi-state investigation into how soil health is affected by management decisions in potato cropping systems, whether methods for improving soil health work well in these systems, and how planting and incorporating biofumigant cover crops such as mustard compare with chemical fumigation in promoting yield and suppressing disease. This Specialty Crop Research Initiative- and USDA/NIFA-funded project is in its fourth year, and the year when we will get at fewest preliminary answers to these questions. Enhancing soil health is a slow and complex process, particularly with a challenging crop like potatoes. Therefore, this study will need to be extended beyond 2022 to learn whether effective strategies continue to improve soil health while controlling soil-borne pathogens over multiple rotations and years.
This research was supported with funding from the Minnesota Area II Potato Council, Minnesota Agricultural Fertilizer Research & Education Council, and TriEst Ag Group, Inc. Results were published in the American Journal of Potato Research September 2021.
Study authors include James Crants, Carl Rosen and Linda Kinkel at the University of Minnesota; José Pablo Dundore-Arias at California State University, Monterey Bay; and Andy Robinson and Neil Gudmestad at North Dakota State University.