There has been a lot of speculation recently about how management practices affect soil health, but not a lot of documented results from long-term studies under western U.S. climatic conditions.
We recently completed a four-year study under a NIFA-funded project entitled “Enhancing Soil Health in U.S. Potato Production Systems” that sought to quantify the impact of rotation length, fumigation, organic amendments, and variety choice on soil health and crop productivity.
While it is hard to draw conclusions about the long-term effects of these practices based on these relatively short-term studies, the results do provide insights on how rapidly the system responds to changes in management.
The Idaho portion of the study was initiated in 2019 at a field near Rupert with a history of potato production. We established separate areas for the two-year (potato, wheat) and three-year (potato, corn, wheat) rotations, with the goal of comparing the impact of rotation length when both areas were planted to potatoes in 2022.
Within each trial area we also evaluated the impact of fumigation with metam sodium at the 40 gallons per acre rate each fall prior to planting potatoes, application of composted dairy manure at 10 tons per acre prior to the wheat and corn crops (a total of 20 tons/acre of compost was applied over the four years in both rotations), as well as incorporation of a full-season green manure crop into the rotation instead of wheat. The majority of the plots were planted to the TX-278 strain of Russet Norkotah, but we also had a comparison to Russet Burbank in the non-fumigated plots.
To get a clearer picture of the chemical, physical and biological characteristics of the field we took separate soil samples in the 2-year and 3-year study areas during 2019 before any of the treatments had been applied. Those values were compared to results from the samples taken in the spring of 2022 to see if measurable changes due to the treatments might help explain any differences in potato yields.
Shortening the rotation length to two years between potato crops reduced marketable yield by over 60 cwt per acre in both Russet Burbank and Russet Norkotah (Figure 1). This reduction in yield was associated with higher populations of Verticillium (one of the casual agents of the early die complex). Pathogen populations increased between 2019 and 2022 in both rotations (Figure 2), but the 2-year rotation started and ended with higher Verticillium populations compared to the 3-year rotation due to the intense potato production (four potato crops in seven years compared to only three potato crops in the longer rotation).
While fumigation increased marketable yield in Russet Norkotah in both rotations, it failed to bring up yields in the short rotation to a level that would match the longer rotation (Figure 3). These results support the argument that if you have a choice between fumigating every other year or increasing your rotation by one year, you are better off with the longer rotation.
Another key finding was that potato yields respond very quickly to the addition of organic amendments like dairy compost. The compost treatment matched the fumigation treatment in terms of marketable yield in both the 2- and 3-year rotations (Figure 3). Surprisingly, there was no benefit to incorporating a full season green manure crop into the rotation, and no additional benefit from the combined green manure and compost treatment.
One of the key things we wanted to do in this study was to identify soil health indicators that were associated with the yield results. One of the effects of adding compost was a measurable change in soil chemistry.
For example, compost treatments significantly increased soil levels of available phosphorus, potassium, zinc and boron. However, soil chemistry alone does not explain the yield results because the 3-year rotation (i.e. highest yields) had lower overall phosphorus levels than the 2-year rotation. Likewise, there were few significant changes in common soil health indicators like organic matter, total organic carbon, microbial respiration, soil protein and aggregate stability.
We hope to continue these studies until at least 2028 to get better information on the long-term impact of management practices on soil health. You can follow progress on the entire U.S. potato soil health project at the following website: https://potatosoilhealth.cfans.umn.edu
(Thornton can be reached at miket@uidaho.edu. Miller can be reached at jeff@millerresearch.com.)