Author: Shane Bugeja, Extension educator, Blue Earth and Le Sueur Counties
Reviewed by Charlie Rohwer, Researcher, Southern Research and Outreach Center
In Minnesota, many tomato producers use organically based fertilizers such as manure or compost to meet the nitrogen needs of their crops. Often, these inputs contain much more phosphorus (P) than the crop can use, which then accumulates belowground. If the P-rich soil moves into waterways, it poses a serious risk for the environment. Some 744 water bodies in Minnesota are considered impaired due to excessive nutrient content, which includes phosphorus.
Adding unnecessary P fertilizer to tomatoes can be costly in 2022. As of this writing, artificial P fertilizers used by row crop farmers are at their highest price in many years. Depending on the farmer’s situation, a high price per pound P on the open market could lead to pricier raw and composted manure. Note that manure value calculators, including UMN Extension’s calculator, include market prices of nutrients in their equations.
Can a higher yielding tomato be part of the solution? If more fruit heads from the field to the market stand, theoretically this should also draw down soil P slowly over time. Grafting and adding starter fertilizer are two techniques that may increase tomato yields, or increase P in the fruit.
Grafting and Starter Fertilizer
Grafting is a process where two different plants are physically put together and grow as one. In tomatoes, the belowground portion of the grafted plant, called the rootstock, is chosen for disease resistance and/or general vigor. The aboveground plant, called the scion, is chosen for its particular fruit qualities. While farmers can graft tomatoes themselves, others pay for the service. While garnering a lot of interest, grafting is not widely used (currently) by Minnesota growers.
Grafted (left) and non-grafted (right) tomato plants. Note the scarring near the bottom of the grafted tomatoes where the rootstock and scion were joined. Photo: Charlie Rohwer
In addition to grafting, starter fertilizers may boost tomato yields. These primarily liquid fertilizers are applied shortly after tomatoes are transplanted. The goal is to ensure the young plant’s early growth is not held back by lack of available nutrients—either through environmental effects or the transplant’s tiny root system. While they contain P in their formulations, if a large enough yield bump occurs, then—in theory—the extra fruit removed could offset any added P.
Shortly after transplanting, this tomato was treated with starter fertilizer. Photo: Charlie Rohwer
2021 Tomato Study Results
To test these techniques, a University of Minnesota led study in two locations, one at the Southern Research and Outreach Center (SROC) in Waseca, and another at a cooperating farmer’s field near Waldorf, MN. Both locations had similar clay loam Webster soils. Three different cultivars of tomatoes were tested, ‘Galahad’, ‘Mountain Fresh Plus’, and ‘Paisano’. Treatments included tomatoes that were grafted and had starter fertilizer added, un-grafted tomatoes with starter fertilizer added, and an un-grafted control with no starter fertilizer. The tomatoes that were grafted were done so commercially with the rootstock 'Estamino’.
One of the tomato trial sites. Photo: Charlie Rohwer
After the 2021 growing season, there were no significant differences between any treatments. In other words, each tomato cultivar yielded about the same no matter if it was grafted, given starter fertilizer, or left untreated. Note that 2021 was a drought filled season for south-central Minnesota, and the grafted tomatoes seemed to be slower to mature than their ungrafted counterparts. It is possible these two factors could have masked any treatment effects. This study will be continued in 2022 to see if these patterns hold.
Marketable tomato yields broken down by grafting and starter fertilizer status. No significant differences were detected between the treatments. Graph: Charlie Rohwer
Cultivar yields (in marketable pounds per plant) broken down by site. No significant differences were detected between the treatments. Graph: Charlie Rohwer
If next year's study shows similar results, a lack of significant results can still help farmers. If these extra grafting and starter fertilizer costs do not add yield, then farmers can put these savings elsewhere on the farm. Going forward, a tailored fertility program that includes soil and manure testing from a certified lab is always a good option to help reduce phosphorus build up. University of Minnesota Extension also has tips for manure as well as soil testing on its website. Follow the SROC Horticulture team on Twitter to stay in touch with the tomato project as it continues its work.
This study is funded by the Minnesota Department of Agriculture’s AGRI Sustainable Agriculture Demonstration Grant
Works Cited
Bie, Z., Nawaz, M., Huang, Y., Lee, J., & Colla, G. (2017). Introduction to vegetable grafting. In Vegetable Grafting: Principles and Practices (pp. 1-21).
Rohwer, C. L., & Fritz, V. A. (2016). Transplant fertilizer solution and early season plastic mulch increase tomato yield in adequate fertility clay loam soil. HortTechnology, 26(4), 460-465.
Rouphael, Y., Kyriacou, M. C., & Colla, G. (2018). Vegetable grafting: A toolbox for securing yield stability under multiple stress conditions. Frontiers in plant science, 8, 2255.
Small, G., Shrestha, P., Metson, G. S., Polsky, K., Jimenez, I., & Kay, A. (2019). Excess phosphorus from compost applications in urban gardens creates potential pollution hotspots. Environmental Research Communications, 1(9), 091007.
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