Potato nutrient management research: 5 things we’ve learned

By: Carl Rosen, Extension nutrient management specialist

In Minnesota, potatoes are typically grown under irrigated conditions on sandy, low organic matter soils. Because of a high nutrient requirement and a relatively shallow root system (most roots are within the top foot), the crop is often responsive to applied nutrients, particularly the three primary macronutrients (nitrogen, phosphorus, and potassium). Nutrient management research over the years has provided some insight into optimizing nutrient inputs to enhance yields and minimize losses to the environment. More recently, improving soil health in potato cropping systems has also been a focus. Here are five things we have learned so far:

1. There are several strategies available to increase nitrogen use efficiency

Of all the essential nutrients, nitrogen (N) is the one most often limiting for potato production. It is also very susceptible to leaching losses as nitrate under conditions of excessive rainfall or unpredicted rainfall following irrigation. There are several strategies that can be used to improve nitrogen use efficacy by potato.

The first is to select a realistic target nitrogen rate. Then, split applications to meet the demands of the crop. This means applying the bulk of the nitrogen at emergence and beyond. The fastest rate of N uptake occurs between tuber initiation and initial tuber bulking. Depending on cultivar, this can occur between 20-30 days after emergence.

Enhanced efficiency fertilizers such as polymer-coated urea or urea coated with nitrification inhibitors applied at emergence can extend nitrogen availability through the bulking period. The benefits of enhanced efficiency fertilizers primarily occur during seasons with excessive rainfall.

To determine nitrogen needs during the growing season, petiole nitrate analysis remains the most sensitive tool. Periodic testing of petioles from tuber initiation through the bulking period is recommended. If petiole nitrate drops below the critical level when conditions are conducive for tuber bulking, application of soluble N via fertigation is suggested.

Current research is evaluating the use of remote sensing with drones to schedule nitrogen applications. The advantage of remote sensing is that spatial variability in nitrogen status within a field is more easily determined than with spot checking of petioles.

2. Fumigation increases nitrogen use efficiency but does not change the optimal N rate

Soil-borne diseases such as Verticillium wilt and root knot nematodes can limit potato root health and the ability to take up nutrients. A short-term fix to these diseases is to use fumigation such as Vapam or Chloropicrin. Our research has found that if soil-borne diseases are present at high levels in a soil, the use of fumigation can increase nitrogen use efficiency but does not significantly affect the rate of nitrogen required to optimize yields. At the recommended N rate of 240 lbs N/acre for Russet Burbank, N uptake in fumigated plots increase by 50 lbs N/acre over non-fumigated plots, indicating that there is better use of applied N and potential for less leaching with fumigation. Unfortunately, the effects of fumigation are short lived and repeated use of fumigation is necessary during the years potatoes are grown. Current research is evaluating the use of cover crops, composted manure, and biofumigation using mustards to reduce or use in combination with chemical fumigation.

3. Improving soil health in potato cropping systems is difficult and takes time but there are some promising strategies that may help

Methods to improve soil health include reducing soil disturbance, keeping the soil covered with residue, keeping living roots in the soil with perennials, increasing crop diversity, and integrating livestock on the land. Soil health is a challenge with potatoes because of the significant soil disturbance that occurs at planting, hilling, and harvest, as well as the need for fumigation to control soil-borne diseases. If you want to improve soil health, you should focus most of your attention on the rotation years when crops can be grown that require less tillage than potatoes.

We were part of a nationwide, multi-institution Potato Soil Health Project to assess the viability of various approaches to improving soil health in potato cropping systems. The field site for the study was used for commercial potato production for several decades. The site was selected because it had a history of potato early dying and low yields, suggesting poor soil health. In the study, a three-year rotation outperformed a two-year rotation in terms of yield, tuber size and tuber specific gravity, with a lower prevalence of Verticillium symptoms in the tubers. Norkotah Russet was especially affected by rotation length. Bannock Russet is resistant to Verticillium wilt and, without fumigation, it yielded better than Russet Burbank did with conventional fumigation. However, Bannock tubers had a much higher prevalence of hollow heart, which may limit the adoption of this cultivar. For Russet Burbank and Russet Norkotah, pro-microbial treatments including biofumigation were as effective as chemical fumigation only in the three-year rotation. These results indicate that improving soil health in a field with known soil-borne diseases will take time and that extending the rotations as long as possible along with use of biofumigation, cover crops, and composted manure may offer the best results in the long term.

4. There are several things to consider if you want to increase phosphorus uptake and supply

Phosphorus (P) is a macronutrient that is essential for all forms of life. A deficiency of P in potatoes is associated with delayed vegetative growth, poor tuber set and bulking, and reduced yields. Excess P seldom affects plant growth, but it can increase eutrophication if it moves offsite into surface water. Phosphorus management is therefore an important consideration from both production and environmental standpoints.

Phosphorus fertilizer recommendations for potatoes are often based on soil test P and yield goal, but numerous studies have shown that economic responses can sometimes occur even on high P testing soils. For this reason, some P fertilizer is usually recommended for potatoes even when soil test P is high. However, a P response is not always guaranteed, even on lower P testing soils.

The reasons why potatoes often respond to phosphorus, even under high soil P conditions, are not entirely known but could be related to several factors. For example, potatoes naturally have a limited root system and the practice of hilling limits that root system even further. Because P is immobile in soil, plant roots depend on an increased root surface area to increase the probability of intercepting soil P in areas that have not yet been depleted. As plant roots take up P, the amount available for further uptake decreases and therefore growing into areas of higher soil P is an important adaptation to increase P supply to the plant.

Soil pH may also be a factor that limits P uptake. Fixation by iron and aluminum in acid soils and calcium in alkaline soils reduces P availability. For some varieties, soils are maintained at a pH below 5.5 to control common scab. While this practice can improve tuber quality, it may also result in lower P availability. Another reason for variable P response may be due to soil-borne diseases such as verticillium and pathogenic nematodes, which can further limit root growth. To control soil borne diseases, fumigation is used but this practice is not specific to disease organisms and may also reduce beneficial microbes such as mycorrhizae. Mycorrhizae are naturally occurring fungi that form a symbiotic relationship with plant roots to increase root surface area, thereby improving phosphorus uptake. Finally, there could be distinct varietal differences in P uptake due to differences in root structure and surface area. Some varieties may be more efficient in P acquisition and uptake than others due to root adaptations to solubilize fixed P or the breakdown of organic P.

In a three-year study with two potato varieties (Russet Burbank and Ivory Russet), both varieties were responsive to P fertilizer application on soils testing greater than 80 ppm Bray P. Phosphorus response was observed under both fumigated and nonfumigated conditions but there seemed to be a greater response to P with fumigation. Banding was effective in two of the three years and may reduce the need for higher P application rates. While potatoes can respond to P fertilizer on high P testing soils, yield increases may not be sufficient to offset the current high cost of P fertilizer, especially if soil-borne diseases are prevalent. Efforts are needed to identify or develop potato cultivars that are more efficient at taking up soil P to reduce the reliance on fertilizer P on high testing soils.

5. Not enough (and too much) potassium can impact potato yield and quality

Potassium (K) is an essential element required in high amounts for profitable potato production. High tuber yields can typically remove over 250 lbs K2O/acre. Rates to apply are based on yield goal and soil test K level. Because irrigated potatoes are grown on sandy soils, soil test K is usually in the low to medium range and therefore some K fertilizer is usually applied to meet the K demand on these soils. Typically, K fertilizer is applied as a broadcast application in the spring or in the previous fall before planting. Potassium deficiency will result in early vine dieback, blackspot bruising, increased disease incidence, and poor tuber yields. On the other hand, high K can result in lower specific gravity and increased shatter bruising, especially when the chloride (Cl) form (0-0-60) is used. The most common K source is potassium chloride (0-0-60). Chloride is also an essential element. It is considered a micronutrient but is taken up by the plant in macronutrient quantities. While much of the chloride taken up remains in the chloride form, plants are known to incorporate chloride in organic compounds. Over 100 organic compounds in plants are known to contain chloride but their function is largely unknown. In addition to studying potassium response in potatoes, we are also taking a closer look at how chloride affects yield and quality. Because Cl is highly leachable, there is also interest in determining if split applications of 0-0-60 would result in greater uptake of Cl and therefore reduce its leaching potential.
For more information, view the U of M’s full guidelines: Potato fertilization on irrigated soils

Thank you to our generous research funders:

• Agricultural Fertilizer Research and Education Council (AFREC)
• Minnesota Area II Potato Research and Promotion Council
• Clean Water Land and Legacy Amendment funding administered through the Minnesota Department of Agriculture
• Environment & Natural Resources Trust Fund as recommended by the Legislative-Citizen Commission on Minnesota Resources
• USDA-NIFA-SCRI grant award # 2018-51181-2870

Carl Rosen will be giving the Leo M. Walsh Distinguished Lecture in Soil Science on Tuesday, October 31, 2023 at 5:20pm at this year’s ASA-CSSA-SSSA International Annual Meeting in St. Louis, Missouri. The topic of the lecture will be “Keeping Nutrients in Place: A Journey of Challenges, Benefits, and Tradeoffs” and will include much of his research on potato cropping systems.

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Support for nutrient management blog posts is provided by Minnesota's fertilizer tonnage fee through the Agricultural Fertilizer Research & Education Council (AFREC). Learn more at MNsoilfertility.com.