Effects of Phosphorus and Calcium on Tuber Set, Yield, and Quality in Goldrush Potato

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1 Effects of Phosphorus and Calcium on Tuber Set, Yield, and Quality in Goldrush Potato Carl Rosen, Charles Hyatt, and Matt McNearney Dept. of Soil, Water, and Climate, University of Minnesota Summary: A field experiment at the Sand Plain Research Farm in Becker, MN was conducted in 2010 to evaluate the effects of phosphorus and calcium nutrient based management practices on Goldrush potato tuber yield and set. Seed planting depth was also examined for its effect on tuber greening. A comparison was made between a standard practices control and treatments that included 0 vs. 150 lb P/A, and/or the addition of 0 vs. 200 lb Ca/A as either gypsum or calcium chloride. Additionally, a deep seed treatment was included which increased the seeding depth by 4 in. The P fertilizer treatments at planting did not significantly affect tuber size or set. The addition of supplemental Ca also did not have a significant effect on tuber size or set as shown by the lack of significant difference in either category. There was no evidence of any combined effect due to P removal and supplemental Ca on tuber set. Gypsum with P or without P tended to result in lower total tuber yield than the other treatments tested. The result of deep seeding on tuber greening was inconclusive due to a general lack of tuber greening in any of the treatments. At equivalent fertilizer rates, deep seeding did not significantly affect tuber yield. Background: Goldrush potato is a fresh market russet that has a high yield potential but is less susceptible to producing misshapen tubers than other cultivars, such as Russet Burbank. Goldrush does, however, have a tendency to produce a second tuber set and also to initiate tubers near the surface of the hill. These problems can result in large numbers of undersized or green tubers. Previous work with Russet Burbank has shown that eliminating phosphorus fertilizer at planting can increase tuber size by reducing tuber set. Research conducted in Wisconsin has shown that calcium application at early hilling can have the same effect. However, no studies have been done to examine tuber set where these two management practices have been combined i.e. the elimination of P fertilizer application at planting and the subsequent application of a calcium source at hilling. In this study, we compared a conventional P application strategy with treatments that included Ca, removed P, or both. Additionally a treatment to evaluate the practice of deep seeding on tuber greening was also included. The objectives of this study were, under field conditions, to 1) evaluate the effect of P and Ca management on tuber set in Goldrush potato, and 2) determine if a greater seeding depth can reduce tuber greening. Materials and Methods The study was conducted at the Sand Plain Research Farm in Becker, Minnesota on a Hubbard loamy sand using the potato cultivar Goldrush. The previous crop was rye. Selected soil chemical properties before planting were as follows (0-6"): water ph, 6.2; organic matter, 1.9%; Bray P1, 24 ppm; ammonium acetate extractable K, Ca, and Mg, 113, 835, and 156 ppm, respectively; Ca-phosphate extractable SO 4 -S, 2 ppm; and DTPA extractable Zn, Cu, Fe, and Mn, 0.7, 0.3, 26.5, and 6.7 ppm, respectively. Extractable nitrate-n in the top 2 ft prior to planting was equivalent to 10.8 lb/a.

2 Whole B seed was hand planted in furrows on April 22, 2010 at a 6-8 in depth, except for the deep seeded treatment, which was planted at a in depth. Four, 20 ft rows were planted for each plot with 18 ft of each of the middle two rows used for sampling and harvest. Spacing was 36 inches between rows and 12 inches within each row. Each treatment was replicated four times in a randomized complete block design. Weeds, diseases, and insects were controlled using standard practices. Rainfall was supplemented with sprinkler irrigation using the checkbook method of irrigation scheduling. Treatments included a conventional management practice control, a deep seeded treatment, and various combinations of reduced P and supplemental Ca as either gypsum or calcium chloride. Six treatments were tested and are listed below (Table 1). Table 1. Phosphorus and Calcium treatments tested in the Goldrush tuber set study. Treatment # N P 2 O 5 K 2 O Ca lb / A None Gypsum None Gypsum Calcium Chloride None (Deep Seed) A starter fertilizer containing 60 lb N/A and 150 lb P 2 O 5 /A as diammonium phosphate (DAP), and 300 lb K 2 O/A and 30 lb S/A as potassium sulfate were applied to treatments 1, 2, and 6 at planting, while treatments 3, 4, and 5 received 60 lb N/A as ammonium nitrate and 300 lb K 2 O/A and 30 lb S/A as a blend of potassium sulfate and potassium chloride. The remaining 180 lb N/A was sidedressed as urea and mechanically incorporated - half at emergence on May 25 and half at hilling on June 2. Calcium treatments of 200 lb Ca/A as calcium chloride or gypsum were sidedressed at hilling on June 2. Plant stands were measured on June 2 and the number of stems per plant was counted on June 9. Tuber numbers were measured by hand-digging five plants before machine harvest on Sept. 7 and separating them into size categories before counting. On Sept. 8, vines were killed via mechanical beating. Plots were machine-harvested on Sept. 14 and total tuber yield, graded yield, tuber specific gravity, and the incidence of scab, hollow heart, and brown center were measured. Tuber greening was also examined at this time through a visual inspection process whereby any noticeable greening on a given tuber from each plot was considered a positive indication of greening. All trials of the experiment were statistically analyzed using ANOVA procedures on SAS and means were separated using a Waller-Duncan LSD test at P = 0.10.

3 Results Rainfall and irrigation amounts are presented in Figure 1. Tuber Set and Yield: Total yields were greatest with the control (150 lb P/A and no supplemental Ca), which resulted in higher yields than either of the gypsum treatments, either with or without P (Table 1). With respect to marketable yield, the control again resulted in the numerically highest yields, although differences were not significant. There were no statistically significant differences among treatments in the numbers of small tubers produced (< 3 oz) or in the number of tubers > 6 oz or > 10 oz, although the deep-seeded treatment produced the numerically highest yields for the last two categories. Though not significant, the trend was for treatments with P additions to have yields with higher percentages of large tubers. There were no significant differences among treatments with respect to the specific gravity of tubers or number of tubers per plant. Tuber Quality: Incidences of hollow heart and brown center were significantly higher in the standard practices control (trmt 1) than in other treatments (Table 2). Scab levels were not significantly different among treatments with means that ranged from 17 (deep seed trmt 6) to 31% (no P trmt 3). There were no significant differences in tuber greening among treatments as the number of green tubers was generally very low (< 2 tubers per 36 ft of harvested row) or zero in all treatments. Conclusions Phosphorus fertilizer at planting did not significantly affect tuber size or change in set. The addition of supplemental Ca also did not have a significant effect on tuber size or set as shown by the lack of significant difference in either category. There was no evidence of any combined effect due to P removal and supplemental Ca on tuber set. Gypsum with P or without P tended to result in lower total tuber yield that the other treatments tested. The result of deep seeding on tuber greening was inconclusive due to a general lack of tuber greening in any of the treatments. At equivalent fertilizer rates, deep seeding did not significantly affect tuber yield.

4 Water inputs between planting and vine kill: (139 days) Rainfall = in (74%) Irrigation = in (26%) Total = in 2.0 Water Inputs (Inches) VK H Days after planting Figure 1. Rainfall and irrigation amounts during the 2010 growing season. Planting (April 22); VK = Vine Kill (Sept 8); H = Harvest (Sept 14)

5 Table 1. Effects of reduced P, supplemental Ca, and Ca source on Goldrush tuber yield and size distribution. Calcium and Phosphorus Treatments Tuber Yield Ca P #1 # 2 Total Trtmt Ca > 6oz > 10 oz Rate Rate 0-3 oz 3-6 oz 6-10 oz oz >14 oz Total > 3 oz > 3 oz Marketable # Source lb Ca / A lb / A cwt / A % 1 None Gypsum None Gypsum Calcium Chloride None (Deep Seed) Significance 1 NS NS NS NS NS ++ NS NS NS NS NS LSD (0.10) NS = Non significant; ++, *, ** = Significant at 10%, 5%, and 1%, respectively.

6 Table 2. Effects of reduced P, supplemental Ca, and Ca source on Goldrush tuber quality and tuber count (i.e. tubers per plant). Calcium and Phosphorus Treatments Tuber Quality 1 Tuber Count Trtmt Ca Ca Rate P Rate Specific HH BC Scab Greening # Source Gravity # of 0-3 oz 3-6 oz 6-10 oz >10 oz Total lb Ca /A lb /A % Tubers 1 None Gypsum None Gypsum Calcium Chloride None (Deep Seed) Significance 2 NS * * NS NS NS NS NS NS NS LSD (0.1) HH = Hollow Heart; BC = Brown Center 2 NS = Non significant; ++, *, ** = Significant at 10%, 5%, and 1%, respectively