Received for publication 25 Apr. 2000. Accepted for publication 10 Aug. 2000. We thank Michael P. Russelle and Satish C. Gupta for their helpful com- ments on an earlier draft of the manuscript. This study was supported by Helena Chemical Co. (USA) and Chisso Co. (Japan). The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper there- fore must be hereby marked advertisement solely to indicate this fact. 1 Graduate Research Assistant. 2 Professor and Extension Soil Scientist. To whom reprint requests should be addressed (crosen@ soils.umn.edu). HORTSCIENCE 36(6):1057–1060. 2001. Evaluation of Polyolefin-coated Urea for Potato Production on a Sandy Soil Francis Zvomuya 1 and Carl J. Rosen 2 Department of Soil, Water, and Climate, University of Minnesota, 1991 Upper Buford Circle, St. Paul, MN 55108 Additional index words. Solanum tuberosum, polyolefin-coated urea, controlled-release fertilizer, economic analysis Abstract. Field studies were conducted on a Hubbard loamy sand (sandy, mixed, frigid Entic Hapludoll) during 1996 and 1997 at Becker, Minn., to evaluate the effect of a polyolefin-coated urea (POCU) fertilizer (Meister, Chisso Co., Japan) on yield and quality of irrigated ‘Russet Burbank’ potatoes (Solanum tuberosum L.). The POCU was a 3:1 mixture of 70-day and 50-day release formulations, respectively, based on historical soil temperatures at the site. The study compared five banded nitrogen (N) rates (110, 155, 200, 245, and 290 kg·ha –1 N) as a split application of urea applied at emergence and hilling, vs. POCU applied at planting. All plants received an additional 30 kg·ha –1 N as monoammonium phosphate band-applied at planting. Yields were higher in 1996 because of cooler temperatures and poor tuber set in 1997. Total and marketable yields averaged, respec- tively, 3.9 and 3.3 Mg·ha –1 higher with POCU than with urea. Total yield was not affected by rate of N application regardless of source, but marketable yield increased linearly with N rate. The yield of marketable tubers larger than 170 g increased linearly with N rate in both years. Gross return was 10% higher with POCU than with urea, but estimated net return showed a significant source × N rate interaction. The net return increased by $3.13 per kg of urea-N applied, but there was no significant change across POCU application rates. In the upper midwestern United States, the potato is often grown on glacial outwash soils that are generally infertile, with low water- holding capacity and rapid drainage. Nitrogen and water requirements of the crop are met through high rates of N fertilizer application and sprinkler irrigation. These factors, together with the shallow root system of potato plants and unpredictable rainfall, can lead to signifi- cant leaching of nitrate (NO 3 – ) to shallow groundwater. Current practices to minimize NO 3 – leach- ing in potato production involve split-applica- tion of soluble N fertilizers, with a small frac- tion of the total N requirement applied at or before planting. Subsequent applications are usually split at emergence and hilling, with later applications based on petiole analysis (Errebhi et al., 1998a, 1998b). However, because of the high N and water requirements of the potato and the use of highly soluble N fertilizers, control- ling NO 3 – leaching is difficult. Another approach that merits further in- vestigation is use of controlled release fertiliz- ers (CRF). Previous studies with potatoes, with polymers that allow N release to be syn- chronized better with crop N demands than when conventional CRFs are used. Release of N from polyolefin-coated urea (POCU) is pri- marily influenced by soil temperature (Gandeza et al., 1991). Since plant growth is also tem- perature-dependent, a more synchronous rela- tionship between N demand and availability may be attained during the season. Use of such fertilizers may be an efficient means to maxi- mize N use efficiency and minimize NO 3 – losses. Results from recent studies demon- strated that polyolefin-coated (POC) CRF not only improved N use efficiency of corn (Shoji et al., 1991), but also decreased NO 3 – leaching (Alva, 1992; Wang and Alva, 1996). The influence of these newer CRFs on irri- gated potato yield has not been documented. The objectives of this research were to: 1) determine the effects of POCU fertilizer on potato yield and quality; and 2) evaluate the economics of using POCU vs. soluble urea as the N source. Materials and Methods Field studies were conducted during 1996 and 1997 at the Univ. of Minnesota Sand Plain Research Farm in Becker, Minn., on a Hubbard loamy sand (sandy, mixed, frigid Entic Hapludoll). During each year prior to estab- lishment of the experiment, rye (Secale cereale L.) was grown without fertilizer N in the experimental field to minimize the potential effects of residual profile NO 3 – -N on crop response to N treatments. The experiment each year was conducted in a different field, but on soils with similar properties. Soil samples were collected from the surface 15 cm each spring for initial soil test analyses before treatment. Residual soil NO 3 – -N was deter- mined in composite samples taken to a depth of 60 cm. After drying at 35 °C and grinding to pass a 2-mm sieve, the soil samples were analyzed for pH (Thomas, 1996), organic matter content (Nelson and Sommers, 1996), extractable phosphorus (Bray P1) (Kuo, 1996), and exchangeable potassium (Knudsen et al., 1982). Nitrate-N in the samples was measured conductimetrically after extraction with 2 M KCl (Carlson et al., 1990). Selected soil chemi- cal properties in the 0–15-cm depth prior to planting averaged: pH, 6.7; organic matter, 2.1%; Bray P1, 36 mg·kg –1 ; and exchangeable K, 137 mg·kg –1 . Nitrate-N in the top 60 cm was 12 kg·ha –1 in 1996 and 20 kg·ha –1 in 1997. The test cultivar was ‘Russet Burbank’, an indeterminate cultivar maturing in ≈120– 130 d, and the one most widely grown for potato processing in the upper Midwest. Cut “A” size seed potatoes were planted by hand the third week in April each year. Each plot received standard basic fertilization recom- mended for ‘Russet Burbank’ potatoes in the area (Rosen and Eliason, 1996). One week prior to planting each year, 225 kg·ha –1 potas- sium-magnesium sulfate and 225 kg·ha –1 po- tassium chloride were broadcast and incorpo- rated. At planting, monoammonium phosphate and additional potassium fertilizer were banded 7.5 cm to the side and 5 cm below each seed however, have generally resulted in lower yields with CRF than with soluble N sources (Maynard and Lorenz, 1979; Waddell et al., 1999). Lorenz et al. (1972, 1974) reported lower yields of ‘White Rose’ potatoes with sulfur-coated urea (SCU) and ureaform than with ammonium sulfate. Cox and Addiscot (1976) reported similar results for ‘Pentland Crown’ potatoes; yields with SCU were lower than those obtained with calcium nitrate. For ‘Russet Burbank’ potatoes, Liegel and Walsh (1976) found that SCU was a better N source than urea only in cases where severe leaching occurred. In general, however, yields with urea were higher than with SCU under normal soil moisture conditions. Based on these stud- ies, Maynard and Lorenz (1979) concluded that N release rates from conventional CRFs such as SCU or ureaform are too slow to meet the N demand of the potato plant early in the growing season. In addition, late season re- lease of N from CRFs cannot substitute for retranslocation of high internal N concentra- tions from early season uptake. Under Minne- sota conditions, Rosen et al. (1992, 1993) found that ≈70% to 80% of the N uptake by ‘Russet Burbank’ potatoes occurs between 20 and 60 d after emergence. Having N available for uptake during this period is therefore criti- cal. If N is available too early in the season, it may be lost by leaching before uptake by the crop; if N is available too late it may not be used efficiently for tuber production. These studies suggest that some soluble N fertilizer may be needed along with a CRF at planting to meet early potato N needs. Recently, several new controlled-release N fertilizer products have been developed (Trenkel, 1997). The new products are coated