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16 POTATO GROWER | JANUARY 2018 TOP 5 (PLUS A FEW EXTRA) TIPS FOR APPLYING NUTRIENTS TO A POTATO CROP By Carl J. Rosen & Peter M. Bierman Optimum potato growth and profitable production depend on many management factors, one of which is ensuring a sufficient supply of nutrients. There are 14 soil- derived elements or nutrients considered to be essential for growth of plants. When the supply of nutrients from the soil is not adequate to meet the demands for growth, fertilizer application becomes necessary. Potatoes have a shallow root system and a relatively high demand for many nutrients. Here, we'll take a look at the six most significant of those nutrients, as well as some—known as micronutrients—needed in much smaller quantities, and the best ways to ensure your potatoes get enough of them during the growing season. NITROGEN The rate of nitrogen to apply to irrigated potatoes primarily depends on the cultivar and date of harvest, expected yield goal, amount of soil organic matter, and the previous crop. Different potato varieties and differences in harvest date will have a pronounced effect on yields and yield goals. Because of earlier harvest and lower yield, early-maturing varieties like Red Norland generally require less nitrogen than later-maturing varieties such as Russet Burbank. In addition to environmental concerns due to excessive nitrogen applications, high rates of nitrogen can detrimentally affect potato yields and tuber quality. Too high a rate will delay tuber initiation and maturity, leading to excessive vine growth at the expense of tuber growth. Excess nitrogen can also increase brown center and the incidence of knobby, misshapen and hollow tubers. High nitrogen rates will induce vigorous foliage, which can lead to an increase in vine rot diseases. On the other hand, lack of nitrogen can increase early blight infestations. Controlling early blight with proper use of fungicides will, in some years, reduce the nitrogen requirement. In other years, use of fungicides increases yield potential and hence the nitrogen requirement is the same or higher when early blight is controlled. PHOSPHORUS Phosphorus is important in enhancing early crop growth and promoting tuber maturity. Phosphorus plays an important role in regulating tuber set, with higher tuber numbers when phosphorus nutrition is high. Banded applications at planting are recommended, because phosphorus movement in the soil is limited. Placing phosphorus close to the seed piece is especially important early in the season when soil temperatures are cool and root systems are undeveloped. In-season application has generally not been found to be beneficial on acidic, sandy soils. Soil pH affects phosphorus availability, which is reduced under both acid and alkaline conditions. Availability is highest at slightly acid to near-neutral conditions, so the practice of growing potatoes at low pH to reduce scab can limit phosphorus uptake if it drops too low. The rate of nitrogen to apply to irrigated potatoes primarily depends on the cultivar and date of harvest, expected yield goal, amount of soil organic matter, and the previous crop. Different potato varieties and differences in harvest date will have a pronounced effect on yields and yield goals. Because of infestations. Controlling early blight with proper use of fungicides will, in some years, reduce the nitrogen requirement. In other years, use of fungicides increases yield potential and hence the nitrogen requirement is the same or higher when early blight is controlled. 16 POTATO GROWER | JANUARY 2018 PHOSPHORUS Phosphorus is important in enhancing early crop growth and promoting tuber maturity. Phosphorus plays an important role in regulating tuber set, with higher tuber numbers when phosphorus nutrition is high. Banded applications at planting are recommended, because phosphorus movement in the soil is beneficial on acidic, sandy soils. Soil pH affects phosphorus availability, which is reduced under both acid and alkaline conditions. Availability is highest at slightly acid to near-neutral conditions, so the practice of growing potatoes at low pH to reduce scab can limit phosphorus uptake if it drops too low.