This article appears in the January 2018 issue of Potato Grower.
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.
Potassium
Potatoes take up significant quantities of potassium, and this nutrient plays important roles in tuber yield, size and quality. High potassium is necessary to prevent blackspot bruising and shattering and to attain good storage quality. However, specific gravity may be reduced if potassium fertilization is too high because it increases tuber water absorption. In-season applications have a greater effect on specific gravity than pre-plant or planting applications, and potassium chloride (0-0-60) can have more of an effect than potassium sulfate (0-0-50) at equivalent potassium rates. Applying significant amounts of potassium during the tuber bulking phase can reduce yields. Potassium is a relatively immobile nutrient in medium- and fine-textured soils but it does leach in sandy soils, particularly when they are acidic and low in organic matter.
Calcium
Calcium deficiency is rare in many agricultural soils, because they have high native calcium levels or are periodically limed to maintain soil pH. Sandy soils, however, do not maintain high calcium reserves, and the practice of growing potatoes at low pH to reduce scab means they are rarely limed. Under these conditions, soil calcium can fall to levels that reduce tuber quality and tuber yield.
Calcium plays an important role in maintaining tuber quality in storage and reducing internal tuber disorders such as brown spot and hollow heart. Low calcium in tubers is often due to inadequate transport of calcium to the tuber, caused by water or temperature stress. This may be a localized calcim deficiency with adequate calcium levels occurring in leaves and the soil testing high in calcium.
Magnesium
Similar to calcium, inadequate magnesium can occur on acid sandy soils that are not periodically limed. High rates of potassium fertilizer, which are often required for potatoes, can also induce magnesium deficiencies, since the two compete for uptake. Magnesium sulfate or potassium-magnesium sulfate are the most common magnesium sources available. They can be broadcast and incorporated prior to planting or banded in the row at planting. Another alternative is to apply low rates of lime during a non-potato year in the rotation. An application of 1,000 pounds of dolomite per acre will meet both the magnesium and calcium recommendations for low-testing soils.
Sulfur
In many soils, sulfur requirements are met from soil organic matter breakdown. Rainwater and irrigation water contain some sulfate and can also provide a significant proportion of the sulfur needed for growth. Sulfate readily leaches through sandy soils, so yield reductions from sulfur deficiency are most common on sandy, low organic matter soils.
Ammonium sulfate, potassium sulfate, magnesium sulfate and calcium sulfate are common sources used to supply sulfur. They can be broadcast and incorporated prior to planting or banded in the row at planting. When ammonium sulfate is used, be sure to account for the nitrogen it contains in meeting the crop’s nitrogen requirement. Elemental sulfur is not an immediately plant-available form and must be oxidized by soil bacteria to sulfate before it can be used by plants. The oxidation to sulfate has an acidifying effect on the soil, but the effect is small at the rates required to meet recommendations.
Micronutrients
Most soils contain sufficient amounts of zinc, boron, copper, manganese, iron, chlorine, molybdenum and nickel to meet plant needs; however, in some areas, micronutrient shortages occur and may limit yields. Calibrated soil tests for mineral soils are only available for zinc and boron. Soil tests for copper and manganese are only reliable for organic soils. Tissue analysis can be used to monitor micronutrient status.
A five-year study on irrigated sandy soil in Minnesota found increases in potato yields with boron and zinc applications, but not with manganese or copper applications. In acid soils, iron, manganese and copper should be available in adequate amounts to meet crop needs. Pesticide sprays often contain enough copper and zinc to meet plant demands for these nutrients. In extremely acid soils (pH lower than 4.8), manganese toxicity may be a problem. Very few potato responses to molybdenum and chlorine have not been reliably studied and reported. Little research has been done on nickel, but required amounts are very low and soil deficiency is probably very uncommon.
This article is excerpted from Carl Rosen and Peter Bierman’s article, “Potato Fertilization on Irrigated Soils,” the full text of which can be found at www.extension.umn.edu.