Potatoes: An Alternative to Consider

by Linzy Carlson
Whether they are fried, baked, mashed, or hashed, potatoes have provided a nutritious, stable food source around the globe for centuries. The potato now commonly grown throughout the world had its origin in the Andes of Bolivia and Peru and arrived in Europe and North America during the latter half of the 16th century. Today, potatoes are grown on 1.3 million acres in the U.S. with over 45 billion pounds produced annually. The national value from production of potatoes is approximately $2.6 billion per year, or about $2000 per acre. The average potato grower invests $1,100/ac in the crop.

Fifty-five percent of U.S. potatoes are produced in the Pacific Northwest states of Idaho, Washington, and Oregon. Other major potato producing states include Maine, Michigan, Minnesota, Wisconsin, New York, Pennsylvania, North Dakota and Colorado.

Life Cycle
Potatoes have four general phases in their life cycle. During the vegetative stage, the eyes break dormancy and produce sprouts, and leaves emerge soon after. Stored soil moisture from preplant irrigation or spring rains is usually enough to carry the crop through this phase, but soil moisture monitoring should begin as soon as the crop emerges. The majority of the root mass lies in the top 18-24" of the profile throughout the plant's life cycle, so soil moisture monitoring should be concentrated in that area. For disease control, irrigation should be avoided between planting and emergence.

Tuber initiation begins with the formation of 15-20 tubers. If the plant does not have enough water during this phase only a few tubers will form, decreasing the overall yield. During tuber bulking, the potatoes increase in size and weight. Between 5 and 10 of the initial tubers actually grow. The rest are either used for nutrition by the plant or absorbed by other potatoes. Moisture stress during this phase causes small potatoes. Stress followed by adequate moisture leads to cracked, misshapen potatoes. During maturation, the canopy begins to die, water use decreases, and tuber growth slows. When the potatoes are nearly mature, producers typically spray the canopy to kill the plant in preparation for harvest.

Water Requirements
Growing potatoes can be tricky. Potatoes are quite sensitive to moisture stress over much of the growing season, so they need relatively high soil moisture levels (60-80% of the available water capacity) to achieve high yields and quality. The average seasonal water use for potatoes is nearly 18", which must be provided by stored soil moisture, rain, and irrigation, usually from center pivots or sprinklers. Water use rates begin at about 0.02" per day at emergence, increase to over 0.25" per day when the potato canopy completely shades the ground, and decrease as the potatoes achieve full tuberization.

The frequency and amount of irrigation depend on the water holding capacity of the soil, the crop growth stage, and the prevailing weather conditions. Potatoes require well-drained, fertile, sandy loam to silt-loam soils. These soils have relatively low water holding capacities, so this factor, combined with the crop's affinity for water and a growing emphasis on ground water protection, makes careful irrigation water management a necessity. If conditions are dry, potato fields need to be irrigated prior to planting and as often as every 2-3 days during peak water use.

Nutrient Requirements
A potato crop makes a large demand on the soil nutrients. An average acre of 300 cwt/ac potatoes will utilize 200 pounds of nitrogen, 60 pounds of phosphorus, and 300 pounds of potassium. One third to one half of these nutrients are found in the vines and returned to the soil. The remainder are removed with the harvested tubers and must be replaced. To preserve potato quality during storage and maximize utilization of nitrogen and other nutrients, fertilizer must be applied in split applications, with one third to one half banded or broadcast prior to planting and the remainder applied through the irritation system later in the season.

Potatoes are subject to a myriad of diseases, primarily fungal and viral pathogens that damage either the leaves or the tubers. No potatoes are completely free of these pathogens, but starting with high quality, healthy seed dramatically increases the chances of having a good crop. Potatoes should not be grown more often than every three years on the same field and should be rotated with other crops such as oats, barley, corn, and beans. The Colorado potato beetle presents a constant problem for potato growers, but in recent years, advances in the field of genetics have resulted in potato plants that are somewhat resistant to these pests.

Rough handling at harvest time can bruise and scratch potatoes, leaving them vulnerable to pathogenic attack and rotting. Because the potato tissue becomes slightly more elastic at warmer temperatures, potatoes should be harvested when the pulp temperature is greater than 50 degrees F, and they should never be dropped more than 6-12". Most commercial potatoes are sprayed with post harvest protectants to help prevent spoilage.

The battle against diseases and pests is an ongoing one for the potato industry, but between conventional chemical treatments and newer biological and genetic control measures, potato farmers have a great many options, ensuring steady potato production. And it's a good thing, because the average American eats nearly 130 pounds of potatoes a year!

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