Potato

PRODUCTION PRACTICES

Potatoes tolerate a variety of soils, but the best soil is a deep, well-drained sandy or silt loam. Even though potato roots grow down two feet, fields without good water-holding capacity must be irrigated. In the eastern states, where irrigating potatoes is less common than in the western states, peat soils are often used for potato production because of their high water-holding capacity. A pH of 5.5 to 6.0 is optimum. If necessary, however, pH can be lowered to 4.8 to 5.0 to reduce the incidence of scab, a serious disease in potatoes. Heavy clay soils lead to misshaped tubers, and, if they are also poorly drained, lenticels enlarge.

Soils that crust are also undesirable because in a cool spring when emergence is slow, heavy spring rains may seal the surface, trapping sprouts below ground. When this happens, growers must break the soil crust until the shoots emerge.

Harvesting one ton of potatoes removes 8 to 15 pounds N, 1.6 to 7 pounds P2O5, and 10 to 26 pounds K2O from the soil.

Potato petiole sap testing for nitrate and potassium

Growth stage	Sap Nitrate-N(ppm)	Sap Potassium(ppm)
Plants 8" tall	1400-1600	4500-5000
First open flowers	1200-1400	4500-5000
50% flowering	1200-1400	4000-4500
100% flowering	900-1200	3000-3800
Tops falling down	800-1200	2500-3000
Tops down	200-800	<2000

Potato nutrient recommendations based on soil tests.

The potato is a heavy feeder in terms of phosphorus and potassium, but nitrogen must be carefully managed to provide adequate but not excessive amounts. As little as 25 pounds of N at planting is often recommended as the plant does not use much nitrogen in its first 4 to 5 weeks of growth. A later sidedressing or broadcasting of 20 to 40 pounds N is sometimes applied after shoots emerge but before they are 4 to 6 inches high. Salt accumulation decreases growth. It is estimated that 40 percent of the eastern North Carolina potato-growing soils have high soluble salts. High salts are estimated to decrease statewide yields by 8 percent. Fertilizer should be applied only after soil testing and only in low-salf formulations. Fertilizer is usually banded at planting.

Management of water and nitrogen is particularly challenging in potatoes. Under dry conditions, plants yellow and grow slowly. Although these plants may appear N-deficient, adding nitrogen to drought-stressed potatoes worsens the problem by causing salt damage and allowing soilborne pathogens such as Fusarium to enter damaged seed pieces. Even where salts are not a problem, high levels of nitrogen and water can lead to hollow heart in large tubers because of rapid tuber expansion. Small, but adequate, nitrogen applications spread out over a long period will reduce the potential for hollow heart and brown center, a related physiological disorder. Excess nitrogen, especially late in the growing season, decreases dry matter content, resulting in poor storage quality and poor texture when cooked. Data shown below presents petiole sap nitrate and potassium levels at different growth stages and plant nutrient recommendations based on soil testing.

Planting

Good, preferably certified, seed is required for good yield. No more than 2 percent of certified seed can have any type of defect and no more than 1 percent can be infected with late blight. With either certified or non-certified seed, any diseased tubers should be graded out before cutting and immediately buried or burned. Most of the seed planted in the southern states is shipped from Maine, Wisconsin and North Dakota. Because these seed tubers were stored at 38 to 40 degrees F and have not yet emerged fully from their natural dormancy, by the time of spring planting in the South, they need pre-sprouting or conditioning before planting. After 7 to 10 days at 50 to 60 degrees F, a slight amount of sprout growth should appear on the surface.

Even if presprouting is not possible, tubers should be at least air-dried prior to storage and stored under conditions that will prevent condensation. Moving warmed air through the storage area will help. Warming seed before planting will also help heal cuts inflicted in storage and transport.

Small whole tubers are sometimes used to establish the field planting, but more commonly, tubers are cut into large chunky pieces of 1 to 1.5 ounces. Because of the proximal dominance of 'eyes,' larger pieces will not necessarily have more sprouts than smaller pieces. After cutting, the pieces should either be immediately planted, stored dry, or 'healed' by holding at 65 degrees F and high humidity for 1 to 2 days. Healing will reduce rotting. In commercial production, seed pieces are also treated with dust-formulated fungicides or cedar sawdust immediately after cutting.

Seed pieces are planted 4 inches deep in high seed beds or are hilled up as they grow with soil thrown on the row. The soil temperature at planting should be at least 45 degrees F but not over 70 degrees F. Rows are typically 30 to 36 inches apart, with seed pieces placed 8 to 12 inches apart in the row. Spacing is determined by the desired size of the tubers to be harvested and the cultivar used. Where cultivars tend to have low 'set' or a tendency to produce oversize tubers or where tubers are grown for seed production, a 7-to-9-inch spacing is used to reduce tuber size.

An 11-to-14-inch spacing is used for cultivars with heavy set which might produce undersize tubers at closer spacings. Wide spacing can increase growth cracking under irregular moisture conditions. Whatever spacing is used, it should be uniform to ensure uniform size of tubers.

Physiological age of the seed pieces will also affect the size and number of tubers. 'Old' potato seed is undesirable because it produces more stems and a larger number of small potatoes than 'young' seed. Seed ages from stresses during the production season, immaturity at harvest, or improper storage and handling. If seed is known to be 'old', spacing should be increased to compensate for the larger tuber numbers produced.

Cultivation

Potatoes should be hilled or ridged at least once after planting to increase the depth of the seed piece to 4 to 6 inches. Deep planting is important because tubers only form between the seed piece and the soil surface. To protect developing tubers from heat and light damage, hills should be flat and broad rather than narrow and peaked. In a clay soil, proper hilling is particularly important because tubers tend to rise to the surface. If herbicides are applied during hilling, care must be taken not to cover the plants with treated soil. The potato has a shallow root system and so should receive a minimum of cultivation to avoid root pruning.

Mulching and Cover Crops

In experiments on the eastern shore of Virginia, a 3-to-5-inch layer of straw added just before the potatoes emerged led to higher yields. Soil temperatures were cooler, soil moisture was higher, and populations of Colorado potato beetle were lower in mulched plots. Although beneficial, the researchers in this study concluded mulching would be most cost effective if the mulch came from cover crops on the same or an adjacent field. Cover crops can also reduce diseases such as verticillium and rhizoctonia, according to studies in Idaho. Consult Chapter 5 for more information on these studies.

Irrigation

Water needs increase linearly from shoot emergence until about two weeks after the period of maximum row coverage. Water needs then hold constant until the vines die back, reducing water requirements. Too much water or too little water will affect tuber number, size, and quality as described in detail in the tuber initiation and growth section.

Integrated Pest Management

Life cycles and monitoring and control practices for some of the common insect pests in the southern states are described in Integrated Pest Management. Some of the important diseases of vegetables in the South and their control are described in Disease Management. The general principles of insect and disease IPM programs are also described in their respective chapters. Cultivar resistances to the more common insects and diseases in the South are listed in Table 15.5. as it may be useful to select cultivars with multiple resistances. The list is not meant to be inclusive, however, as new resistant cultivars are constantly being released.

Weeds

The first 4 to 6 weeks after the shoots emerge are the most critical for weed control. Potato yields were reduced 19 percent when only one redroot pigweed per meter of row competed with the crop for the entire season. Weeds emerging after the vines covered the rows did not compete directly with the potato plant.

Experiments in New York showed grain rye residues reduced weed biomass in no-till potato fields. It was not known if weed suppression resulted from allelopathic effects of the rye residues or from other effects. In follow-up experiments, rye residues in field plots were found insufficient to suppress weeds through allelopathic effects. In fact, greenhouse experiments showed twice the biomass of rye produced in the field was necessary to significantly reduce weed biomass. The weed suppression previously noted in the field, therefore must be due to other factors such as reduced soil disturbance, allelopathy or creation of microclimatic conditions that prevent weed germination.

Potato cultivars with insect and disease resistance