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Potential potato yield in any area is determined by the amount of radiant energy available, number of frost-free days, and the amount and uniformity of the water supply. All other known needs can be added. The many important factors that determine production can be divided into those over which a grower has some degree of control and those over which he has little or no control. The maximum potential production is limited by the uncontrollable factors. The ability of each grower to manage the controllable factors and the degree to which they are optimized, determine the actual level of production.
Factors not controlled by growers include: frost-free period, air temperature, soil temperature, light intensity, day length, humidity, and soil type.
Grower controlled factors are: soil moisture, crop pests, days grown, fertility, seed quality, seed piece size, plants per acre, timely operation, and soil condition.
Soil Requirements
Potatoes grow well on a wide variety of soils. In some areas where potatoes are commercially grown, the soils are acid, whereas, in others they are alkaline. Ideal soil for potato growing is deep, well-drained, and friable. The soil is a water and nutrient reservoir through which air exchange between the soil and atmosphere must readily occur. Without oxygen the roots do not efficiently absorb either water or nutrients. In areas where potatoes receive moisture entirely from rainfall, the most desirable soils have a high water-holding capacity without a tendency to become puddled when wet or cloddy when dry. In the irrigated areas, especially where sprinkler irrigation is used, soil type is less critical because water can be applied as needed in quantities sufficient to meet the needs of the plants without undue runoff, leaching of nutrients, or puddling.
Soils high in clay content require special treatment such as, proper crop rotation, cover crops, and timely tillage operations to keep them productive over long periods of time. Sandy soils, which contain little clay or little organic matter and have almost no soil structure, when properly irrigated and fertilized will produce high yields of tubers with excellent culinary and processing quality. Wind erosion can be a problem on sandy soils; however, cultural practices where cover crop residues are left on or near the surface provide good control of wind blown soil.
The cultural practices desirable in any production operation must be adapted to the soil in which potatoes are being grown. The potato plant requires an adequate supply of moisture, nutrients, and air throughout the growing period. The amount of moisture and nutrients which must be provided is determined by the fertility status of the soil and the rooting depth of the plants. Any interruptions in growth, regardless of the cause, can result in poor quality tubers. It appears to be especially easy to cause malformation of tubers during the early stages of growth.
Temperature and Moisture
All processes of living plants are governed by enzymes and all enzymes function faster at high temperatures, until eventually at some maximum temperature the enzymes are inactivated. The potato has long been classified as a short day, cool season crop, but does very well at high temperatures when water is supplied in uniform quantities sufficient to meet evapotranspiration demands. The highest yields are currently being produced in areas where the daytime temperature is often over 100F (38C) during the hottest part of the growing season, such as in Washington's Columbia Basin, eastern Oregon, California and Nevada. The 'Russet Burbank', the predominant variety in these areas, is not unique in its adaptation to these environmental conditions, as evidenced by the fact that new cultivars, as well as many older varieties, produce their top yields in these areas. The critical factor is a supply of water at soil moisture tensions low enough to keep the stomata open during the heat of the day.
Yield potential or photosynthate produced is the result of rate/hour times the number of hours per day, times the number of days, times the number of functioning plants per production unit. Yield is the difference between the photosynthate produced as expressed in the formula above minus the amount used up by the living plant (respiration). Cool night temperatures are an asset because they reduce respiration. The vine temperatures change more rapidly and to a Heater degree than tuber temperatures because of the latent heat capacity of the soil.
Contrary to expectations in a hot environment, high specific gravity (high dry matter) tubers and high yields can be produced at the same time. As a result the tubers from high yielding plants are excellent for most forms of processing.
Specific gravity of the tubers can be influenced by the relative rates of four physiological processes, i.e., respiration, transpiration, photosynthesis, and water absorption. Respiration pertains to the "rate of living," the higher the temperature, the faster the utilization of carbohydrates produced by photosynthesis. The amount of carbohydrates produced depends upon the rate of photosynthesis and the length of time that it continues. Consequently, there is a direct relationship between length of the growing season and production of high yields of high dry matter content (high specific gravity) potatoes. There is also a direct relationship between high yield and amount of nutrients in a crop. Fertilization may have little direct effect but a large indirect effect on the specific gravity because of the effect of size of the plant on the relative rates of water lost by transpiration and water absorption by the roots.
The specific gravity of potatoes attached to living plants in the field can change rapidly because of water movement into and out of the tubers. When transpiration (water loss through the leaves) exceeds the rate of water absorption by the roots, the vines draw water from the tuber, causing the tuber to decrease in weight, shrink in size, and at that point in time have an increased specific gravity. This process will continue until the leaf cells lose their turgor pressure, the leaves wilt, the stomata close, and photosynthesis ceases.
If the rate of water absorption by the roots exceeds water loss by transpiration, the excess water is pumped into the tubers and they expand, increase in weight, become more brittle, and the specific gravity decreases. The increase in weight of potatoes and decrease in specific gravity, because of absorbed water, are at times sizeable.
Rotations
Potatoes should be grown in a crop rotation that will enhance the soil fertility, maintain a loose friable soil condition, reduce weeds, increase organic matter level, conserve soil moisture, and reduce crop loss from insect damage and plant diseases. No specific crop rotation can be given, as combinations vary from place to place. Potato growers must develop the rotation best suited to local environmental conditions, crop alternatives, and market prospects.
In general, long rotations (potato crops planted three or more years apart on the same land) are particularly good for reducing potato losses caused by soil-borne organisms causing such diseases as scab, verticillium wilt, and fusarium wilt. Wheat wireworms (Agriotes mancus) increase in abundance in fields planted for several years to hay and grass crops and may be a serious threat to a potato crop immediately following the plowing under of a heavy grass sod, alfalfa, or clover. On the other hand, the Pacific Coast wireworm (Limonius canes) increases in fields under intense cultivation, but the population decreases when fields are planted to hay crops.
Soil Preparation and Improvement
A well prepared seedbed is desirable, but over preparation wastes energy and destroys soil structure.
Plowing the soil has been a conventional practice in preparing the land for potato planting. Depending on soil type, amount of precipitation, and other factors, fields should be plowed just prior to planting or in some areas fall plowed. In areas where spring plowing is practiced, it is often customary to have a soil leveling implement such as a clod buster attached to the plow. In such areas, potatoes are often planted without further tillage.
Fall plowing aids decay of the plant debris and leaves the soil exposed
to the natural weathering processes during the winter months. How deep
to plow depends largely on the depth of the surface soil and the character
of the subsoil. Deep and fertile surface soil should be plowed to a depth
of 8 to 12 inches (20 to 31 cm), but shallow soil should be plowed no deeper
than an inch below the plow sole. A 14-inch (36 cm) wide or larger plow
is required to do a good job of plowing for potatoes. If much trash is
to be turned under, plows with 16- and 18-inch (41 to 46 cm) bottoms and
with large clearance are needed (Fig. 8). It is preferable to plow sloping
lands in very late fall or in the spring and leave them rough to help prevent
erosion. Strip and/or contour plowing of sloping lands will also help control
erosion
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FIGURE 8. Plowing with roll-over moldboard plows is a popular practice which eliminates back-furrows and dead-furrows. Often a cloud buster, spring tooth harrow or other smoothing implement is attached to the plows to prepare soil for planting. |
Fall-plowed land should be tilled the following spring as soon as the land is dry enough to work. Implement traffic over the field should be limited to a minimum to help prevent soil compaction but adequate for weed control and obtaining a loose friable seedbed. On some fields it may be necessary to final till the land with the spiked-tooth harrow to break clods and level the ground. The spiked-tooth harrow is also effective in eradicating small weeds.
In general, heavy soil benefits most from fall plowing because the action of frost, snow, and winter rain on exposed soil makes it more mellow. Fields from which soil is likely to be washed away by winter rains and rapidly melting snow, however, should be plowed in the spring.
Prevention of soil erosion, either by wind or water, cannot be over-emphasized. Any waste of topsoil ultimately means a serious loss of capital to the potato grower.
In some parts of the West, alfalfa is turned under just before potato-planting and neither the cutoff crowns nor the surviving alfalfa plants give trouble. Thorough plowing of alfalfa soil is essential. The plow should be equipped with alfalfa shares, so that no plants will be left uncut.
Many growers find it beneficial to loosen heavy soil to a depth of 16 to 20 inches (41 to 51 cm) with a land chisel. Precision tillage, shanking 16 to 24 inches (41 to 61 cm) directly below where the seedpieces are to be planted has resulted in an increase in soil aeration and improved drainage of some soils.
Soil should not be plowed when too wet. If plowed when too wet, the land is likely to remain in poor physical condition throughout the growing season, with slow water infiltration, poor soil aeration, poor drainage, and clods at harvest time. Where heavy rainfall may occur, potato land can be ridged to facilitate better runoff of excess water and to provide better drainage.
Basin listing is sometimes a good way to prepare an area for dryland potato growing. The basin lister makes a furrow with small earthen dams at regular intervals. The dams keep rainwater from running off the surface and thus increase the quantity of moisture that penetrates the soil. The same method has been adapted to uneven sprinkler-irrigated land in the West. In some cases, use of damming or pitting has been helpful on soils with low surface infiltration rates. If fields are to be furrow irrigated, proper leveling of the land is essential before final preparation of the seedbed.
Building Up Soil Organic Matter Content
One soil problem in practically all potato regions, except those with muck and peat soils, is that of supplying and maintaining a desirable level of organic matter in the soil. In warm regions with long growing seasons, the decomposition of organic matter in the soil is rapid and almost continuous. In colder regions, decomposition takes place in the summer and does not occur appreciably during the winter. The advantages of organic matter in the soil are many. An ample supply of decaying organic matter helps to keep the soil loose and mellow and thus reduces soil compaction. Potato tubers develop and maintain normal shape better in loose, well-aerated soils. Organic matter facilitates plowing and cultivating; it enables roots of potato plants to penetrate the soil more readily, and it improves water retention; it provides food energy for the growth of desirable soil micro-organisms and supplies plant nutrients.
Potatoes, as a crop, provide little organic matter to be returned to the soil. General methods of getting organic matter into the soil include a crop rotation in which legumes or other crops are grown and plowed under as a green-manure crop. Such crops include crimson or red clover, vetch or a combination of peas and vetch, soybeans, cowpeas, rye, oats, barley, wheat, millet, Sudan grass, field corn, or other suitable crop plants. The application of barnyard manure and the plowing under of the organic residues of all crops produced will contribute to the organic matter content of the soil.
Barnyard manures help improve the physical condition of the soil, provide plant nutrients, and increase bacterial activity. Fresh manure should not be applied just before the potato seed is planted. Manures may produce a favorable environment for common scab development, particularly if the soil is near pH 7.0. If large quantities of manure are used, there may be enough salt present to injure young plants.
Liming
Some soils are too acid for successful potato production. The pH of these soils can be increased by the application of lime. Several nutritional problems, such as phosphorus fixation, certain micronutrient deficiencies, and aluminum toxicity are associated with soils that are too acid. In some potato areas, soil acidity is adjusted to about pH 5.0 to 5.4 to control common scab.
It is a good policy to periodically have soil tests to check the soil pH to avoid pH-related problems.