This section is from the book "The Potato: A Compilation of Information from Every Available Source", by Eugene H. Grubb, W. S. Guilford. Also available from Amazon: The Potato: A Compilation Of Information From Every Available Source.
R. W. Thatcher, Director of the Agricultural Experiment Station of the State College of Washington, at Pullman, Wash., says that the industry of growing potatoes is a very extensive one in that state, and potatoes are grown commercially at every elevation from below sea level to nearly the snow line; and with every variation of rainfall from so little that irrigation is necessary up to 120 inches annually; and on almost every type of soil known to agriculture.
The peculiar climatic and soil conditions of Washington are especially favorable for the production of potatoes," writes A. G. Craig, Assistant Horticulturist of the same institution in 'Bulletin 11.'
In many large sections of the state the atmosphere is so dry during the summer that conditions are very unfavorable for the growth of fungous diseases on the foliage. We do not have in this state the Colorado beetle (potato bug), which is so destructive a pest in other potato-growing districts. There is little danger of overstocking the market for potatoes here. The Eastern demand for Washington grown potatoes is good and has rarely allowed the price to fall below $10 per ton in car lots in the past. In addition to this there is a rapidly increasing market for our potatoes in Alaska. There is no crop now grown in Washington which shows greater variation in yield per acre than the potato. This is largely because of an erroneous idea that the potatoes as a crop do not need much attention. Many farmers give time and care to the potato crop only when there is nothing else to be done, and as a result the potato is neglected. This crop responds to good treatment to as great a degree as any other, and the grower who exercises proper care with his potatoes is always repaid in the yield and quality of his crop.
There are thousands of acres of land now devoted to summer fallow which might produce good crops of potatoes with very little additional expense, and yet leave the soil in better condition for wheat than it is under the present methods of summer fallowing. The average cost of producing potatoes in eastern Washington is a little less than five dollars per ton. The plowing and harrowing which would have to be done on the summer fallow land if potatoes were not grown is included in the cost. Therefore, the potatoes grown in the place of summer fallow can be sold for a very low price and still leave a good balance. If the market remains as high as it has been for many years a net profit of $15 to $20 an acre from what would otherwise be idle land can easily be secured.
A deep, friable, mellow loam, rich in humus, is the ideal soil for potatoes. Heavier soils may give good results if well manured or by plowing under a clover or alfalfa sod. If the soil has any tendency to pack, the tubers are restricted in their growth and are often misshapen. Supplying humus to such soils not only remedies this difficulty by making them more friable, but increases their water-holding capacity, thus insuring larger yields per acre. The soil should always be put in good physical condition, as potatoes respond very profitably to good soil conditions. Light soils can be worked earlier in the spring than clay soils and also favor early maturity of the crop. They are, therefore, better for early potatoes. Potatoes should not be grown repeatedly on the same soil, as most of the potato diseases live over in the soil.
Potato land should be plowed in the fall and allowed to lie rough during the winter. This favors the catching of winter moisture and allows the subsurface soil to settle, and the surface can be worked earlier in the spring. If fall plowing is impossible, the land should be disked in the fall so that the surface may be rough and open through the winter. Deep plowing usually gives better results than shallow. The plowed land should be well harrowed early in the spring and if not immediately planted it should be frequently harrowed in order to conserve moisture and kill the weeds which start after the first harrowing. Spring plowed land should be harrowed immediately after the plow, to prevent loss of moisture. In the drier sections, some form of subsurface packer should follow the plow, and this should be immediately followed by the harrow to work up a surface mulch. If the soil plows up cloddy, a plank clod masher may profitably be used.
The time of planting should be governed largely by the climate and the purposes for which the potatoes are grown. The potato plant needs ample moisture when the tubers are "setting," hence the grower should endeavor to have the plants reach that stage of development at the time when the moisture supply is likely to be favorable. For early new potatoes, the seed should be planted as early in the spring as the soil will permit, on light, warm soil. For late potatoes they may be planted as late as the middle of June, provided the moisture supply is ample and continuous, but where summer rains cannot be depended upon, the earlier the potatoes are planted the better, if danger from frost is guarded against.
Enormous yields of potatoes have been secured under irrigation, but their cultivation is attended with some difficulty. No other crop is so much dependent upon the skilful use of artificial water. The quality of irrigated potatoes may, or may not, be as good as that of those grown without irrigation. Good varieties, if well grown, will be good in quality either with or without irrigation.
Winter irrigation may be practised very successfully in potato growing. The fields should be flooded before plowing, and allowed to dry to a tillable condition. This insures perfect condition of the soil for working and for the early growth of the potato plants. The ordinary methods of cultivation may then be followed, without further addition of water, until about the time the plants blossom. At this stage of development the tubers are set, and it is then that an abundance of water is needed to give them good growth. After the water is once applied to the soil, it should not be allowed to become dry again until time for the crop to mature. If the soil is allowed to become dry at any time after the first application of water and a subsequent irrigation is given, the tubers are sure to make a second growth and become knobby. Water should not be applied too late in the season, or the potatoes will not ripen properly. In all applications of irrigation water, care must be taken to avoid bringing it in direct contact with the growing tubers, as under such conditions the tendency for the potatoes to become scabby is increased.
If winter irrigation is not practised, the first water should be applied immediately after the seed is planted. Irrigated potatoes should be hilled, and the water applied between the rows. In ordinary soil, water applied in the middle of rows three feet apart satisfies the requirements of the growing potatoes. The cultivator should follow each application of water.
Sub-irrigated' lands, when not too wet or too strong with alkali, are most satisfactory for raising potatoes. There are some localities where soils receive just enough seepage from irrigation ditches, or other water supplies, to keep in moist, friable condition throughout the season. These, with frequent shallow cultivation, produce the finest, smoothest tubers, with the least trouble and expense. To produce uniform moisture conditions in the soil is the secret of successful irrigation, and this is the absolutely essential condition for the most profitable potato growing under irrigation."