CHAPTER IV ROTATION In some cases potatoes are grown continuously for several years on the same soil, but a rotation of crops is preferable for many reasons-among others, to lessen the dangers of attacks of diseases and insects, and to bring the soil into a suitable physical condition for growing this crop. Some rotations suggested by Wheeler, of the Rhode Island Experiment Station,' are as follows: three-year rotation-potatoes, winter rye, common red clover; four-year rotation-corn on clover sod, potatoes, winter rye, clover. This can be made into a five-year rotation by seeding timothy and redtop with the clover, and leaving the mixture down two years, thus reducing the labor bill to some extent. Trials of these and other rotations were made on land so poor that corn attained a hight of but 4 or 5 inches, while the first crops of salable potatoes were but 65 bushels per acre. During later years, with management similar to that given the first year, and the application of a similar amount of fertilizers, the yields ran up to 350 bushels of salable potatoes per acre. A common Maine rotation is a four-year course of potatoes, oats, clover and grass, the latter for two years-it being noted that clover thrives on good potato land. In deciding upon the rotation it is important 1 R. I. Bul. 74, 75, 76. to note the influence of each crop upon the moisture content of the soil (see p. 50); thus, rye removes less moisture from the soil than wheat. Oats draw heavily upon the moisture content. The potato crop is not usually considered to be a heavy water consumer. It leaves the soil in a relatively moist condition; hence, the wisdom of the Maine four-year course, in which oats succeed potatoes. This course requires but one deep plowing in four years, that for the potatoes, and in this it is economical. Peas use a relatively small amount of water, and would leave the soil in good shape for potatoes. In Wisconsin,' while potatoes grown in rotation yielded 342.8 bushels per acre, a crop grown on an old alfalfa sod yielded but 277.7 bushels per acre, although the rainfall was considered adequate to produce a full crop. In some cases clover tends to leave the soil drier than some other crops, and its use as the preceding crop for potatoes may be detrimental. In most cases, however, a leguminous crop is the best to precede potatoes. In Florida' cow-peas preceding potatoes increased the yield 40 per cent. The Ohio Station' found that in the three-course rotation-potatoes, wheat, clover-whenever good crops of clover were grown the economy of using nitrogenous fertilizers for the potatoes was questionable, thus showing that a good rotation is equivalent to manuring. Plowing under a leguminous crop is held to be good practice on farms where an adequate supply of manure is not forthcoming and little stock is kept; thus, at the 2 Fla. Report, 1900-1901, p. 28. 1 Wis. Report, 1902, p. 188. 3 Ohio Bul. 125, p. 132. Maryland Station,' plowing under a crop of crimson clover increased the yield 34.4 bushels per acre, or 50 per cent., and the average gain for two years was 27 bushels per acre, or 45 per cent.; the Storrs' (Connecticut) Station reports that clover sown in corn at the last cultivation had a high value when used to plow under as manure for potatoes, even though it only attained a hight of three or four inches; in Germany' the sweet clover (Melilotus alba) is found to be a valuable green manure; while in another German experiment,* where clover was seeded in rye which was grown for grain, the clover being plowed under the following spring, it was noted that the yield of rye was dimished, but the yield of the succeeding crop of potatoes was increased. The yields of rye and potatoes As green manuring for poor sandy land on Long Island, N. Y., Professor Stone, of Cornell University, suggested sowing a bushel of cow-peas and ten pounds of crimson clover per acre, in July, with some fertil 1 Md. Bul. 38, p. 58. 3 E. S. R., V., p. 701. 2 Conn. (Storrs) Report, 1900, p. 65. E. S. R., VI., p. 292. izers. The cow-peas were killed by the first frost, but the clover persisted; the crowding, however, was such that the plants of neither crop got too large before being plowed under the following spring. For farther north a combination of half a bushel of buckwheat and a peck to half a bushel of rye per acre, sown together, has given good results. Rape sown at the rate of four to five pounds per acre is useful. Other crops will suggest themselves. In parts of New York, especially on heavy loams, buckwheat is esteemed as the preceding crop for potatoes. It crowds out weeds. and leaves the soil in excellent physical condition. CHAPTER V MANURING AND FERTILIZING LAND is manured and fertilized either to increase or to maintain its crop-producing power. Whether this is secured by the direct effect of the chemical ingredients in the manure or fertilizers, or by their influence upon the physical properties of the soil, or both, is an unsettled scientific problem, but all agree that under certain conditions the addition of manures, fertilizers, and water to the soil is profitable. Whether it will be profitable on a particular farm or field, and the manure, fertilizer or combination of fertilizers which will be most profitable to use, are questions the grower must settle for himself by trial. No chemical examination of the soil yet conducted has shown why two soils, apparently identical in chemical composition, should not produce similar yields of crops. Experience has shown that the chemical composition of the soil is no guide to its crop-producing power. Hence, all that can be given in this chapter is to submit mixtures of fertilizers that are used and the rôle the different important ingredients are believed to play in the plant economy. In addition to water, which is treated elsewhere, four elements are frequently applied in various chemical forms as fertilizers-nitrogen, phosphorus, potassium, and calcium. The potato through its life requires liberal supplies of the first three of these elements, and its behavior in regard to these is similar to that of |