PLOW. 125 PLOW. and so arranged that when one series is in opera- tion the other is elevated above the soil. The steam plows used in America are of the gang class drawn by a traction engine as shown in Fig. 9. Attention of American inven- The time and manner of plowing will be de- termined by the special object to be attained. In soils more or less impervious to water it is advisable to plow to different depths at different seasons of the year in order to prevent the forma- tors has been directed mainly toward develop- tion of hardpan, which occurs when plowing to ing a cheap, light, and durable engine of sulli- the same depth for several seasons, due to the r^&^0^j^^:^^^ Fig. 7. DOUBLE-ENGINE OR DIRECT METHOD OF STEAM PLOWING. cient traction and adapted to different kinds of land. On the Western wheat ranches there has, according to Taylor, been "'developed a special form of engine for plowing, harvesting, and sim- ilar work. These engines have assumed a tri- cycle form, the weight of the boiler and engine pressure of the plow and the trampling of the teams. If the soil is very pervious it is some- times desirable to promote this formation of hardpan to arrest a too rapid percolation of water. Plowing promotes aeration of the soil and nitrification (q.v.), and otherwise improves -i^3idi- SINGLE-ENGINE OR EOUND-ABOCT METHOD OF STEAM PLOWING. resting on two very high, wide tread wheels with a third wheel of castor type in front for easy steer- ing. A high-pressure, force-draught boiler is used, and small, high-speed engines, developing from 40 to 80 horse-power. Such engines draw from 12 to 18 twelve-inch plows, and turn over 25 to 40 acres per day." A system using electricity on the trolley plan has been introduced in Germany with .some success. It is claimed for the better ■systems of steam and electric plowing that they are cheajier and more efficient than the ordinary methods. This may be true where horses and feed are expensive, fuel cheap, and lands level and otherwise well suited to these systems; but that it is universally true remains to be demon- strated. the physical and chemical condition of soils, re- sulting in increased productiveness. A deep tilled layer of soil is desirable, but this should be brought about gradually, since the plowing should not be so deep as to bring up raw sub- soil. Plowing to a greater depth than 6 or 7 inches is rarely desirable. The time and man- ner of plowing will be determined by climatic conditions, character of soil, and kind of crop to >e raised. Sod may be plowed wetter than stubble land. In midsummer and fall deep plow- ing is desirable, in early spring rather shallow plowing is usually best. Manures and similar material should not be turned under deeply in the spring. If there is danger of too rapid evap- oration, surface tillage should follow very closely
