With these data a close estimate can be made of the cost of tractor ploughing under different conditions, as all the basic costs are given. The average soil resistance in the Dakotas may be assumed to be 8 lb. per sq. inch. In heavier soils the area ploughed per day will be less in substantially the inverse ratio of the soil resistance (provided the latter is not excessive). This is borne out by estimates of the ploughing capacities of the tractors which competed in the Lincoln trials of 1919. Averaging the estimates for heavy clay soil (soil resistance 11.5 lb. per sq.in.), and reckoning on the basis of a 1 o-hr. instead of an 8-hr. day, the following results are obtained: two-plough tractor, 41 acres, three-plough tractor,
acres, four-plough tractor, 63 acres.
To the American and Canadian farmer the advantage of the tractor is not so much that it reduces the cost of ploughing as compared with horses, as that it enables one man to work a much greater acreage. Approximately one-third of several hundred tractor farmers in Illinois circularised by the U.S. Dept. of Agriculture had increased their acreage by purchasing tractors. Another great advantage is that the tractor permits work to be done quickly when the weather is favourable. Power farming has proved particularly advantageous in the wheat belt of Kansas where for the best yield the land must be ploughed in August, when the temperature is often above 100° F in the shade and very little work can be accomplished with horses. The tractor works as efficiently at this temperature as in winter. Moreover, the tractor can be used also for threshing, and the old practice of contract threshing has been largely superseded by the plan of four or five farmers owning a small threshing machine co-operatively and helping each other thresh their grain, each using his own tractor as the motive power. Sometimes it is necessary to get a crop into the ground very quickly, and, with a double shift of operators, tractors can then be operated day and night.
The ordinary farm tractor was designed primarily for ploughing and is not suited to the cultivation of row crops, such as Indian corn, cotton, sorghum, etc. For this work a special type of motor cultivator was devel oped, and while a certain degree of success was achieved with it, most of the machines of this type brought out disappeared from the market. A compromise design, the Farmall, was developed by the International Harvester Company. It had two steering wheels rather close together at the front, on opposite sides of a vertical steering column, designed to pass between adjacent rows; and two driving wheels at the rear, with a tread of 74 in., which was sufficient to straddle two rows of corn. At the front the frame was supported on the steering column above the front wheels (of 25 in. dia.), while at the rear the axle was arched, the centre portion being considerably higher than the wheel spindles, giving an axle clearance of 3o inches. One of the ob
jections to many items of farm equipment is that there is use for it only a few days per year and it lies idle all the rest of the time. An all-purpose machine of this type evidently overcomes that objection to quite an extent. A large variety of tools were especially designed and others adapted for use with the Farmall.
Tractors for use in orchards, on which a number of American manufacturers specialise, must be of lower build than those for general use. A distinct type is the garden tractor, for the cultiva tion of row crops and general work in market gardening. The Beeman, the first tractor of this type, was put on the market in 1915, and in 1926 the total number of such tractors produced in the United States (by 13 manufacturers) was 3,921.
Interest in farm tractors increased greatly in Europe during and following the World War. In 1919 and 1920 tractor trials were held at Lincoln, England, the first under the auspices of the Society of Motor Manufacturers and Traders and the second under that of the Royal Agricul tural Society. The French Government in 192o paid a bonus of 25% on the purchase price of tractors of domestic manufacture and o% on tractors of foreign manufacture. French engineers paid particular attention to tractors which must be very narrow for use in vineyards. Tractor development and application in Europe also received a severe setback about the time of the world wide business depression of 1921, chiefly owing to the high cost of the fuel. The tractor trials of the Royal Agricultural Society were not repeated after that date. To reduce the fuel cost item a number of compression-ignition type engines (Diesel and hot bulb) were brought out, including the Avance in Sweden, the Lanz Acker-Bulldog and Benz-Sendling in Germany, and the Helios in Holland, these tractors using heavy liquid fuels.
In France efforts to reduce the fuel cost of tractors were directed along other lines, the machines being fitted with gas generators operating on charcoal or prepared solid fuels. Most of the tractors exhibited at the Paris Agricultural Show in 1928 had such generators. Considerable development work was done in Germany during the decade following the war, but most of it was along lines that had been covered previously in the United States. Prof. Gabriel Becker of the Charlottenburg Technical College made extensive tests and investigations on American and German tractors and published his results. The German Govern ment in 1927 initiated a scheme for extending credit to farmers for the purchase of (German-built) tractors. The Soviet Govern ment of Russia during the period 1925-1927 bought thousands of tractors from leading American firms and prepared to manufacture similar tractors in Russia.