DRY-FARMING. In many parts of the world there are large tracts of land suitable for the growth of crops but in which the rainfall is deficient. They lie in a different category from ac tual desert areas which are, for the most part, so lacking in plant food as to require more than water to make them capable of pro ducing vegetation. What are commonly termed "arid" or "semi arid" districts need only an adequate supply of water to make them productive. A district is usually classed as "arid" when its annual rainfall is less than loin. and "semi-arid" when the rainfall exceeds Io and is less than 20 inches. These limits are elastic, as other factors of climate and of soil have to be taken into account. Dry-farming is a system of cultivation designed to conserve and utilise to the utmost possible extent the limited amount of moisture falling upon the land in arid regions, without resorting to extraneous supplies of water. It is practised in those regions of low rainfall where, for physical or economic reasons, irrigation is not practicable. The conservation of moisture is effected by care ful tillage of the surface-soil and the subsoil separately. The sur face is kept loose and finely pulverised to allow the ready percola tion to the subsoil of such moisture as may fall in rain or snow. The subsoil is also finely pulverised but it is made as firm and compact as possible to increase its capacity for holding water and render its condition suitable for the germination of seed and the development of plant roots.
By this system of cultivation the land is first deeply ploughed, and the subsoil consolidated by a machine termed a sub-soil packer. The surface soil is thoroughly harrowed and scarified. The best results are obtained if the land is left fallow for a year. During the winter and early spring it is covered with snow. When the snow melts, the water, instead of draining away or evaporat ing, is collected and stored in the packed subsoil. As soon as the surface is sufficiently dry it is harrowed over, and this operation is repeated after every rainfall until seed-time. The seed is drilled at the right depth to place it in the packed subsoil where the moisture has been accumulated. After sowing, the land is har rowed after each rainfall until the crop is so far advanced that this cannot be done without injuring it. As soon as the crop is har vested preparation for the next seeding begins. The plough fol lows close on the harvester and cuts a furrow seven inches deep. The subsoil packer follows immediately after the plough. The land is then harrowed after every rainfall, but never when it is dry, until the sowing of the next crop.
Crops are grown under this system either annually or biennially, this being determined mainly by the amount of rainfall, and also by the nature and texture of the soil.
Wheat is the crop mainly grown by the dry-farming system. W. P. Rutter in Wheat-growing in Canada, the United States and the Argentine (1911) states that under dry-farming the nitrogen content of the wheat is large and the grains flinty and trans parent, whereas under irrigation the grain is light-coloured, opaque, starchy and soft. The yield on irrigated land is greater but the quality of the wheat is better on land which is dry farmed.
The United States Department of Agriculture (Yearbook, 1923) estimates that 587 million acres, or nearly one-third of the total area in America available for crops, pasture and forest, are arid or semi-arid pasture and range. About 51 million acres were stated to be irrigable, of which 20 million acres had been irri gated in 1920. California, Colorado and Idaho have the largest areas of irrigated land, comprising more than half the area irri gated. Dry-farming is practised chiefly in the Great Basin, the Columbia river valley, the interior valley of California and por tions of the Great Plains.
The arid or semi-arid region of the United States extends over the Canadian border, a tract of some 8o,000 square miles west of the Io2nd meridian and east of the Rocky mountains being sub ject more or less to these conditions. Here also dry-farming is practised and methods of cultivation to conserve moisture in the soil are continuously studied.
The system of dry-farming has been mainly developed in North America, but in other areas having deficient rainfall it is also practised although on a less extensive scale.
See W. Macdonald, Dry Farming: Its Principles and Practice (1909) ; J. A. Widtsoe, Dry-farming (1913) ; D. E. Stephens, "Experi ments in Wheat Production on the Dry Lands of the Western United States," U.S. Dept. Agric. Bull. No. 1173 (1923) ; R. A. Oakley, "Forage Crops in Relation to the Agriculture of the Semi-arid Portion of the Great Plains," U.S. Dept. Agric. Bull. No. 1244 (1924) . (R. H. R.) the trade-name for solid carbon dioxide, is a dense, snow-like substance having a temperature of 109° F. below zero or lower. Carbon dioxide gas is changed to a liquid when subjected to a pressure over five times that of the ordinary atmosphere and at 7o° F. below zero. When this liquid is allowed to expand rap idly from the high pressure to atmospheric pressure, carbon di oxide snow results. This snow is compressed into cakes. Instead of melting it evaporates to a dry gas that has no corrosive action if moisture free. It makes possible the shipping of perishable prod ucts (meats, ice cream) long distances. One pound of dry-ice will absorb a little less than twice as much heat as one pound of water ice. A block of dry-ice loin. x loin. x loin. in size weighs about 45 lb.