Cultivators for stirring the land on which crops are growing are not used extensively on dry-farms. Usually the spring tooth harrow is employed for this work. In dry-farm sections, where corn is grown, the cultivator is frequently used throughout the season. Potatoes grown on dry-farms should be cultivated throughout the season, and as the potato industry grows in the dry-farm territory there will be a greater demand for suitable cultivators. The cultivators to be used on dry-farms are all of the riding kind. They should be so arranged that the horse walking between two rows carries a cultivator that straddles several rows of plants and cultivates the soil between. Disks, shovels, or spring teeth may be used on cultivators. There is a great variety on the market, and each farmer will have to choose such as meet most definitely his needs. The various forms of harrows and cultivators are of the greatest importance in the development of dry-farming. Unless a proper mulch can be kept over the soil during the fallow season, and as far as possible during the growing season, first-class crops cannot be fully respected. The roller is occasionally used in dry-farming, especially in the uplands of the Columbia Basin. It is a somewhat dangerous implement to use where water conservation is important, since the packing resulting from the roller tends to draw water upward from the lower soil layers to be evaporated into the air. Wherever the roller is used, therefore, it should be followed immediately by a harrow. It is valuable chiefly in the localities where the soil is very loose and light and needs packing around the seeds to permit perfect germination. Subsurface packing The subsurface packer invented by Campbell is [shown in Figure 83--not shown--ed.]. The wheels of this machine eighteen inches in diameter, with rims one inch thick at the inner part, beveled two and a half inches to a sharp outer edge, are placed on a shaft, five inches apart. In practice about five hundred pounds of weight are added. This machine, according to Campbell, crowds a one-inch wedge into every five inches of soil with a lateral and a downward pressure and thus packs firmly the soil near the bottom of the plow-furrow. Subsurface packing aims to establish full capillary connection between the plowed upper soil and the undisturbed lower soil-layer; to bring the moist soil in close contact with the straw or organic litter plowed under and thus to hasten decomposition, and to provide a firm seed bed.