. Principles of agricultural chemistry [microform] . ous hard pan at the depth of aboutfour feet. On account of the arid climate, the roots of trees mustbe able to penetrate to a greater depth than this. D. The water level is about three feet. This prevents rootpenetration and restricts the use of the soil to shallow rootedcrops. This condition may arise from the leakage of water fromirrigation ditches. E. The soil is underlaid by a layer of coarse sand or gravel atthe depth of about four feet, through which roots will notpenetrate to the water below. A large number of orchards havedied from t
. Principles of agricultural chemistry [microform] . ous hard pan at the depth of aboutfour feet. On account of the arid climate, the roots of trees mustbe able to penetrate to a greater depth than this. D. The water level is about three feet. This prevents rootpenetration and restricts the use of the soil to shallow rootedcrops. This condition may arise from the leakage of water fromirrigation ditches. E. The soil is underlaid by a layer of coarse sand or gravel atthe depth of about four feet, through which roots will notpenetrate to the water below. A large number of orchards havedied from this cause. Soil Temperature.—Processes of plant and animal life can goon only between certain limits of temperature. The temperatureto which the plant is subject depends upon both the soil andatmospheric conditions, the latter being perhaps the controllingfactor. 1 The Soil, p. 177. io8 PRINCIPLES OF AGRICULTURAL. CHEMISTRY The earlier the soil warms up in the spring, the earlier it can beplanted. Some few seeds begin to germinate at the freezing. Fig. 30.^—Fruit grown against a wall so that it will ripen. France,point, but most seeds require a higher temperature. Haberlandt^ Landw. Versuchs-stat., 17, p. 104. PHYSICAL PROPERTIES OF SOILS 109 tested various seeds at several temperatures and found the lowesttemperature at which germination took place was as follows:Alfalfa, beets, barley, beans, red clover, oats, peas, turnips, 32-40° F. Indian corn, carrots, sorghum, sunflower, timothy 40-51° F. Cucumbers, melons 60-65° F. The time required for germination decreases as the temperaturerises, until the optimum temperature is reached. For example,corn required ii;l4 days to germinate at 50^, 3^ days at 60°, and3 days at 65° F. While the sunflower seeds required 25 days at51°, at 60° they required only 3 days to germinate. The growth of the plant also depends upon the temperature towhich it is subjected. Between the extremes of heat and coldfatal to plants, is an
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