50/15 AND THEIR PROPERTIES 65 



natural condition. Calculated to the weight of soil per 

 acre, taken to a depth of eight inches, one acre would weigh 

 a thousand tons ; or to two decimetres, a million kilograms. 



Sources of Heat to the Soil.— Although under con- 

 ditions of market gardening and the use 01 the warm 

 frame the amount of heat produced by chemical action 

 may be appreciable, in large-scale agriculture the onh' 

 important source of heat is from the sun. The chief fluctua- 

 tions of heat arise in (i) the photosphere of sun (" sun 

 spots "), which produces indifferent harvests about once 

 ever}' ten or twelve years and fortnightly alternations of high 

 and low temperatures ; (2) the resistance of the atmosphere 

 to the passage of solar radiant energy, a resistance which 

 is greatly increased by clouds, moisture and fog; (3) the 

 angle of incidence of the sun's rays upon the surface of 

 the earth, which angle will vary with the season, the latitude, 

 and. the slope of the soil. Within the limits of the tropics, 

 that is, 23° north and south of the equator, at some period 

 of the year the sun's ^-ays are vertical, and, according to 

 the proximity of the equator, the sun even passes away 

 still further from the vertical. In the temperate zones the 

 sun is never absolutely vertical, but owing to the increase 

 in the length of days during the summer, the total amount 

 of solar radiation received within the twenty-four hours 

 exceeds that received in the tropics. The highest tempera- 

 tures are recorded in latitudes of 30° or thereabouts : at 

 latitudes over 60°, solar radiation does not reach the opti- 

 mum for plant production. Slopes having a southerty aspect 

 in the northern hemisphere, or a northerly aspect in the 

 southern hemisphere, are advantageous, since a definite 

 quantity of solar radiation has then a smaller area to 

 distribute itself over. In the northern hemisphere what the 

 southern slope of a hill gains the northern slope loses. 



Altitude is an important consideration in the growth of 

 plants. In high altitudes the sun's rays fall upon the ground 

 through a shorter, less dense, and clearer column of atmosphere. 

 On the other hand, considerable lowering of temperature 

 is produced on high altitudes by ascensional currents of 



D. 5 



