74 SOILS AND MANURES 



culated on the same basis, the quantity of water transpired 

 by the former is equal to about 5 inches of rain and by the 

 latter to about 8 inches. 



TEMPERATURE. 



The temperature of the soil has an important influence 

 on fertility. Not only does it affect the crops directly at 

 all stages of growth, but it also has a great effect upon the 

 chemical and biological changes within the soil, upon which 

 fertility largely depends. The vital activity of most vege- 

 table organisms is entirely arrested at temperatures below 

 the freezing point of water and above 50 C. The most 

 favourable temperature is generally above the mean of 

 these two, and considerably higher than the mean tempera- 

 tures commonly recorded in this climate. Anything which 

 tends to raise the temperature of the soil will therefore, as 

 a rule, increase the fertility, and anything which tends 

 to lower it will have a contrary effect. 



The great source of heat in the soil is, of course, the sun, 

 which pours its rays upon the earth. Physical and chemi- 

 cal changes within the soil contribute a certain amount, 

 but experiments have shown that, under ordinary circum- 

 stances, this heat has a very small effect on the tempera- 

 ture. Some heat is also derived from the interior of the 

 earth ; the amount is unknown but it may be considerable. 

 In all probability it is constant throughout the year and 

 varies but little from place to place. 



The climate of a place depends mainly upon the situa- 

 tion latitude, altitude and aspect the character and 

 direction of ocean currents, distance from the sea, direction 

 of prevailing winds, height and direction of mountain 

 ranges, and afforestation. 



Influence of Latitude. Any portion of the surface of 

 the earth receives most heat from the sun when the rays 



