WATER OF SOILS 189 



while the movement of the plant itself aids the circulation of the 

 water within the plant, and so increases transpiration. 



(3) Low humiditv is favorable to transpiration. 



(4) The character of the soil is an important factor. More 

 transpiration takes place from plants on poor soils than on those 

 well supplied with plant food. 



(o) Transpiration increases with the brightness of the 

 sunshine. 



(0) The more moisture there is in the soil the greater is the 

 transpiration. 



The Supply of Moisture in Soils. 1. Rainfall. The supply 

 of moisture in soils depends primarily on the rainfall. The excep- 

 tions are where artificial irrigation is practiced or where water is 

 brought to a region through some porous substratum and then 

 reaches the surface by hydrostatic pressure. The following table 

 shows the annual precipitation for different portions of the earth's 

 surface : 



Precipitation on Earth's Surface 5 



Annual precipitation PerreutaRo of earth 1 



land surf arc- 



Under 10 inches 25.0 



From 10 to 20 inches 30.0 



From 20 to 40 inches 20.0 



From 40 to 60 inches 1 1 .0 



From b'O to 80 inches 9.0 



From 80 to 120 inches 4.0 



From 120 to 160 inches. . 0.5 



Above 160 inches 



0.5 



100.0 



It is seen from the table that r>o per cent of the land area 

 receives less than *^0 inches of rainfall annually, while only 5 pel- 

 cent receives over SO inches. In the I'nited States fully 50 per cent 

 of the total area receives less than ^o inches of rainfall, and over a 

 very large part of this the growing of crops is practicallv impossible 

 except under irrigation. (Sec Rainfall Map, Fig. S!>. ) In almost 

 all regions whero crops depend upon rainfall, its unequal distribu- 

 tion, or the frequent recurrence of periods of drouth, results in 

 reduced yields. 



In almost every season in some parts of even so small an area 

 as a single State crops are injured to a greater or less extent by 

 drouth. In some cases the dry weather occurs early in the spring. 



