58 AGRICULTURE ON THE PACIFIC SLOPE 



the plant. It is dissolved in the soil water in the form 

 of saltpeter. These and the carbon dioxide of -the air 

 make up the food needed by the plant. 



Acids from the roots. Some plants have grown to 

 seed in well water constantly renewed. So much 

 water as this is rarely received by plants in our fields 

 and gardens; but they help themselves by the acids 

 which they give out from their roots, and which act 

 upon the soil particles by dissolving plant food for the 

 use of the roots. 



Moisten some blue litmus paper and allow seeds to 

 germinate on it to see if they turn it red. What does 

 this show about acid excretions? 



Soluble salts 1 in soils. Water dissolves from the soils 

 of the humid regions about two hundred and fifty to 

 three hundred pounds of mineral matter per acre. In 

 getting these figures we suppose the soil to be only three 

 feet deep. It never stops dissolving a little as long as 

 the washing is continued (see page 56). In California, 

 and the arid region generally, about ten times as much 

 is easily dissolved at the first washing even in ordinary 

 soils. There are more water-soluble matters (" salts ") 

 in the arid soils, because the amount of rain is too small 

 to wash the salts out of the soil as completely as in the 



1 The word "salts/' as used in this book, means the mineral sub- 

 stances readily dissolved by water; such as, common salt, epsom 

 salt, Glauber's salt, sal soda, saltpeter, and the like. In chemistry the 

 word has a much wider meaning, which need not be explained here. 



