168 BOARD OF AGRICULTURE. [Pub. Doc. 



of a soil (sand), consisting chiefly of quartz, and the finest 

 particles (clay), consisting mostly of silicate of aluminum, 

 carry little or no plant food. Between sand and clay, as 

 regards size, is silt. This is composed of different minerals, 

 varying with the kind of rock from Avhich the soil was made. 

 This usually contains much of the mineral constituents of 

 plants, particularly phosphoric acid, lime and potash. The 

 silt and the humus by chemical changes were supposed to 

 supply the necessary food for plant growth, and the function 

 of the sand and clay was believed to be chiefly to give a soil 

 proper physical characteristics, so as to allow circulation of 

 air and water. The plant obtains its food materials through 

 the leaves and roots. This food material is of three kinds ; 

 water, chemical substances and gas. Water is an actual 

 necessity to plant life, ])oth as a food and food solvent and 

 transporter. The chief chemical sul)stances that affect plant 

 life do not enter into the plant as such, but combined with 

 other substances and dissolved in water. The gases essential 

 to plants are carl)onic acid, hydrogen, oxygen and nitrogen. 

 By the action of chlorophyll the leaves absorb carbonic acid 

 from the air. The hydrogen and part of the oxygen is ob- 

 tained by the decomposition of water ; the remainder of the 

 oxygen is obtained from the air of the soil. Both of these 

 gases are received through the roots. Nitrogen can prol)ably 

 be used only in the form of a nitrate, and it is taken up 

 through the roots. Very little soil nitrogen is in the form of 

 nitrate, and what little there is, is subject to loss. By chem- 

 ical action the soil nitrogen is oxidized to nitrate. But for 

 the most part the snpply of nitrate nitrogen must be kept 

 up by applying it directly as nitrate, or as annnonia, or as 

 opofanic nitroijen, in farm manures and other materials. In 

 the two last instances the nitrogen was supposed to be 

 changed by chemical forces into nitrate nitrogen, and thus 

 made available to the growing plant. 



Limitation of the Chemical Explanation. 

 This chemical theory explained the action of mineral fer- 

 tilizers, including nitrate nitrogen; and, founded upon this 

 theory, elaborate field experiments with fertilizing materials 



