PRINCIPLES OF AGRICULTURAL CHEMISTRY 



and perhaps die. The varying needs of different kinds of plants 

 and their varying powers of satisfying these needs, permit plants 

 to flourish in nature under a great diversity of conditions, as in 

 the tropics, or in arctic regions, in shade or in sunshine, in water 

 or in deserts. The conditions of temperature, light, or water 

 favorable to cultivated plants are more limited than those of wild 

 plants, but still the range is wide. 



The simple conditions we have named are rendered more com- 

 plex by the varying degree in which different classes of plants 

 require them, and the varying powers they have of supplying their 

 needs. The varying powers of soils to supply the needs of the 

 plant growth thereon, and the necessity of maintaining the 

 fertility of the soil, render the matter still more complex. 



Relation of the Plant to the Atmosphere. The atmosphere has 

 its part in supplying some of the conditions for the growth of 

 plants, the more important being light, heat, carbon dioxide, and, 

 indirectly, water. The atmospheric conditions are less susceptible 

 to control than soil conditions; nevertheless, sometimes a partial 

 control is established, as temperature and humidity in green 

 houses, light in the growth of plants by artificial light or under 

 shade, and the prevention of frost by smoke clouds. The 

 atmosphere indirectly supplies the plant with small quantities of 

 combined nitrogen, through the soil. 



Relation of the Plant to the Soil. The relation of the plant to 

 the soil is more complex than its relation to the atmosphere. 

 The functions of the soil are primarily to support the plant, sup- 

 ply it with water and certain necessary elements, and maintain a 

 favorable temperature. These are, however, fulfilled in a very 

 complicated manner. 



Methods of Experiment. The methods of studying the problems 

 of agricultural chemistry must be varied to suit the end in view. 

 At various points we shall bring in experimental evidence in sup- 

 port of certain views, thus illustrating by example some of the 

 more important methods. The earnest student is advised to study 



