S6 THE SOILS AND CEOPS OF THE FARM. 



regards the elements of plant food which pass into the 

 air when the plant is burned or decays. There is a 

 great abundance of carbon, in the form of carbonic 

 acid, in the air and in most soils. Oxygen and hydro- 

 gen as water, and oxygen in combination wich many 

 substances, are usually in abundant supply. Nitrogen, 

 however, although it forms the larger part of the air, 

 is often only sparingly found in either the soil or air 

 in such a state that plants can feed upon it, most 

 plants being able to make use of nitrogen only when 

 it is combined with some other element — in such com- 

 pounds as ammonia and nitric acid. Hence, while no 

 more essential to plant growth than is oxygen, hy- 

 drogen or carbon, nitrogen is commonly spoken of as 

 the most important of the combustible parts of plants. 



In discussions concerning manuring or the value of 

 different artificial manures or commercial fertilizers, 

 nitrogen, as ammonia or as nitric acid; phosphorus, as 

 phosphoric acid, and potassium, as potash, are referred 

 to very much more than any other or than all 

 other substa Aces. 



Soils differ much in their fertility, or power 

 of producing crops. This may be because in one 

 soil there is a more abundant supply of one or more 

 of the chemical elements which have been named 

 as essential to the growth of plants. A soil may, how- 

 ever, have an abundant supply of each of these ele- 

 ments, and yet not produce good crops, because some 

 of them may not be in a condition in which the plants 

 can use them. This may be because of poor physical 

 conditions or because the elements of plant food are 

 in such chemical combinations as to make them use- 

 less for the "present, at least. Much of the soil is not 



