42 The Ottawa Naturalist. [June 



Secondly, we have nitrogen. The percentage of this element 

 in the tissues of plants will vary from , i to 3.0, the largest propor- 

 tion being found in the seeds. With the exception of the legumes, 

 farm crops, indeed all plants, obtain their necessary supply of 

 nitrogen from that contained in the humus of the soil. This so- 

 called organic nitrogen is not directly assimilable, but must first 

 be converted by certain soil micro-organisms into compounds 

 known as nitrates. The process by which this chonge of inert 

 nitrogen into valuable food forms takes place, is known as nitrifi- 

 cation and is one of the most remarkable and important in the 

 whole field of agriculture. 



The amounts of nitrogen as nitrates consumed by crops is 

 variable; while some remove not more than 20 lbs. per acre, others 

 utilize 100 lbs. or more. Of the legumes (clover, peas, beans, &c.) 

 and the source of their nitrogen we shall speak more particularly 

 later on. 



The cropping of the land, therefore, we are to understand, 

 depletes it more particularly of certain amounts of potash, 

 phosphoric acid, and nitrogen — the so-called essential elements oj 

 fertility. To maintain productiveness, it is essential that the 

 stores of these elements in available forms be preserved ; to in- 

 crease productiveness they must be added to. 



The rate of soil exhaustion is indicated by the subjoined data 

 of an orchard and field crop. 



Essential Elements of Fertility Removed Approximately 

 IN 20 Years from an Acre of Soil. 



Nitrogen. Potash. Phos. Acid. 



Lbs. Lbs. Lbs. 

 Apples, fruit, leaves and wood (trees 



in full bearing) 1,300 1,800 300 



Wheat, grain and straw 700 700 400 



Mangels, roots and tops 1,50° 3»oo<^ 7oo 



The Nature of Soils. 



Having now taken this cursory review of the plant's require- 

 ments, we must turn our attention to soils, and learn somewhat of 

 their nature and the manner in which they are affected by culti- 

 vation. All arable soils consist chiefly ot two classes of con- 



