110 AGRICULTURE 



of growth. Not only do plants require to 

 be fed with suitable substances, but their 

 growth is also determined by other necessary 

 conditions, as, for example, a plentiful supply 

 of oxygen, a suitable temperature, and the 

 requisite degree of moisture. If, for instance, 

 oxygen is deficient in the soil, no conditions 

 of temperature, however favourable, will 

 compensate for insufficient aeration. Or 

 everything may be satisfactory save tempera- 

 ture, and, again, growth will be unsatisfactory 

 or non-existent until this limited factor is put 

 right. 



According to the manurial element that 

 they contain, artificial fertilizers are classified 

 as nitrogenous, phosphatic, or potassic, or a 

 combination of these terms. The two most 

 important manures that furnish nitrogen 

 alone to plants are Nitrate of Soda and Sulphate 

 of Ammonia ; but to these there have recently 

 been added two others, nitrate of lime and 

 calcium cyanamide, which are prepared by 

 inducing the free nitrogen of the atmosphere 

 to enter into chemical combination under the 

 stimulus of electric discharges. In addition 

 to the four purely nitrogenous manures just 

 mentioned, there are such subordinate sub- 

 stances as Rape Meal, Blood Meal, etc., but 

 these are of comparatively little importance. 



