312 ELEMENTARY STUDIES IN BOTANY 



the free nitrogen of the air. Such plants, therefore, not only 

 do not draw upon the nitrogen supply of the soil, but they are 

 the means of adding to it. 



20. The soil complex. The above paragraphs give only 

 a glimpse of the complexity of the soil, with its chemical con- 

 stituents, its physical properties, and its active bacterial life. 

 It is a great complex in which changes are always taking 

 place, and which can be thrown out of balance in a number 

 of ways. Left to itself and to natural vegetation, it becomes 

 better for plants with each succeeding year ; but interfered 

 with by man, who rarely appreciates what he is doing, it 

 frequently gets into bad condition. This is notably true 

 when attempts are made to remedy unknown troubles by 

 " fertilizing " with unknown substances, especially with the 

 so-called " chemical fertilizers," which are as dangerous to 

 the soil in the hands of inexperienced people as are strong 

 medicines in the hands of the untrained. While destroying 

 the chemical or physical equilibrium of a properly balanced 

 soil is bad enough, any interference with the bacterial life of 

 the soil is worse. These statements merely serve to empha- 

 size the fact that any efficient manipulation of the soil, 

 especially in adding materials to it, requires experience and 

 considerable knowledge. 



21. Tillage. The preceding paragraphs have outlined in 

 a general way what the soil supplies to plants we cultivate. 

 It is well known, however, that all the appropriate materials 

 may be present, and yet the soil must be " worked " for plant- 

 ing seeds and also to help the growing plants. This " work- 

 ing " of the soil is called tillage, and its purpose is to put the 

 soil into the best possible physical condition. Tillage of the 

 soil is the first and principal thing, and often the only neces- 

 sary thing. If any " fertilizers " are to be added, this is a 

 iratter of secondary importance. In fact, there is much 

 experience to show that proper tillage reduces and often 

 eliminates the need for fertilizers. Of course proper tillage 



