CHILDREN'S GARDENS 



heavy soil refer to the ease of tillage, which is 

 necessary to give the soil proper physical con- 

 ditions and make the plant-food locked up in the 

 soil available for plants. When a crop is har- 

 vested the elements secured from earth and air, 

 locked up in the plant-tissue, are removed and 

 the soil depleted to that extent, and fertility must 

 be renewed. This may be done by proper tillage, 

 application of pure mineral fertilizers, barn- 

 yard manures or green cover crops, which when 

 plowed in will add humus to the soil. Nitrogen 

 is the most important of all soil ingredients, and 

 is most rapidly depleted by faulty management. 

 It exists as free nitrogen in the soil, and is essen- 

 tial to the life of certain microscopic forms of 

 plants, which use it and render it available to 

 higher plants. It occurs temporarily in a tran- 

 sition state as ammonia and nitrous acid, which 

 pass rapidly into nitric acid, from which most 

 of the higher plants derive their chief supply. 

 The humus in soil is supplied by roots of field- 

 crops, especially clovers and the leguminous 

 plants, and passes through a process of fermen- 

 tation, supplying nitric acid, which is soluble 

 and available to plants. 



One of the chief sources of the supply of 

 nitrogen is the free nitrogen-fixing bacteria, 

 which infest the roots of leguminous plants. 

 They are micro-organisms which settle on the 



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