ORCHARD MANAGEMENT 



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available to the plant. There are various nitrogenous, phos- 

 phorous, and potassic fertilizers. A complete fertilizer is one 

 carrying all of these three elements, but the term does not in- 

 dicate that the elements are present in any definite proportion. 



215. Nitrogen is usually the first of the three elements 

 to be exhausted and is, therefore, one of the most important 

 of the commercial fertilizers. A scarcity of nitrogen is ac- 

 companied by a dwarfed growth, while in the absence of this 

 element plants will not grow at all. This has been proved by 

 many unsuccessful efforts to raise crops under conditions 

 where it was not possible for them to secure any nitrogen, 

 though they could obtain the other plant-food elements. 

 Nitrogen is especially important when the growth of plants 

 is mostly above ground, as in the case of 



lettuce. When the growth is underground, 

 however, as in the case of the potato, carrot, 

 and the like, nitrogen must be used more 

 sparingly. 



216. Sources of nitrogen. The simplest 

 and cheapest method of supplying nitrogen 

 to the soil is by the use of leguminous cover- 

 crops. (A legume is a plant which bears 

 pods, such as the pea, bean, alfalfa, clover, 

 vetch.) These plants have on their roots 

 small galls or nodules (Fig. 89), caused by 

 bacteria. The bacteria take nitrogen from the 

 air, and form compounds which are later 

 transferred to the roots of the plants, and from 



them distributed throughout the plant tissue. FIQ 89 _ Nod 

 When these legumes are plowed under, the on roots of legume, 

 nitrogen is plowed under with them, enriching the soil. Where 

 water power is abundant and electricity is cheap, nitrogen 

 for use in fertilizers is obtained from the air. In this process 

 an electric spark causes nitrogen and oxygen in the air to 

 unite chemically. At the same time, water dropping through 



