FERTILIZERS 165 



Since such small amounts of the mineral elements are 

 used by growing plants, the research in this field has been 

 long and exacting. Most of these studies have been made 

 by growing plants in water to which the various elements 

 are added. Only recently manganese, boron, zinc, and 

 copper have been added to the Hst of essential elements and 

 other studies are in progress with other elements. This prob- 

 lem has been studied for several centuries as illustrated by 

 the work of Van Helmont (1577-1644), who grew a sixty- 

 nine-pound willow tree from a sprout in a weighed tub of 

 soil, and when he found a loss of only two ounces in weight 

 of the soil, he concluded that plants were made entirely of 

 air and water, a mistake for which we must not censure him 

 too severely. 



Of the eleven elements supplied by the soil, only the first 

 four, nitrogen, phosphorus, potassium, and calcium, are 

 commonly added as fertilizer, since the last seven are needed 

 in such small amounts by the plant, that enough is available 

 in most soils to supply the needs of the plant. Probably no 

 one has analyzed the chemical composition of strictly garden 

 plants so completely as Latshaw and Miller have analyzed 

 corn, but the results would be comparable. They found 

 about 1 pound of nitrogen in 225 pounds of corn plant and 

 much smaller amounts of each of the other elements; in fact 

 they found nitrogen was almost equal in amount to the total 

 of the other ten necessary elements. Since nitrogen is used 

 in such large amounts in plant growth and is the one element 

 that can be added by certain bacteria found in the soil and 

 by leguminous plants, the next chapter has been devoted to 

 this one element. 



The chemical composition of plants varies widely depend- 

 ing on several factors, four of which will be described: (1) 

 Newton found that the characteristics of plants vary in their 

 absorption of elements when grown under the same condi- 

 tions as shown by the composition of some common plants, 

 as follows : 



