CYRIL GEORGE HOPKINS 



Iron is used by the plant in such extremely minute quanti- 

 ties, and the supply in the soil is so large, that it need never 

 be added to the soil as a plant food. While the plant require- 

 ments for sulfur are rather large and the supply in the soil 

 comparatively small, the amount added to the soil by rainfall 

 is adequate, not only to meet the needs of the plant but also 

 to offset the loss in the soil thru drainage. 



There remain, then, as Doctor Hopkins stated, five elements 

 of plant food which must receive careful consideration by the 

 farmer in the construction of any system of permanent soil 

 fertility that may be proposed. These are calcium, magnesium, 

 potassium, phosphorus, and nitrogen. Since there is no nat- 

 ural provision for the restoration of these elements to the 

 soil when once removed by crops, a system which assumes 

 to be permanent must provide for their return, unless as some- 

 times happens, they are present in the soil in unusual quanti- 

 ties sufficient to provide for the maximum production of crops 

 for indefinite periods of time. 



The inorganic plant foods calcium, magnesium, potas- 

 sium, and phosphorus are removed by the plants in compara- 

 tively large quantities. Chemical analyses have shown that 

 an ordinary rotation of wheat, corn, oats, and clover removes, 

 per acre, in the maximum production of crops, 77 pounds of 

 phosphorus, 320 pounds of potassium, 68 pounds of mag- 

 nesium, and 168 pounds of calcium. Since these elements are 

 obtained by the plant entirely from the soil, Doctor Hopkins 

 recognized the fundamental importance of knowing the 

 amounts of these materials which occur in the soil and of deter- 

 mining their relation to the requirements of crops. 



Various chemical methods have been proposed from time 

 to time for analyzing the soil. Most of these have been based 

 upon the fantastic claim that they determine the available 

 plant food in the soil. Doctor Hopkins, however, early real- 

 ized the futility of such a claim, and concerned himself only 

 with the determination of the total amount of plant food in 



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