3<3 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



per acre; manganous chloride 60 pounds; potassium iodide one- 

 third ounce; sodium fluoride one ounce. 



The importance of these stimulating compounds in practical 

 agriculture remains to be demonstrated. Such field tests as have 

 come to the notice of the writer are contradictory. It has been 

 suggested that the copper hydroxide and similar substances used 

 for combating certain plant diseases, instead of killing the fungus 

 direct, stimulate the plant and increase its vigor sufficiently to 

 resist the disease, but we know of no experiments supporting this 

 hypothesis. It is also possible that "stimulating" compounds may 

 occur in certain soils in quantity sufficient to be injurious. 



Essentials for Plant Life in Addition to Food. Conditions other 

 than plant food which are essential to plant life are 

 mentioned here for the sake of completeness. The requirements 

 of foliage, fruit, or roots are different, but the entire plant suffers 

 if a part suffers. 



Water, in considerable quantity, is essential to plant life. Only a 

 small portion of the water used by a plant is used as plant food 

 and goes to the formation of organic material. Most of it is 

 evaporated by the leaves of the plant. 



Light is essential to the formation of organic matter by the 

 leaves of plants. Too much or too little light affects the develop- 

 ment of the plant. Plants vary in their requirements for light. 



Temperature. Extremes of heat and of cold destroy plants. 

 For their best growth a certain temperature is most favorable, 

 and it varies with the kind of plant. Temperatures which do not 

 destroy the plant may yet interfere with its growth. 



Favorable soil conditions are essential ; the proper degree of 

 moisture, of soil atmosphere, absence of deleterious influences, 

 etc., are necessary to the best growth. 



The Law of Minimum. In the experiment of Wolff on oats, 

 cited previously, the size of the oat crop varied with the quantity 

 of phosphoric acid supplied to it, other conditions being favor- 

 able. In the following experiment of Hellriegel, 1 various 

 1 Exp. Station Record, 1893-4, p. 853. 



