126 THE BOOK 01? WHEAT 



it required his work to bring about a full appreciation of plant 

 requirements and of the important office of the soil. Through 

 the vehement discussions of his work, Boussingault, Lawes, 

 Gilbert and others were led to a critical study of these problems. 

 The exact needs of plants for mineral nutrients were carefully 

 investigated by means of experiments of water-culture and sand- 

 culture. This work was carried on by the foreign experiment 

 stations between 1865 and 1873, and its results contributed very 

 materially to the subsequent development of the enormous in- 

 dustry of manufacturing and selling commercial fertilizers. 



With prophetic vision Liebig said: "Manufactories of ma- 

 nure will be established in which the farmer can obtain the 

 most efficacious manure for all varieties of soils and plants. 7 ' * 

 Systematic work in the chemical analysis of soils in the United 

 States began in 1850, when D. D. Owens made an extensive 

 chemical examination of the soils of Kentucky in connection 

 with its geological survey. The most recent developments seem 

 to show that the amount and proportion of the elements con- 

 tained by the soil are of less importance than was formerly 

 supposed. It is of far greater importance that such elements 

 as are present should be in a form available for plant food. 

 Just what form an element must assume to be most available 

 seems to be in a large measure an unsolved problem yet, but 

 evidently the texture and the structure of the soil are fully as 

 important as the chemical condition of its elements. By texture 

 is meant the relative sizes of soil grains, and by structure the 

 arrangement of these grains under field conditions. After ex- 

 haustive investigations on many types of soil, the conclusion has 

 been reached "that on the average farm the great controlling 

 factor in the yield of crops is not the amount of plant food in 

 the soil, but is a physical factor, the exact nature of which is 

 yet to be determined." 



Most of the fertilizing which has been done has been accord- 

 ing to the theory that the soil is a lifeless mass composed of so 

 many elements, and that some elements were absent, or not 

 present in sufficiently large proportions, it being the object to 

 contribute in the form of fertilizer the elements which were 

 needed. While the benefits of fertilizers have been unquestioned 



1 Yearbook U. S. Dept. Agr., 1899. p. 340. 



2 U. S. Dept. Agr., Bu. of Soils, Bui. 22 (1903), p. 63. 



