INTRODUCTION 7 



the original papers that mark important steps in agricultural 

 science. 



The problems of agricultural chemistry are often so complex 

 and interrelated as to render their solution very difficult. In the 

 study of them, one should endeavor to vary one factor at a time 

 and keep the others constant. Let us take, for example, the es- 

 sential elements in plants. By chemical analysis we can ascertain 

 that plants contain certain elements. Which of these are 

 necessary to the plant and which are not? The solution of the 

 problem is obtained by growing the plants under the most favor- 

 able conditions, with an ample supply of all the elements found in 

 the plant except one. If the plant does well, then this one is not 

 needed. If it does very poorly, and all the conditions are most 

 favorable, then the element is necessary. The difficulties in fol- 

 lowing out this method of experiment will be presented later. 



It is obvious that if two variables are present, it would be im- 

 possible to tell which one produced a given effect, or what part 

 each had in it. The conditions of agricultural experimentation 

 are sometimes such that it is difficult to reduce the experiment to 

 a variation in one variable, and sometimes proper precautions are 

 not taken to eliminate other variables. Take, for example, a field 

 experiment on corn, or any other crop. Variables are weather, 

 insects, seed, soil, etc. We attempt to eliminate them by sub- 

 jecting the entire field to the same conditions, but it is very 

 difficult to make all conditions uniform. 



Agricultural investigations must be brought to the test of ac- 

 tual conditions. Conditions in the laboratory or in pot experi- 

 ments, are often radically different from those which prevail 

 in practice. The gap between the two must be bridged by ex- 

 periment, rather than by theory. 



Observation and Experience. Agricultural knowledge is 

 largely based upon observation and experience. General agricul- 

 tural practice is based upon experience, passed on from one 

 generation to another. Experience is, indeed, based upon ex- 

 periments, though the experiments are not always consciously 



