56 
THIRTEENTH REPORT. 
justified by the instrumentation they place in our hands for the manage- 
ment, i. e., control and efficiency of the soil as a source of food. 
The great fundamental problem of agriculture is crop production. 
Efficiency in crop production is not always recognized, nor even what 
efficiency means. There is an important distinction between quality of 
crop and quantity, the attainment of the optimum in either direction 
being not always compatible one with the other. Frequently if not gen- 
erally these ends are antagonistic, and it is a question of nice judg- 
ment so to adjust the several factors determining crop production as to 
obtain the best compromise between quantity and quality. For such a 
judgment, and such an adjustment, there is requisite a knowledge of 
the several factors, their several functions and their relations to one 
another. Crop production is the resultant of many factors. It de- 
pends upon the biological characteristics of the plant, upon the via- 
bility and germinating power of the seed, the species and varietal dif- 
ferences. It depends upon climate especially, the amount and distribu- 
tion of rainfall and of sunlight, and upon temperature variations. It 
depends upon the soil, including the composition and character of the 
mineral components and of the organic substances, including the yet 
mysterious complex, humus. It depends upon the physical properties 
of the soil, its textural characteristics, its absorptive powers and water 
movements; upon the biological properties of the soil, including bacteria, 
molds, enzymes. With these natural factors may be included others of 
which at present no opinion can be ventured other than the possibility 
of their existence. To these in the case of cultivated crops, must be 
added three artificial factors which comprise nearly all that man has 
yet devised for the control of soil and crop, namely, tillage methods, 
rotations, and fertilizers. The use of wind brakes, shades, glass houses, 
etherization, spraying, etc., are all very special practices for special 
cases, and need not be considered in a general discussion, as they have 
no wide application to field crops. 
Consideration of the factors determining crop production and the 
experimental investigations of them, brings a realization that the re- 
sultant of all these variables is complex. Not only are the factors 
numerous but they are interdependent, and no one of them can be 
changed without producing corresponding changes in all the others. 
These are points of great importance, for up to the very recent past it 
has been assumed that crop production is a simple matter, dependent 
primarily upon the amount of plant food available, although it was 
recognized that the weather sometimes interfered and that some tillage 
was necessary to the production of any crop at all. 
Let us confine ourselves for the present to a consideration of the 
soil factors. The soil is the resting place for the products of practic- 
ally all the activities taking place on the face of the earth. Agricultur- 
ally we may confine the term soil to that portion of the solid crust 
of the earth which is or can be adapted to the growth of crop plants, 
and in it are to be found results of these various activities, natural 
and artificial. The soil contains a vast array of mineral parti- 
cles. It is true that sometimes certain minerals or groups 
of minerals will predominate and give convincing evidence of the rock 
origin of the soil. Sometimes, however, the soil shows no obvious rela- 
tion to any particular rock. But investigations to which attention only 
