4 BULLETIN 102, PART 2, UNITED STATES NATIONAL MUSEUM. 
in some instances, particularly along mechanical lines, as in irriga- 
tion, terracing, the prevention of erosion, and the like ; some advances 
were even realized in the use of artificial accelerators or fertilizers. 
But agriculture remained a rule-of-thumb procedure, locally effec- 
tive because based on accumulated experience, yet broadly inefficient, 
until the development of modern chemistry gave a foundation of ex- 
act knowledge upon which further advance could be built. 
Scientific agriculture came to the western world none too soon. 
It made possible the growing population essential to industrial de- 
velopment — a population increasing at a rapid rate because no 
longer held down by the waves of pestilence that swept over Europe 
before the days of preventive medicine. In the East, grim nature 
has periodically lopped off the excess of population by depressions in 
soil productivity; and there to-day this inexorable process con- 
tinues among the hordes of India and China. At the present mo- 
ment a new and unnatural factor has come to upset the balance in 
the West — the World War has placed an increased and unprecedented 
burden upon intensive and scientific agricultural development. As 
at first, so to-day, the securing of an adequate food supply is the 
most pressing problem that faces mankind. 
The amount of food that may be realized from any region de- 
pends upon two obvious factors — the area of soil and its fertility. 
In a new or pioneer country land is plentiful, and the need of an 
increasing food supply for a growing population is met at first 
by enlarging the area cultivated; but, sooner or later, there comes 
a time when further enlargement is impossible and food must be 
imported or else the yield of the soil must be increased by artificial 
means. The world has already reached the stage where in all regions 
of dense population the latter is imperative — a fact which should 
have the widest and most thorough appreciation. 
THE THEORY OF FERTILIZERS. 
Various means of increasing soil productivity are practiced, but 
the most important gain is attained by adding certain products di- 
rectly to the soil. These substances are the so-called fertilizers. A 
glance at the nature of soils and the chemical reactions that take 
place in them to produce plant growth will make clear the part that 
fertilizers play in accomplishing their important function . 1 
To begin with, soil does two things : It forms a mechanical medium 
for supporting and protecting the growing plant, and it supplies the 
plant with some of the chemical material to be built into its structure. 
1 It is, perhaps, needless to say that this will be outlined in the most general 
way only. To go into the matter in technical detail would involve material in- 
appropriate for the present purpose. The theory of fertilizers is by no means 
thoroughly understood. 
