392 POPULAR SCIENCE MONTHLY. 



study are thus acquired, with the serious incidental result of weak- 

 ening the retentive power which depends so much upon interest and 

 concentration. "With the substitution of the oral for the book method, 

 reliance upon the memory during the memory period will perma- 

 nently strengthen the child's power of retention. 



The period between the ages of five and ten years is an important 

 one in the child's life. It is the time when the " let-alone " plan of 

 education is of most value, for the reason that nearly all our educa- 

 tional devices beyond the kindergarten are more or less attempts to 

 make men and women out of children. If the child at this age must 

 be put into the harness of an educational system, his course of study 

 will not be impoverished by the omission of reading and writing. To 

 teach him to speak and to listen, to observe and to remember, to 

 know something of the world around him, and instinctively to do the 

 right thing, will furnish more than enough material for the most 

 ambitious elementary school curriculum. 



*«»■ 



SOILS AXD FERTILIZERS.* 



By CHARLES MINOR BLACKFORD, Je., M. D. 



THE word " soil " is used in several arts and sciences to denote 

 the material from which something derives nourishment. The 

 meat broths and jellies on which bacteria are grown are soils for 

 them, as the earth of a field is a soil for the ordinary farm crops ; but 

 in general we mean by soils the various mixtures of mineral and 

 organic substances that make up the surface of the earth. 



The object of this paper is to show as briefly as possible the way 

 it was formed, of what it is composed, the manner in which it nour- 

 ishes plants, and the rules that should guide us in replenishing its 

 nutritious matter when exhausted. So broad a field can be but 

 lightly touched, and the effort will be to give only hints from which 

 rules for specific cases may be deduced. 



"When a sample of ordinary fertile soil is analyzed, it is found 

 to consist of a number of minerals, of carbon, nitrogen, and phos- 

 phorus in various combinations, water, and certain other ingredients 

 dependent on the locality. Among the minerals the most important 

 are potassium, sodium, lime, iron, and silicon, and the history of these 

 is of the greatest interest. 



Scientific students are generally agreed that the surface of the 

 earth is but a shell inclosing a liquid, or at all events a highly heated 



* An address delivered before the Richmond County (Georgia) Agricultural Society, on 

 February 19, 1898. 



