934 



SCIENCE 



[N. S. Vol. XXXIII. No. 859 



foundation of germane data, thoroughly and 

 logically cemented. The 121 tables of founda- 

 tion data are mostly the results of well con- 

 ceived, long continued, carefully measured 

 and verified experiments, executed both in 

 Europe and in this country. They are not only 

 put into efFectively illuminating form by the 

 author and discussed, for his purpose, with 

 good discrimination, but they will be found 

 most valuable references for many other pur- 

 poses. 



Some will question the wisdom of recalcu- 

 lating the data of some of these tables on the 

 basis of the elements but there can be no doubt 

 that this method serves much better for com- 

 parative study, and had earlier investigators 

 adopted the plan no serious criticism would 

 be urged. We feel that the author should 

 have gone a step farther and expressed all re- 

 sults on the basis of weight, and of dry weight, 

 wherever possible. The amounts of plant food 

 elements and of water removed from the soil 

 by a bushel of potatoes, oats and wheat have 

 no proper comparative relation, as they do 

 when expressed on the basis of dry substance. 



Within the last few years there has been 

 such a pronounced growth of public apprecia- 

 tion regarding the importance of the main- 

 tenance of soil productivity that plant physi- 

 ologists, pathologists and geologists, with 

 others, are following the lead of bacteriologists 

 in efforts to see if in their fields there may 

 not exist relations which will enable their ob- 

 servations to shed important light on the 

 complex problems involved. This is progress 

 along hopeful lines and such investigators 

 may feel that the volume before us does not 

 give sufficient prominence to these newer lines 

 of research. But as Professor Hopkins has 

 written for the educated farmer, and from 

 the standpoint of every-day field practise, he 

 has wisely chosen not to obscure that which 

 has been proven by prolonged careful research, 

 checked by practical experience, with the pre- 

 sentation of theories suggesting practises not 

 yet tried, which may or may not prove helpful. 



Ten chapters are devoted to science and 

 soil, in which fundamental facts and prin- 

 ciples are stated, followed by presentations 



regarding plant food elements and compounds ; 

 plant food and the earth's crust as an original 

 source; and soil formations and classifica- 

 tions, two maps being given, one for Illinois 

 and the other for the United States. The 

 composition of soils in general is treated, fol- 

 lowed by that of soils of eastern, central, 

 northern, southern and western United States, 

 expressing the results in amounts contained 

 in two million pounds of dry surface soil, or 

 approximately that contained in an acre to 

 the usual depth of plowing. Ten residual 

 soils of Maryland are shown to carry 720 to 

 1,500 pounds of phosphorus, while the Illinois 

 soils range between 810 and 2,030 pounds per 

 two million pounds of dry soil. These 

 amounts Professor Hopkins considers would 

 be removed by approximate maximum crops in 

 a rotation of wheat, corn, oats and clover in 

 from 37 to 105 years; or if only the grain is 

 sold, in from 68 to 192 years. 



It is highly probable that no normal soil 

 can experience the withdrawal of any con- 

 siderable proportion of its phosphorus without 

 suffering material reduction in producing 

 power, and if so, even if it be true that the 

 phosphorus in the second, third and fourth 

 feet participates equally and continuously in 

 crop production, permanent agriculture with 

 undiminished yields must be impossible ex- 

 cept through restoration in some manner. 

 Professor Hopkins considers that usually 

 there is no natural process of restoration 

 sufficient to maintain high yields, and closes 

 the first part, only after considering crop re- 

 quirements for nitrogen, phosphorus and 

 potassium, together with sources of plant food, 

 thus providing quantitative data for the dis- 

 cussion of systems of permanent agriculture, 

 treated in seven chapters. 



In introducing the subject he says : 



For practically all of the normal soils of the 

 United States, and especially for those of the 

 central states, there are only three constituents 

 that must be supplied in order to adopt systems 

 of farming that, if continued, will increase, or at 

 least permanently maintain, the productive power 

 of soil. These are limestone, phosphorus and 

 organic matter. The limestone must be used to 



