682 



SCIENCE. 



[N. S. Vol. XXIV. No. 622. 



understood that throughout all of these ex- 

 tended studies soils have been considered 

 broadly (1) from the standpoints of geology 

 and climate; (2) from that of their physical 

 composition, condition, moisture and aeration; 

 (3) from that of chemistry and plant nutri- 

 tion; (4) and finally from that of the correla- 

 tion of native vegetation to virgin soil condi- 

 tions as indicative of relative productive 

 power a very important contribution to agri- 

 cultural science may well be anticipated as 

 the outcome of such preparation, especially 

 when, as is the case here, the author fully 

 avails himself of other investigations in set- 

 ting out the subjects discussed. 



Another feature which must give special 

 importance to Hilgard's views as guardedly 

 presented in this volume is the fact that 

 throughout his half century of study he has 

 been able to deal almost entirely with soils in 

 their unfertilized or entirely virgin condition, 

 whereas the whole of the soil literature of 

 Europe had grown up on fields centuries old 

 which had been long fertilized before the 

 studies began; and what is more significant, 

 our author, from the outset and throughout, 

 was keenly alive to the fact that he was deal- 

 ing with fundamental conditions which must 

 be widely different from those which European 

 students have met and from which they have 

 drawn their conclusions. 



The author groups his subjects into four 

 parts: (1) the origin and formation of soils, 

 covering 79 pages; (2) physics of soils, to 

 which is devoted 237 pages; (3) chemistry of 

 soils, appropriating 131 pages and (4) soils 

 and native vegetation, to the consideration of 

 which are given 62 pages. Those who have 

 been in the habit of attributing to Hilgard 

 the view that physical properties and condi- 

 tions of soils play but a small part in deter- 

 mining their productive capacity will be sur- 

 prised at the amount of space devoted to soil 

 physics. This is done, not because of any 

 change of views on the part of the author, 

 for throughout all his writings no one has 

 been more insistent regarding the greatest im- 

 portance being attached to proper physical 

 conditions as the first essential to a productive 

 soil ; and no one in America has done as much 



as he to establish the causes of the physical 

 differences in soils which are productive and 

 which are unproductive, and to point out prac- 

 ticable methods for correcting evils when they 

 are known to exist. It has been his strong 

 insistence during these later years, when it 

 has become a fashion to ignore chemical dif- 

 ferences, that these too are very important in 

 influencing productive capacity, that has given 

 the impression to some that Hilgard regards 

 physical differences as of comparatively slight 

 moment. 



In Chapter I. the physical processes of soil 

 formation are discussed, followed in the sec- 

 ond by a presentation of the chemical, which 

 includes the exchanges of bases in zeolites and 

 the action of plants and their remnants in 

 soil formation. Next come two chapters treat- 

 ing first of the major soil-forming minerals 

 and then of the chief soil-forming rocks, where 

 special stress is laid upon the nature, origin, 

 determination and importance of colloidal clay 

 as greatly influencing both the physical and 

 chemical nature of soils; while in the last 

 chapter of Part I. the minor minerals and 

 ingredients of soils are treated, including 

 those used as fertilizers and also those which 

 are unessential or are injurious. 



Part II. begins with the physical composi- 

 tion of soils and ' since clay is the substance 

 whose functions and quantitative proportions 

 influence most strikingly the agricultural 

 qualities of land' it is first discussed. Then 

 follows an important characterization of sands 

 in humid and in arid regions, showing why 

 the latter are usually and naturally so much 

 more productive; the chapter closing with the 

 methods, purposes and importance of mechan- 

 ical analysis of soils. Chapters VIII., IX. 

 and X., treating of soilsi and subsoils from 

 the standpoints of causes and processes of 

 differentiation; of organisms influencing soil 

 conditions; of their relations to vegetation, 

 and of the origin, nature and distribution of 

 humus, will be found among the most lumin- 

 ous, important and suggestive, to both the stu- 

 dent and the investigator, of all the valuable 

 presentations which the book contains. Espe- 

 cially instructive is Eig. 27, contrasting a 

 type of eastern soils with two of those of 



