272 University of California Publications in Agricultural Sciences [Vol. 1 



Arid California with her deep soils of well diffused humus 

 and richness in plant food thus possesses very great advantages 

 over the more humid east where a depth of six to nine inches 

 is very generally accepted as that of a true upland surface soil, 

 containing practically all of the humus and marked by a very 

 sharp change of the black humus color to the gray and yellow 

 subsoil with its inappreciable amount of humus. ^^ In the sedi- 

 ment lands of the streams and some of the black prairies the 

 humus is of course found at a greater depth. 



CONCLUSIONS 



1. The depth of the soils of California is indefinite because of 

 the presence of humus and available plant food to and beyond 

 twelve feet below the surface. It may be several times twelve 

 feet, being limited only by ground water, country rock or heavy 

 beds of gravel ; layers of hardpan at a depth of several feet 

 would limit the depth for small cultures, but may be broken up 

 with dynamite and the depth greatly increased for the extension 

 of tree roots. There are in reality none of those difficultly 

 permeable clay subsoils that characterize humid soils and limit 

 their depth to but a few feet. The sharply defined change of the 

 black humus color to gray at six or nine inches that marks the 

 depth of humid soils is present in some of the heavy clay soils 

 in California at the depth of three feet, but for the most part 

 the change in tint is very gradual downward through many feet. 

 The upper three feet may, however, properly be called the soil, 

 for in that depth the greater part of the feeding roots of plants 

 are found. 



2. The soils of California are richer in humus than has 

 generally been supposed; in their depth of three feet (the soil 

 proper) they contain more than in the humid soils, and in the 

 entire columns of twelve feet or more they have double that of 

 the humid soils. 



13 See Hilgard's Soils, p. 164, and Agricultural Science, 1892, p. 263; 

 King's The Soil, p. 29; Hall's Soils, p. 45; Minn. Agr. Expt. Sta. Bull. 

 30, 41, and 65 contain analyses of 121 subsoils, in only sixteen of which 

 is humus reported lyhough found in all of the corresponding surface soils 

 which were taken to depths of nine inches. (Bull. 30, p. 164.) 



