822 EXPERIMENT STATION RECORD. 



designate that portion of the organic matter of the soil soluble in dilute solu- 

 tion of ammonia and sodium hydroxid after removal of the lime and mag- 

 nesia by treatment with dilute hydrochloric acid. 



The centrifugal method was used in maliing the determinations, a prelimi- 

 nary comparison of this and the official method having shown that by using 

 the centrifuge the time was considerably shortened and the results were equally 

 accurate. The humus ash was a little lower for the centrifugal method. 



Summarizing his results the author concludes that " continuous cropping to 

 corn, mangels, and wheat causes a depletion of humus; on the other hand, 

 field peas increase the amount of humus. Generally rotation of crops increases 

 the amount of humus, the increase being greatest when clover is plowed under. 

 There is a fair agreement in regard to the direction of the changes in humus 

 as compared with the changes in total nitrogen and humus-nitrogen; continuous 

 cropping causing depletion, and rotation of crops an increase. 



" Except in the case of wheat, no marked decrease in total phosphoric acid has 

 occurred in the continuously cropped plats. Generally, under systems of crop 

 rotation, the plats have maintained the total phosphoric acid content for the 

 10-year period. The humus-phosphoric acid has decreased in all the plats, 

 with one exception, both under continuous cropping and systems of crop rota- 

 tion ; the depletion being gi-eater in the continuously cropped plats. 



" No definite conclusions can be drawn from the results obtained for total 

 potash. However, a much greater depletion occurs in the case of wheat than 

 in the other continuously cropped plats. In general, a marked decrease in 

 humus-potash has occurred in the continuously cropped plats, as compared 

 with the rotation plats. In the rotation plats a decrease of humus-potash has 

 occurred in all the plats, except those which received applications of stable 

 manure. The relatively large proportions of nitrogen, phosphoric acid, and 

 potash associated with the humus indicate the great importance and value of 

 tlie latter as a source of i)lant food." 



The distribution and activities of bacteria in soils of the arid region, C. B. 

 IjIpman (Univ. Cal. Pubs. Agi: Sci., 1 (1912), No. 1, pp. 20). — Recognizing the 

 important differences in physical and chemical characteristics between soils 

 of humid and those of arid regions as shown by the work of Hilgard and 

 liOUghridge, the author undertook a study of the ammonifying, nitrifying, and 

 nitrogen-fixing powers of different soil types of the arid region of California 

 as comi)ared witli those of humid sections. The determinations were made for 

 every foot of soil to a depth of 12 ft. A modified Remy solution method was 

 used, since the direct soil culture method was not considered feasible for ob- 

 taining in an uncontaminated condition such large amounts of soil as these 

 experiments required. The samples of soil were obtained " from a hole 12 ft. 

 in depth with at least one vertical wall, the latter when sterilized being 

 sampled." 



It was found that these soils, which are considered typical of the arid region, 

 (Showed ammonifying powers at all depths studied but generally most vigorous 

 in the first 6 or 8 feet. " In one case ammonification was noted in soil from a 

 depth of 15 ft, or adjoining the water-table." 



Nitrification occurred commonly down to a depth of 5 to 6 ft. In one case 

 soil from the 8 ft. depth showed a vigorous nitrifying power. There was no 

 nitrogen fixation through Azotobacter "below 2 ft. in the soil usually, but [it] 

 lias been found in some soils at 3 ft. and in one soil down to 4 ft. Many soils 

 in the arid region, otherwise favorably constituted, do not contain Azotobacter 

 organisms." 



In general, the lower layers of arid soils differed markedly from I hose of 

 humid regions in ammonification and nitrification and to a less extent in nitro- 



