31 8 Journal of Agricultural Research voi. xii, No. 6 



METHOD OF ATTACK 



It is evident that in any soil a large number of complex factors deter- 

 mine production. These factors, besides influencing the final crop 

 obtained, can also conceivably modify the condition of the plant food 

 in the soil itself. It becomes very clear then that a chemical study of 

 the soil solution can hope to obtain a fair degree of success, only if a 

 number of these modifying factors are subordinated or made comparable. 



Prominent among such factors are the influence of physical texture, 

 climate, and moisture. To reduce the effect of physical texture, two 

 types of soils were chosen: silty clay loams and fine sandy loams. The 

 representatives of each class were selected with as uniform a physical 

 texture as possible. Comparisons can therefore be made between the 

 soils within each class, with the assurance that the physical factor is 

 reduced to the minimum. 



The effect of climate was made uniform by transporting exceptionally 

 large samples of surface soil to Berkeley. The samples were then 

 sifted and placed in containers which will be described later. Moisture 

 conditions were made as uniform as possible by adding just sufficient 

 distilled water to keep the soils at their optimum content. 



SELECTION OF THE SOILS 



The soils used in the investigation were chosen according to the map- 

 ping of the Bureau of Soils of the United States Department of Agri- 

 culture. The silty clay loams were all chosen from the Yolo series. 

 Of these, three were taken from the Sacramento Valley and three from 

 the Santa Clara Valley. The past history of all these soils was very 

 different, as shown in Table I. The crops grown on them were equally 

 divided between orchard crops (prune, almond, peach) and field crops. 

 None of the orchards had received any special treatment. Among the 

 field soils, the sample from the University Farm at Davis was the only 

 one which had been manured, though its cropping was no more varied 

 than No. 4 from the Santa Clara Valley. Soil 3 from Yolo had a less 

 varied history than any other in either set. This soil had been under 

 cultivation since about i860, and, except for two years in sugar beets, 

 191 1 and 1912, it had been steadily cropped with wheat or barley each 

 year. Its past production was stated to be good, fully the average for 

 that section, though no exact record had been kept of the yields. 



