244 University of California Publications in Agricultural Sciences [Vol.3 



given soil, so that the results reported at the present time relate only 

 to one phase of the biochemistry of soils. But few attempts have been 

 made by investigators to determine actually the magnitude of varia- 

 bility in those products of microorganic activity which are capable of 

 quantitative measurement and in no instance has a mathematical inter- 

 pretation been attempted with such measurements. There are but 

 three references *' 2 ' 3 in the literature, as far as the writer is aware, 

 dealing with this phase of the biochemistry of soils, and none of them 

 is extensive enough to be of any value as statistical studies of varia- 

 bility. A number of papers 4 have appeared dealing with the variation 

 in the weight of the crop produced over different parts of an appar- 

 ently uniform field. Such variations reflect the variability of the soil, 

 serving simply as a substratum for the growth of plants, but it is 

 evident that the variations between such measurements as those given 

 do not depend upon the soil as the only variable factor. Any attempt 

 to correlate the crop produced on any given soil with the chemical 

 composition of that soil, for instance, must necessarily take into account 

 variations in both crop and soil. In fact, it appears to the writer that 

 any correlation which may exist between the properties of any given 

 soil and its crop-producing power can only be worked out by the 

 statistical interpretation of data obtained as recorded below, together 

 with the data secured in a similar manner for the crops produced on 

 that soil. The problem of variability in the soil itself is worthy of 

 careful experimental study, both from the standpoint of soil investiga- 

 tions in themselves and from that of their possible bearing upon the 

 problems of variability in field experiments with crops. The results 

 of such a study, as regards nitrate production, are presented below. 



Methods 



The field selected was one on the University Farm at Davis. For 

 the three years preceding 1917, corn, Sudan grass, and grain sorghum 

 had been grown in the order named. In 1917, the field was allowed 

 to lie fallow and at the time the samples were taken (Oct. 20), was 

 free of vegetation of any kind. No rain had fallen since April so that 

 the surface soil was practically air-dry, the subsoil, however, being quite 

 moist. The soil is classified as a silty clay loam. The particular area 

 chosen was apparently as uniform as one could well find, being level, 

 of uniform texture and color, and free from small local depressions 

 of any kind. 



