1914] Loughridge : Humus and Nitrogen in California Soil Columns 197 



The Fresno and Tulare soils have been under cultivation for 

 the past few years and the alkali has been kept below the surface ; 

 as a consequence, root growth was greater and its decay and 

 humification produced more humus than in the Miramonte and 

 Tulare Lake soils. There was also less of alkali salts. 



The percentage of humus in the first foot of each of the 

 columns, except that of the lake, is not very much lower than 

 in other gray soils of the Tulare plains. It is distributed through 

 the entire column, except in the water-soaked lower part of the 

 white-ash lands, and the total amount is greater than in some 

 of the alkali-free columns of the valley. 



The results, then, apparently show that neither carbonate, 

 sulfate, or chlorid of soda have any injurious effect on humus 

 itself, but that they do so retard or even kill the growth of 

 surface plants and root systems as to cut off the supply of 

 humus-forming material. The humus in the Tulare Lake bed 

 column is exceptionally low in nitrogen from some cause not 

 now apparent, and in all of the soils the nitrogen percentage 

 falls below the minimum required for fertility. 



The conclusion is plain, then, that where a green-manure crop 

 can be grown by keeping the injurious alkali salts below a 

 depth of three or more feet by irrigation its conversion to humus 

 under favorable condition is not interfered with. 



Black Adobe Clay and Loam Lands. — A large area of black 

 adobe land occupies a region in San Joaquin County extending 

 from two miles north of Calaveras River south to French Camp 

 slough and from the tules or marshes of San Joaquin River east- 

 ward toward the hills, and is timbered with oaks. The character 

 of the soil is shown in a column twelve feet in depth taken two 

 and one-half miles southeast of Stockton. 



The region continues southward in a narrow belt along the 

 base of the hills into Kern County, being known in Tulare 

 County as "dry bog" because of its tendency to break up into 

 small fragments when dry. It is here underlaid by a reddish 

 clay loam, from which it is sometimes separated by a whitish 

 calcareous and silicious lime and magnesia bed of varying thick- 

 ness. Two columns of this black clay adobe were secured from 

 near Porterville ; one is from the Williams orchard with a depth 



