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



weeds have been followed to the depth of eleven feet below the 

 surface, and alfalfa roots have been observed at a depth of 

 twenty-six feet in the adobe of Berkeley. It is extremely plastic 

 when wet, but when dry it absorbs water with extreme slowness. 

 Because of the black color it was supposed that the humus content 

 would be found to be quite high in all localities, but the dis- 

 appointment following the examination of many columns has 

 induced us to present them in a table for comparison ; the figures 

 for humus and for amount of clay are taken from the report on 

 the several agricultural regions already given. The "adobe" 

 soils in which there is less than 35 per cent of clay are omitted 

 from the table. 



It is a matter of much surprise that the heavy black clay or 

 adobe soils of the state should, generally, have so little of humus 

 in the upper three feet and especially in the surface soil. Only 

 two of the eleven columns chosen as typical of the various black 

 clay regions have as much as 2 per cent in the upper foot. The 

 black adobe of the Sacramento and San Joaquin valleys have far 

 less humus than was found in those of the Coast Range. It is 

 also a surprising fact, well shown in these soils, that a black 

 soil does not always owe its color to the humus that it contains, 

 for all of those given in the table are very black and yet have 

 but little humus; this is especially noticeable in those from 



