1914] Loughridge: Humus and Nitrogen in California Soil Columns 263 



It is interesting to note in each of these composite columns 

 of soils the quite regular decrease in the amount of humus from 

 the surface to the twelfth foot, thus indicating that in each the 

 humus has been derived from the decay of plant roots rather 

 than from vegetable material deposited during the building-up 

 of the soil strata. It also gives testimony to the deep rooting of 

 plants of all kinds, notably grasses and weeds during the ages 

 that have passed. 



Another interesting point is the regular increase in the humus 

 percentages from a minimum in the very sandy to the maximum 

 in the clay and then a decrease in the adobe clays, not only in 

 the surface foot of these composite soil columns but also in each 

 succeeding foot down to the twelfth, the only exceptions being 

 slight ones in the sandy loam series which are a little less than 

 in the sandy. 



It is clearly evident, then, that as a rule the clay soils, except- 

 ing the adobe clays, of the state have a higher percentage of 

 humus than any other class ; and that the humus content in 

 the series increases with the increase in clay up to a certain 

 point when there is a decrease. 



The tables show that the lower half of the heavy adobe clays 

 has a smaller percentage of humus than any other class except 

 the very sandy ; the inference being that the close, compact, heavy 

 clay strata have prevented the extension of plant root systems 

 to the lighter soils. 



It will also be noted that one-half of the total humus in each 

 of the composite columns is held in the upper three feet, with 

 the exception of the sandy, in which it is held in four feet. 



Comparison of Individual Heavy Black Adobe Clay Soils 



The heavy clay soils loiown as adobe clay and containing from 

 35 per cent of clay upward to as high as 62 per cent are found 

 in isolated areas throughout the state, the largest being in the 

 region of Stockton. They form a distinct class possessing 

 peculiar physical properties. The surface soil is usually black 

 to a depth of three or more feet, changing to a lighter color 

 below and generally resistant to the easy movement of water, 

 plant roots, and tillage implements, though the roots of ordinary 



