NATURE OF DRY-FARM SOILS 59 
difference between arid and humid soils. In humid 
regions plants cover the soil thickly; in arid regions 
they are bunched scantily over the surface; in the 
former case the decayed remnants of generations of 
plants form a large percentage of humus in the 
upper soil; in the latter, the scarcity of plant life 
makes the humus content low. Further, under an 
abundant rainfall the organic matter in the soil rots 
slowly; whereas in dry warm climates the decay 
is very complete. The prevailing forces in all coun- 
tries of deficient rainfall therefore tend to yield soils 
low in humus. 
While the total amount of humus in arid soils is 
very much lower than in humid soils, repeated investi- 
gation has shown that it contains about 34 times 
more nitrogen than is found in humus formed under 
an abundant rainfall. Owing to the prevailing sandi- 
ness of dry-farm soils, humus is not needed so much 
to give the proper tilth to the soil as in the humid 
countries where the content of clay is so much higher. 
Since, for dry-farm purposes, the nitrogen content 
is the most important quality of the humus, the dif- 
ference between arid and humid soils, based upon 
the humus content, is not so great as would appear 
at first sight. 
Soil and subsoil.—In countries of abundant 
rainfall, a great distinction exists between the soil 
and the subsoil. The soil is represented by the upper 
few inches which are filled with the remnants of 
