NATURE OF DRY-FARM SOILS 59 



difference between arid and humid vSoils. In humid 

 regions plants cover the soil thickly; in arid regions 

 they arc bunched scantily over the surface; in the 

 former case the decayed I'cmnants of generations of 

 plants form a large percentage cjf 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 soHs is 

 very much lower than in humid soils, repeated investi- 

 gation has shown that it contains al^out 3-| times 

 more nitrogen than is found in hiunus 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 fillctl with the remnants of 



