36 BULLETIN 1105, U. S. DEPARTMENT OF AGRICULTURE. 



underlying causes. Further investigations are required for a com- 

 plete explanation, but the following observations will serve to call 

 attention to some of the factors involved. 



MOISTURE. 



The entire soil problem in the Southwest revolves about the ques- 

 tion of moisture. We have abundant evidence that practically all 

 of the soils in the yellow pine type of this region will grow yellow 

 pine from the seedling stage to maturity if the water supply is 

 sufficient. But a limited supply of moisture is more effective in 

 some soils than in others. This is especially important during the 

 seedling stage, because the seedling is not equipped with an adequate 

 root system to meet its demands when the supply of available mois- 

 ture in the soil is low. It is a well-known fact in agriculture that 

 sandy soils, though having a lower water-holding capacity and 

 lower retentive qualities than clay soils, are able to deliver moisture 

 to plant roots in drought periods when none can be secured from 

 the clay soils. It is also a well-known fact that pines thrive better 

 in sandy than in clay soils. The reasons for this are that sandy 

 soils permit deeper root penetration, absorb rainfall more readily, 

 are better aerated, and, as stated above, furnish a greater propor- 

 tion of the moisture to the plant roots in droughts than clay soils. 



But the sandy soils are not really concerned in the reproduction 

 problem of the Southwest, because where the soil is sandy, reproduc- 

 tion of western yellow pine is not, as a rule, difficult to secure. The 

 reproduction problem is limited mainly to the basaltic clay soils which 

 are prevalent on the Coconino and Tusayan National Forests. On 

 the southern portion of the Coconino, and on the Sitgreaves and the 

 Apache Forests, soils apparently similar to the basaltic soils above 

 mentioned bear excellent stands of reproduction, but close examina- 

 tion will show that they are more sandy or gravelly and, as a rule, 

 have a deeper loamy surface layer than the Coconino and Tusayan 

 soils, which bear poor reproduction. The important fact is that, even 

 though the soil proper may be classed as a clay, reproduction will suc- 

 ceed if liberal quantities of stones .are mixed in the soil mass. There 

 are a number of obvious advantages due to this admixture of stones. 

 Where they occur on the surface in considerable quantities they check 

 run-off. They also act as a mulch against evaporation. Water per- 

 colates deeply in the soil by following the surfaces of partially em- 

 bedded rocks. Haasis (6) has pointed out that there is often a distinct 

 line of cleavage between the surface soil and the rock, caused by frost. 

 He also calls attention to the fact that the volcanic rocks, because of 

 their porous structure, absorb considerable quantities of water, which 

 may become available after the surrounding soil has become relatively 

 dry. He has found that the roots of pine seedlings often follow the 



