136 BULLETIX 1059, IT. S. DEPARTMENT OF AGRICULTURE. 



Summary of Soils Discussion. 



The preceding discussion has attempted to bring out the theoreti- 

 cal considerations which make the study of soils very important in 

 forest^, from the standpoint of initiation of seedlings, later com- 

 petition between individuals and species, and the rate and ultimate 

 limit of height growth on any particular site. In the main all other 

 soil conditions have been considered in their bearing on the supply 

 of soil moisture. In view of the length of the discussions, it would 

 appear desirable to repeat the salient points, as follows : 



1. It is believed that from every ecological aspect the important 

 soil condition is the availability of the soil moisture. 



2. Plans for the study of this soil condition have been based on 

 the assumption that the relation between the plant and the soil in 

 which it grows can best be demonstrated if, at any time, the status 

 of either may be expressed in terms of osmotic pressures. 



3. The generally coarse character of forest soils, and the presence 

 o£ rocks which are as characteristic as any other part of the soil and 

 can not properly be eliminated, give rise to the need for special 

 methods of examining forest soils, and particularly for methods 

 adapted to larger samples of the soil than have commonly been used 

 in agricultural investigations. 



4. The total moisture of the soil, while not directly making possible 

 the comparison of sites, if there be any variation in soil composition, 

 must be had for most of the indirect methods of comparison; and in 

 forest studies it must be determined periodically through one or more 

 seasons in order to discover the conditions that are critical. The 

 quantity may sometimes be found through ordinary methods of sam- 

 pling and drying the soil samples ; but often, because of mechanical 

 difficulties, and to insure greater physical uniformity in the samples 

 from time to time, it is desirable to have " wells " of prepared soil 

 from which successive samples will be taken. 



5. If it seems desirable to compute the moisture of the natural 

 soil from that found in a soil well, this may be done, at least approxi- 

 mately, by comparison of the capillarities, moisture equivalents, and 

 wilting coefficients of well soil and natural soil, respectively. It seems 

 probable, however, that up to a high moisture content osmotic equi- 

 librium is more likely than capillary equilibrium between the well 

 and the natural soil, so that if the moisture of the former may be 

 expressed in terms of osmotic pressures, it is unnecessary to compute 

 the moisture of the soil. 



6. For any study of the critical situations in soil moisture, either 

 for seedlings or for older trees, it is necessary to know the wilting 

 coefficient of each soil under consideration. The moisture content at 

 which a plant may wilt, however, varies widely not only according to 



