130 BULLETIN 1059, U. S. DEPARTMENT OE AGRICULTURE. 



tion having been paid to the subject because of the almost universal 

 belief that the requirements of trees are satisfied by almost any soil. 

 That this is probably true of such trees as the pines is evidenced by 

 their adherence to light, sandy soils. In fact, rather low require- 

 ments may be assumed for all of the evergreens on theoretical grounds, 

 because of the fact that the green, functioning parts are of long life 

 and the main product, cellulose, is a purely organic compound. 



Ecologically speaking, evidence of any direct part played by soil 

 fertility in the distribution of species, and especially of forest trees, 

 is rarely found. This may be partly explained by the fact that 

 forest soils are usually young and potentially fertile, so that other 

 characteristics, especially water-holding capacity, come into greater 

 prominence. Much careful work must be done, however, to deter- 

 mine where and when soil fertility becomes an important ecological 

 factor. 



Much difference of opinion exists as to how the fertility of the 

 soil should be measured. There is potential fertility in practically 

 all of the soil mass except the silicon, and actual available fertility 

 only in those substances which are currently in solution with the 

 soil water. As has been pointed out, notably by Hoagland(127), the 

 quantity of all substances in solution varies not only with the drain 

 upon these substances by plants, but with the quantity of the soil 

 water. For practical purposes these substances may be said to be 

 soluble only to a limited extent. 



The ordinary complete quantitative analysis of a soil involves the 

 treatment of all of the mass susceptible to chemical action, with a 

 view to discerning potential fertility. In some young soils, espe- 

 cially if formed in situ, these potentialities may be arrived at by the 

 experienced person through examination of the mother rock. Where 

 there is any question, however, the ordinary investigator, because of 

 the great amount of equipment and technique involved, should refer 

 samples for analysis to some well-equipped laboratory, such as that 

 of the Bureau of Soils in Washington. Four or five pounds of the 

 soil are required for complete anaylsis. The samples should be thor- 

 oughly air-dried when taken from the ground, freed of rocks, 

 and shipped either in jars or in heavy canvas sacks from which the 

 fine material will not be lost. 



To obtain a measure of the total soluble salts readily available in 

 the soil solution, where the chemical make-up of the soil is gen- 

 erally known, or may be assumed to be adequate for all needs, ex- 

 traction of the solutes by leaching may be employed. For com- 

 parative purposes, the amount which may be extracted with five 

 volumes of distilled water (1 liter for 200 grams of soil) will serve 

 as well as a more thorough extraction. The soil is placed on a 



