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 
