14 BULLETIN 355, U. S. DEPARTMENT OF AGRICULTURE. 
practical methods of farming. Of course, the percentage that can be made available 
will vary very much with different seasons, with different soils, and for different 
crops; and yet with normal soils and seasons and for ordinary crops the above 
percentages represent roughly about the proportion that is liberated from our com- 
mon soils of the elements that limit the yield of the crop. 
The meaning and value of chemical soil analysis. — Chemical analysis 
of soil is a means of helping to determine how areas of soil which 
are unproductive should be managed. A few things should be 
understood with regard to soil analysis: (1) It is highly important 
that the surface and subsurface soil samples be representative of 
the area examined. To this end it is advisable to get directions from 
the analyst before taking the samples of soil to be analyzed. (2) The 
chemical analysis of a sample of soil will probably not detect a bad 
physical condition which may be an important factor of its non- 
productiveness. For example, poor drainage of a soil may not 
be evident from its chemical analysis. (3) A soil may be dead, so 
to speak, due to microbiological inactivity, or other causes. The 
regular process of soil analysis probably would not detect this con- 
dition. (4) Chemical soil analysis does give the amounts of nitro- 
gen, phosphorus, and potassium hi the samples of soil analyzed, and 
if the samples are representative the total quantities of these essen- 
tial elements of plant food per acre to a stated depth can be quite 
accurately estimated. The supplies of these elements available for 
plant growth may also be indicated by the analysis, but the reliabil- 
ity of the methods used in determining availability is still a matter 
under discussion by soil chemists. It is safe to say that chemical 
soil analyses often indicate what is the limiting factor in crop pro- 
duction in the soil. (5) In soil analysis a test is made for acidity, 
and if acid is found this is stated in terms of the amount of lime 
necessary to correct the condition, and from this the application most 
practical for the cropping system in use may be estimated. 
The relation between the terms nitrogen, phosphorus, and potas- 
sium, and the corresponding terms ammonia, phosphoric acid, 
and potash, commonly used by soil analysts, will be explained in 
subsequent lessons. 
The possibility of exhaustion of soil nutrients (Ref. No. 3, p. 419). — 
It is a matter of common knowledge that the cultivated soils of the 
United States, under the ordinary farm practices, frequently become 
less and less productive. There are various causes for this decline 
in productiveness. The removal of plant-food materials in cropping, 
which has already been referred to, is one of these. The leaching 
of soluble compounds into the drainage water of soils is likewise a 
source of considerable loss. It has been found in general that soils 
have greater retentive power for compounds containing phosphorus 
and potassium than for compounds containing nitrogen. Sodium 
