14 BULLETIN 355^ U. S. DEPAETME]SrT OF AGEICULTUEE. 



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 Uberated 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 determme how areas of soil which 

 are unproductive should be managed. A few things should be 

 understood w4th 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 m 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 determinmg 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 apphcation 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 



