VI.] NECESSARY CONVENTIONS 161 



coarser particles, which could not reach the plant for 

 generations, and that which exists as very small particles 

 or as a coating on the larger ones, and is therefore open 

 to attack by the water in the soil. The nutrient material 

 of the soil can only reach the plant in the dissolved 

 state, and in dealing with slightly soluble substances 

 such as constitute the soil, the amount which goes 

 into solution is practically proportional to the surface 

 exposed. But the surface exposed increases as the 

 material is subdivided, one gram of soil in pieces 

 i mm. in diameter would only expose one-thousandth 

 of the surface exposed by the same amount of soil in 

 particles o-ooi mm. in diameter, so that to all intents 

 and purposes the stones and coarser particles con- 

 tribute such a small proportion of the surface of the 

 soil that the material dissolved from them can be 

 neglected. 



For these reasons the small surface exposed by 

 the larger particles and the unweathered nature of 

 the compounds within them the stones above a 

 certain size are not included in the analysis, nor is 

 any attempt made to bring into complete solution even 

 the selected material. Hence it becomes necessary 

 in soil analysis to accept certain "conventions" as to 

 the preparation of the soil for analysis, the nature of the 

 acid used for solution, and the duration and temperature 

 of the attack ; all of which factors so affect the mineral 

 matter going into solution that results are only com- 

 parable when obtained in the same way. It must 

 always be remembered that soil analysis is only a 

 relative process, by which soils that are unknown can 

 be compared with others whose fertility has been 

 tested by experience ; no means exist of directly 

 translating the results into terms of the crop the soil 

 will carry. The methods of analysis that are indicated 



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