VI.] INTERPRETATION OF RESULTS 151 



inferior limit of 0-5 per cent, is the lowest that is safe. 

 In the case of soils containing about this proportion 

 much will depend on how finely it is disseminated, 

 05 per cent, in visible pieces will not be so effective 

 as o- 1 per cent, of the amount in particles of the same 

 order of size as the clay or silt particles. For this 

 reason it is advisable when analysing a doubtful soil of 

 this kind, to make a rough separation of the finer 

 particles, by pestling up 10 grams of the soil with water, 

 and pouring off the supernatant liquid after one minute's 

 standing, as in a mechanical analysis. Having washed 

 away the finer portion of the soil two or three times in 

 this way, the residue is dried and the carbonates which 

 remain are estimated as before, thus a rough idea is 

 obtained of their distribution among the finer or coarser 

 sets of soil particles. 



Interpretation of the Results of a Soil Analysis. 



Though much may doubtless be learnt by a com- 

 parison of the analysis of a given soil with the analysis 

 of others whose fertility has been proved by experience 

 or by actual manurial experiments, there are yet many 

 considerations which prevent much weight being attached 

 to the results thus obtained. 



A comparison of the total amount of any of the 

 elements of plant food in the soil with the amount that 

 is withdrawn by an ordinary crop shows at once that 

 even in the poorest soils there is sufficient material for 

 something like a hundred average crops. 



The density of the surface soil has already been dis- 

 cussed ; it will be sufficiently accurate for our purpose 

 if we consider that the top 9 inches of one acre of 

 an ordinary arable field weighs 2,500,000 lbs. On this 

 basis, and without taking into account the fact that 



