204 § 108. AlS^ALYSIS OP SOILS AND ROCKS. 



amounts of sand and clay may be used ; one box of each 

 kind of soil should have nothing added to it, so as to have 

 a standard with which to compare the effect of the ma- 

 nuring in other soils of the same nature. From the aver- 

 age crops of the preceding year in all the boxes, an esti- 

 mate may be made of the quantity of plant-food to be 

 added. 



The soil to be manured is taken out of the box to the 

 depth of 30 cm. ; ^ |^ of this is intimately mixed with the 

 aqueous solution of the salts to be added, and this in its 

 turn is intimately mixed with the remaining ^|^ of the 

 soil, by rubbing the two together carefully between the 

 hands ; the whole is then put back into the box. 



Instead of the solution of the above salts, the actual 

 practice with manures may be more closely imitated by 

 making an aqueous extract of a superphosphate, deter- 

 mining the amount of phosphoric acid in the solution, and 

 dissolving therein the proper amounts of crude potassic 

 chloride, sodic nitrate, and magnesic sulphate. 



In a similar manner, the effect of an increased propor- 

 tion of humus in the soil may be studied, by letting some 

 sawdust of a soft wood, free from resin, partly decay, 

 making an aqueous extract of this, or of some other suit- 

 able substance rich in humus, and mixing this solution 

 with the soil in the way already described. 



Other matters that might be profitably studied in this 

 connection are, the effect of lime, of the concentrated 

 commercial manures, and the relation between the co- 

 efficients of absorption of the soil for the various ele- 

 ments of plant-food, and its fertility. 



