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I 



of soil from different depths, the greatest care must be taken to insure 

 that the samples truly represent the exact depth supposed. The mode 

 usually adopted of taking samples of an indefinite area, perhaps not 

 to a definite depth, and ahncsfc certainly not of uniform breadth or 

 width to the depth taken, is obviously quite inapplicable for the 

 purposes of any such inquiry as tliat here supposed. 



Another difficulty is that, in the case of subsoils, with a low actual 

 percentage of nitrogen, the variations in the amount in different 

 samples are often proportionally great, and obviously unconnected 

 with the special history of the plot. 



Unfortunately, the few samples of soil that were collected in the 

 early years of the Rothamsted field experiments were not taken in 

 such a manner as to afford results applicable to our purpose. Com- 

 mencing in 1856, however, the mode adopted has been, after carefully 

 levelling the soil, to drive down a square frame, made of strong sheet- 

 iron, open at top and bottom, and of an exact area, and of an exact 

 depth, to the level of the surface. The inclosed soil is then carefully 

 taken out, and its weight determined. The soil around the frame is 

 then removed to the level of its lower edge, aiid it is again driven 

 down, and the inclosed soil removed ; and this process is repeated 

 until the desired depth of sampling is reached. 



Of surface soils, samples are taken from three, four, or as many as 

 eight places on the same plot. A portion of each such sample is kept 

 separate, as a means of testing the range of variation, and, if need be, 

 of correction in case of any abnormal results due to accidental animal 

 droppings, or other causes. Another portion of each separate sample 

 of the surface soil is used to make a mixture of all. In the case of the 

 subsoils, the separate samples of corresponding depth from the same plot 

 are, as a rule, at once mixed. Surface soils are sometimes taken of an 

 area of 12 by 12 inches, but frequently of only 6 by G inches, and 

 subsoils almost invariably of the smaller area. The depth of each 

 sample is generally 9 inches ; but in some special cases it has been 

 only 3 inches, and in some 6 inchcF. It is perhaps to be regretted 

 that the depth originally fixed upon did not more nearly represent that 

 to which the soil is more directly affected by the mechanical operations, 

 and by the application of manure, say G inches. But having originally 

 adopted 9 inches, it has been necessary to adhere to this depth sub- 

 sequently, in order, as far as possible, to obtain comparable results at 

 different dates. 



The soils when brought to the laboratory are first broken up, and 

 then partially dried in a stove-room at a temperature of about 130° F., 

 to arrest nitrification, which would be liable to take place if the soils 



