104 
INVESTIGATIONS ON ROTH AMSTED SOILS. 
The probable limit, then, denoting phosphatic deficiency for cereals 
seems to be, as deduced from this investigation, between 0.01 per 
cent and 0.03 per cent of citric-acid-soluble phosphoric acid in the 
surface soil; that is to say, a percentage as low as 0.01 seems to denote 
an imperative necessity for phosphatic manuring, while as much as 
0.03 per cent would seem to indicate that there is no such immediate 
necessity. For root crops, more especially turnips, the limits would 
probably be higher. 
Brief Examination of the Results Obtained for Individual Plats. 
In view of the fuller discussion of the detailed results in the paper 
presented to the Royal Society, it will suffice here to confine our atten- 
tion to some of the more striking indications obtained by comparison 
of the analytical results with the estimated removal or accumulation 
of phosphoric acid during the fifty years. 
Plat 3, it will be remembered, is continuously unmanured. Plat 4 
received some mineral dressings in its earlier years, but in 1893 it had 
been for forty-two years unmanured. Assuming the phosphoric acid 
originally present in the two soils to have been alike, plat 4 should 
now contain 445 pounds more phosphoric acid per acre than plat 3. 
We actually find by analysis 155 pounds more of total phosphoric acid 
in the surface soil, 58 pounds of this being soluble in dilute citric- 
acid solution. 
Plats 10a and 10b have both continuously received ammonium salts 
without phosphates, except that plat 10b had some mineral dressings 
in its earlier years. Plat 10b should contain GO pounds per acre more 
phosphoric acid than plat 10a. We actually find by analysis in the 
surface soil 78 pounds more. The citric-acid-soluble phosphoric acid 
is the same in the surface soil in both plats, but in the second and 
third depths plat 10b shows an excess of 41 pounds per acre as com- 
pared with plat 10a. 
We shall see, hereafter, from the analyses of the earlier samples, 
that on plats 3, 10a, and 10b, especially the two latter, the surface 
soils have probably readied the stage at which the quantity of mineral 
food annually rendered available by natural processes, including the 
decay of roots and stubble, balances the annual output in t he crops. 
For the various chemically manured plats receiving phosphoric acid 
the results are tabulated below: 
