102 
INVESTIGATIONS ON ROTH AMSTED SOILS. 
The four plats in the first group contain, on the average, over 3,000 
pounds of phosphoric acid per acre in the first inches of soil, to say 
nothing of a further 0,000 pounds in the subjacent 18 inches. This is 
equivalent to nearly 3 tons of phosphate of lime per acre in the top 
soil, with 6 tons lying below, well within reach of the plant in its later 
stages of growth. In face of this enormous quantity of total phos- 
phoric acid, the wheat crop, which under the most favorable circum- 
stances does not need more (and usually needs less) than some 30 
pounds of phosphoric acid per acre, is unable to do more than eke out 
a half -starved existence; the only explanation of which is that, of the 
total phosphates present, only a small proportion are in a form in which 
the} 7 can be utilized. Even in ordinary farming it is a familiar fact 
that, on soils containing a good deal more phosphoric acid than this, a 
few hundredweights of superphosphate per acre will often make the 
difference between a full crop of roots and one that is all but a failure. 
This being so, it is evident that such ratios as those just given for 
total phosphoric acid would have little significance if we wished to 
compare soils from different fields, or to judge of their relative min- 
eral fertility, or forecast the probable advantage of applying phos- 
phatic manure or the econonry of withholding it. 
If, on the other hand, we take the phosphoric acid dissolved by a 1 
per cent solution of citric acid, we find differences of an altogether 
different character; for, while the ratios for total phosphoric acid 
between phosphatieally manured and phosphatically unmanured plats 
were all comprised within a ratio of 2:1, we find the citric-acid-solu- 
ble ratios, as will be seen from the following table, to show approxi- 
mately such numbers as 4:1, 5:1, and nearly 8:1. 
Number of plats. 
Four plats 
Five plats 
One plat . . 
Do .... 
Do .... 
Mode of manuring. 
Ratio of phos- 
phoric acid 
soluble in 1 per 
cent citric-acid 
solution to 
that of plats 
receiving no 
phosphates. 
No phospates 
Phosphates and nitrogen, with and without alkaline salts 
Phosphates and alkaline salts only 
Dung f>0 years 
Dung 9 years 
1.00:1 
5.46:1 
7.83:1 
6.83:1 
3.01:1 
Clearly the percentage of citric-acid-soluble "phosphoric acid gives 
us overwhelmingly clearer qualitative information as to the condition 
of the soils than any that could be arrived at from a study of the 
mere total percentages, apart, as aforesaid, from a priori topograph- 
ical and historical knowledge. 
Pkoi'.amlk Limit Dknotinu Pnosenoiiic-Acii) Deficiency in Soils. 
Ii will have been noticed that in only one case, on the phosphatic- 
ally unmanured plats, docs the phosphoric acid soluble in citric acid 
