136 
INVESTIGATIONS ON KOTHAMSTED SOILS. 
As far as the total' phosphoric acid is concerned, it is sufficient to 
compare the first 9 inches of soil, the irregularities in the second and 
third depths being such as in many cases to overwhelm altogether 
such differences as would be due to the effects of manuring and crop- 
ping. In the case of the citric-acid results, the second and third 
depths in some instances afford an interesting subject for comparison. 
Beginning with plat 3, which, it will be remembered, is continuously 
unmanured, we see from period to period, as indicated by the analy- 
ses of the 1865, 1881, and 1893 samples, a steady decrease in the total 
phosphoric acid. The same thing is seen in plat 10a, which receives 
only ammonium salts without minerals. 
Plats 11, 12, 13, and 14 all show the progressive increase in total 
phosphoric acid due to the accumulation of residues of unused phos- 
phatic manure, and the dunged plat, 2b, also shows a steady increase 
in total phosphoric acid from period to period. 
When we regard the citric-acid-soluble phosphoric acid, we must 
bear in mind that the samples of 1865 and 1881 were not examined 
immediately after collection, but only in 1893, when they were, respec- 
tively, twenty-eight and twelve years old ; and it is not certain whether 
the citric-acid-soluble phosphoric acid may not have undergone some 
modification during that time notwithstanding the fact that the sam- 
ples were stored in an approximately dry state. Nevertheless they 
afford features of considerable interest and on the whole agree excel- 
lently with the history of the various plats represented. 
In the case of the unmanured plat the citric-acid-soluble phosphoric 
acid decreased appreciably between 1865 and 1881, but does not appear 
to have decreased since. The same thing is to be noticed in the case 
of plat 10a, which receives only ammonium salts. In fact, in both 
these instances there appears, if anything, to be a slight increase 
be1 ween 1881 and 1893, and it would appear as though the quantity of 
citric-acid-soluble phosphoric acid in the surface soils of these plats 
had become reduced to a sort of natural level, at which the decompo- 
sii ion oi* crop residue and other influences producing readily available 
phosphoric acid from the natural resources of the soil approximately 
balance the annual quantity removed in crops. 
In the case of plats 10a and 10b, the effect of the early mineral 
dressings on plal LOb seems to be clearly apparent in the 1881 samples. 
The gradual accumulation of mammal phosphoric acid during the 
three periods on plats 11-11 is very strikingly seen in the citric-acid 
figures for these various plats, and this is also the case with the dunged 
plat, 2b. 
When we come to the upper subsoil or second 9 inches we find in 
the unmanured plal (3) and the ammonium-salts plat (10a) a diminu- 
tion in citrie-aeid-soluble phosphoric acid between 1865 and 1881, and 
then w hat appears to be a distinct reenfqrcement between 1881 and 
L893, suggesting that possibly as the surface soil has grown more 
exhausted the roots Of the wheal have taken to deeper growth to seek 
