106 
INVESTIGATIONS ON EOTHAMSTED SOILS. 
we shall presently see that this variability bears an interesting rela- 
tion to the fertilizers whichhave been applied together with the super- 
phosphate. 
In the second and third depths, except in two cases, none of the 
phosphoric acid which may have descended appears now to exist in 
the citric-acid-soluble condition. Some of the phosphoric acid which 
may have descended may well have reverted into a condition insolu- 
ble in citric acid; but the deficiency of citric-acid-soluble phosphoric 
acid in the lower depths may very possibly be related in some cases 
to the more vigorous plant growth on these plats, as compared with 
tlic unmanured plat. Being manured with both nitrogen and phos- 
phates, the roots of the wheat may have developed more vigorously 
in the second and third depths of the soil, producing a greater strain 
on the phosphoric-acid resources of the subsoil, notwithstanding the 
abundance of phosphatic food above. 
It will be seen that two of these plats differ notably from the others 
in subsoil contents of citric-acid-soluble phosphoric acid, namely, 
plats 5 and 7. These two plats alone, in addition to phosphoric acid, 
have had persistently applied to them potassium, sodium, and mag- 
nesium salts, 400 pounds per acre per annum in the aggregate. To 
tli ret 1 of the other four plats one or other of these salts has been 
applied, but only to these two plats have all three salts been given. 
One of the two, plat 7, has received ammonium salts also, in the 
same quantities as plats 11 to 14. The other, plat 5, has received 
the same full phosphatic and saline dressing, but without ammonium 
sails. 
The last-named plat, getting no nitrogen, has yielded a very small 
average crop, and has naturally accumulated a far larger quantity 
of phosphoric acid, nearly 500 pounds more 1 per acre being found by 
analysis in the first 9 inches than in the average of the other plats. 
As might be expected, there is also a much larger accumulation of 
citric-acid-soluble phosphoric acid, and the proportion of citric-acid- 
soluble to total accumulation is greater than in the average of the 
other plats. Further, in both the second and third depths we find a 
tangible excess of citric-acid-soluble phosphoric acid beyond that in 
the unmanured plat, showing that the available phosphoric acid in the 
subsoil lias been in excess of the demands of the crops. This appears 
to he due not merely to the absence of nitrogen and consequent small 
growth and small assimilation of phosphates, for plat' 7, which 
receives ammonium salts with the same full mineral dressing, lias 
persistently yielded a much larger crop both of grain and straw than 
any of its companions. Its output of phosphates lias consequently 
been greater, and its accumulation less; but instead of being poorer 
in available or Citric-acid-solnble phosphoric acid than its compan- 
ions, il is now richer to the extent of some hundreds of pounds per 
acre, though, as might be expected, it is not so rich as plat 5. 
