THE BBOADBALK WHEAT SOILS. 
139 
In the ease of t he unmanured plat il would appear that in 1805, 
After over twenty years of unmanured cropping, the available potash 
hail already been reduced to a low ebb, and that in the sin face soil it 
lias since fallen still lower, though in the second and third depths 
it would seem to have increased. 
This decrease to a practically stationary poinl in the surface soil, 
accompanied by an increase in the Lower depths, was also not iced in 
the ease of the phosphoric acid, but the increase, especially in the 
third depth, is more marked in the ease of the potash. It has already 
been surest ed that this may to SOine extent be attributable to the 
deeper extension of root growth as the surface soil has become poorer 
and to the accumulation Of root remains thus formed. 
In the case of plats and 10b, receiving ammonium salts only, 
we find that the very early dressings of minerals (which included 300 
pounds of potash per acre), applied in L848 and L850 1<> plat 10b, 
seemed to show in L881, as in 18!»3, an appreciable effect on the citric- 
acid-soluble potash, both in the lirst and second depths. Plat 10a 
seems to have reached in l s ( .»:i t he same Level of available potash as 
the unmanured plat after a diminution between L865 and L881. Asa 
matter of fact, in I860 plat L0a was showing Little indication of potash 
exhaustion, for in 1866 its crop yielded pounds of potash per acre, 
while thai of plat 3 contained but L6 pounds of potash per acre. It 
is possible that the ammonium salts acted as solvents on the soil pot- 
ash. r>y L881, however, plat LOa set m> 1<» have become appreciably 
poorer in potash than plat :> in all three depths. Bui by 1893 there is 
a recovery throughout in plal LOa, possibly owing to the greatly dimin- 
ished out put of crop on account of t he failure of phosphal ic food and 
to the continued solvent action of the ammonium salts, and possibly 
(for the increase is mainly in the subsoil) partly owing to the effects 
of deeper root development. 
On plats LI, 11', and 14, which receive ammonium salts and phos- 
phates without potash, there is in the surface soils a uniform diminu- 
tion between 1805 and isu:j. and in two of the three cases between 
1SS1 and lS'.i:;. If we compare the lSDo and 1 S!»:j figures for the sec- 
ond depths of the same plats we also see, on the whole, a diminution. 
When the results for the potash-manured plat (13) are examined 
the effect of the potash salts is strikingly seen in all three sets of 
samples not only in the first depth of the soil but also in the second 
1) inches. In the third 9 inches the difference is clearly apparent only 
in the L893 samples. 
The comparative significance of the results obtained in the samples 
drawn at the end of each of the three periods may be studied in 
the following table, which I also quote from the paper presented to the 
Royal Society. 
As in the corresponding phosphoric-acid table, the estimated excess 
Or deficiency of potash on each plat is obtained by taking- into account 
