THE BROADBALK WHEAT SOILS. 
103 
roach 0.01 per cent of tin- surface soil, the average number for these 
four plats being 0.0082 per pent, corresponding to a little over 200 
pounds per acre. This figure corresponds almost exactly with the 
mean figure obtained for the soils of the eight phosphat ically starved 
barley plats from lloos field, from the results of which T ventured to 
draw the tentat ive conclusion that when a soil is found by analysis 
to contain as litt le as about o.oi per cent of phosphoric acid soluble 
in a 1 percent .solution of citric aeid, used as described, it would be 
justifiable (as far as cereals air concerned) to assume that it stands 
in immediate need of phosphat ie manure. That conclusion, there- 
fore, appears to be well maintained by the new results obtained from 
ihe wheat soils. How far we may go in the other direction, in ventur- 
ing to fix a limit which shall be indieal ive of the nonnecessity of phos- 
phat ie manure, is a more difficult question to decide. The soils of the 
various superphosphate-manured wheat plats in Broadbalk field, like 
those of the corresponding barley plats in Boos field, have been abun- 
dant ly or overabundant ly supplied, and would probably grow undimin- 
ished crops for some yea is t o come, if the annual BUpplyof phosphatic 
manure ceased, nit rogen (ami other minerals) being supplied as usual. 
Those Included in our series average from <U>4 to O.O.") per cent of 
citric-acid-soluble phosphoric aeid — almost identically the averages 
found for the various corresponding barley soils. Thai this is indica- 
tive of superabundance of phosphates is evident from the fact that 
plat 8, receiving the same dressing of mineral manures, bm receiving 
half as much again of nit rogen (»'»'»(» pounds ammonium salts per acre), 
has consistently yielded considerably larger crops. We seem thus in 
a position to say that the limit of phosphatic sufficiency for cereal 
crops is somewhere below o. of [mm- cent of citric-acid -soluble phos- 
phoric acid, while as Little as about 0.0] per cent indicates phosphatic 
starvat ion. 
Tlie plat (zb) continuously dunged for fifty years gives 0.056 per 
cent, and is probably saturated with mineral fertility. Its yield is 
inferior to that of the most Liberally manured chemical plat of the 
field (not represented in our table), owing, it may be supposed, to an 
insufficient annual supply of nitrogen in a rapidly available form. 
I Mat L } a has been dunked for over nine years. Previously, for nearly 
thirty-live years, if was unmanured, except for partial dressings of 
alkaline sulphates. Its yield is about equal to the average of the 
chemically manured plats included in our table, though inferior to 
the best of them. It contains 0.03 per cent of citric-acid-soluble phos- 
phoric acid. From various considerations it appears probable that 
the lower yield of plat 2a, as compared with the much longer dunged 
plat 2b, is rather due to a deficiency in the yearly supply of available 
nitrogen than to deficiency in the present supply of mineral food. If 
this were the case our limit indicative of phosphatic sufficiency would 
be reduced from 0.04 to 0.03 percent of citric-acid-soluble phosphoric 
acid. 
