3i8 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1926 
’ 
lime in the form of calcium sulphate. Repeated tests show that after 
the application of aluminum sulphate to a soil of this type, the first 
leachings contain only a trace of aluminum but an abundance of 
calcium sulphate. The change in the soil reaction from neutrality 
or alkalinity to acidity is doubtless due at first to the acidity of the 
aluminum sulphate itself, but the continuation of the acid reaction 
is due apparently to the fact that the calcium and other substances 
that could neutralize soil acidity arising from aluminum salts or 
other causes, have been removed by the treatment described. The 
resulting condition is substantially that which occurs in a peat soil, 
the particular characteristic of which is acidity caused by the pres- 
ence of organic substances. In the two lots of plants illustrated in 
Plate 4 the soil of the healthy plant at the right, seven weeks after 
the last application of aluminum sulphate, had an acidity of 10 on 
the Wherry scale,®° while the soil of the untreated and sickly plant 
at the left was neutral. To summarize the matter, the application 
of aluminum sulphate may be regarded as an effective and rather 
inexpensive means of changing the reaction of a soil from neutral 
or alkaline to acid. 
APPLICATION OF THE EXPERIMENTS 
The aluminum-sulphate experiments described in this paper have 
not yet been extended by the writer to large plants growing in the 
deeper soils of outdoor plantings. For such situations amounts of 
aluminum sulphate up to a pound per square yard may be applied 
advantageously and safely if the soil is a loam of the ordinary fertile 
type, the application being repeated if the soil is not made acid by 
the first application. In applying ground aluminum sulphate to an 
outdoor bed the material should be distributed evenly over the 
ground, and mixed into the surface soil with a rake. The bed should 
then be watered thoroughly with as much as 2 to 3 inches of water in 
order to dissolve the sulphate and carry it deeply into the soil. The 
water should be so applied that it will not run off the surface but will 
sink through the bed past the roots, and leach out underneath. For 
greenhouse experiments 1 part of aluminum sulphate to 200 parts 
of soil, by bulk, may be taken as a standard experimental mixture. 
Persons desiring to experiment with sickly outdoor rhododendrons 
or other acid-soil plants are advised to apply the aluminum sulphate 
to only a portion of a planting, always leaving another portion un- 
treated for comparison. 
If a soil is already sufficiently acid, the application of aluminum 
sulphate is useless. 
° Bdgar T. Wherry, 1922, “ Soil acidity—its nature, measurement, and relation to plant 
distribution,” published in the Smithsonian Report for 1920, pp. 247 to 268. Also, by 
the same author, “ Soil reaction in relation to horticulture,” published in May, 1926, as 
Bulletin 4 of the American Horticultural Society. 
