ACID-SOIL PLANTS—COVILLE 379 
Outdoor experiments with aluminum sulphate should not be tried 
in mixed plantings unless it is known that all the plants are suited 
to a strongly acid soil, because the ordinary plants of horticulture, 
which require a soil with a neutral or alkaline reaction, are likely to 
be severely injured, or killed, by the aluminum sulphate. 
Crude aluminum sulphate, such as was used in these experiments, 
is commonly known in the trade as sulphate of alumina. It is em- 
ployed extensively in the chemical industries and is not expensive. 
In large quantities it can be purchased at about $30 a ton. 
Experiments that have been in progress for several years have 
shown that soil acidity is required not only for rhododendrons, frank- 
linias, and blueberries, but for azaleas, kalmias, heather, trailing- 
arbutus, wintergreen, and practically all the plants of the heath 
family, besides pink ladyslipper, sweet ladies-tresses, and many other 
orchids, and numerous other plants of ornamental horticulture that 
are commonly regarded as difficult of cultivation, such as bunch- 
berry, vernal iris, birdsfoot violet, painted trillium, galax, pitcher- 
plant, and Venus flytrap. There is every reason to expect that these 
other plants also can be made to thrive in ordinary soils through the 
use of aluminum sulphate, provided the soil does not contain too 
much clay, for a heavy clay soil is unsuited, for other reasons, to 
most acid-soil plants even after it has been acidified. A knowledge 
of the usefulness of aluminum sulphate in the culture of acid-soil 
plants is likely to be of importance at the present time when the 
importation of these plants has been greatly curtailed through the 
plant-quarantine laws, and nurserymen are now trying to grow the 
needed plants inside the United States. Before the aluminum-sul- 
plate treatment is applied extensively to ericaceous plantings, how- 
ever, it remains to be determined whether the treatment if long con- 
tinued may not lead to the development of unforeseen difficulties, 
such as the formation of compounds of sulphur injurious to erica- 
ceous plants. For the present the aluminum-sulphate treatment 
should be regarded as experimental. 
NATURALLY ACID SOILS PREFERABLE 
Readers of this publication are especially asked not to conclude 
from these experiments that the best way to grow rhododendrons and 
other acid-soil plants is to put them in a neutral or alkaline soil and 
then apply aluminum sulphate. The best way to grow such plants 
is to remove the neutral or alkaline soil and put in its place a bed 
of naturally acid soil, such as sand mixed with peat, rotting wood, 
or half-rotted oak leaves. (PI. 13.) Detailed directions for the 
preparation and maintenance of such acid soils are given in other 
