382 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1926 
basis will necessarily be a slow procedure, the outcome of careful experi- 
mentation, but fortunately a general though not infallible guide to the need 
of soil acidity for a particular species is already in existence in such well- 
known works on gardening as Nicholson’s Illustrated Dictionary of Garden- 
ing and Bairey’s Standard Cyclopedia of Horticulture. European gardeners 
have learned from long and cumulative experience that certain plants thrive 
best when supplied with peat, and this knowledge has been handed down to 
us in garden literature, and in garden practice when conducted intelligently, 
but never apparently with any suggestion that the essential quality of the 
peat was its acidity. The statement in any reliable work on gardening that 
a particular species requires peat may be taken as good evidence that this 
species is an acid-soil plant. In very many cases, however, especially in 
American works, even this evidence is lacking. Fortunately there has been 
published very recently (May, 1926) by Dr. Edgar T. Wherry a paper that 
contains lists of plants classified according to the degree of soil acidity at 
which they thrive best.* ” 
CONCLUSION 
If, contrary to the advice in the preceding paragraphs, a planting 
of acid-soil plants has been made in a nonacid bed, the plants can 
probably be saved by proper applications of aluminum sulphate. If 
an acid-soil bed has become neutral as a result of the use of hard 
water, or by reason of the excessive decomposition of the peat or the 
leaves originally placed in the bed, or from any other cause, treat- 
ment with aluminum sulphate will probably prove beneficial. If the 
cost of preparation of an acid-soil bed is prohibitive, in a locality 
in which the necessary materials are not easily available, then the 
acid-soil plants may be tried in an ordinary fertile neutral soil after 
it has been acidified by means of aluminum sulphate. 
1 Soil reaction in relation to horticulture,’ 1926, Bull. 4, American Horticultural 
Society, 14 pp. 
122 Much of the information contained in the eight preceding paragraphs has been used 
for several years past, under the title ‘‘ Experiments in rhododendron culture,” to answer 
letters of inquiry on this subject addressed to the United States Department of Agri- 
culture. It was published in 1923, so far as the experiments up to that date permitted, 
on pages 336 to 341 of L. H. Bailey’s ‘The cultivated evergreens,” under the heading 
“ Acid soils for certain broad-leaved evergreens.” 
