ACID-SOIL PLANTS——COVILLE 377 
been published, but a brief description of it will be instructive in its 
relation to the present subject. On May 26, 1923, 13 small plants of 
the Japanese variety of hydrangea known as Otaksa were potted in 
5-inch porous earthenware pots in a soil consisting of rotted turf 
loam 1 part, well-rotted cow manure 1 part, sand 1 part, and pounded 
crocks 3 parts. On July 18, 1923, one of the plants bloomed. Its 
flowers were pink. Ground aluminum sulphate was applied to the 
surface of the soil in each pot at the rate of 1 part, by bulk, to 200 
parts of the soil. In the fall the plants were placed in a coldhouse 
for chilling, and on January 12, 1924, they were repotted in 6-inch 
pots in the same soil mixture as before, and moved to a warmhouse, 
55° to 70° F. Two days later an application of aluminum sulphate 
was made, in the same proportion as the first, and washed into the 
soil by repeated sprinkling with water as at the first treatment. Other 
plants were given the same soil and treatment, except that no alumi- 
num sulphate was applied. Others were given the same treatment, 
but in a soil consisting of 2 parts of kalmia peat, 1 part of sand, and 
3 parts of crocks, or instead of crocks soft-coal cinders. 
Plate 12, from a photograph made May 14, 1924, shows two plants 
typical of the lots they represent. The flowers at the left, of a pink 
color, were grown in the loam mixture without aluminum sulphate; 
those at the right, deep blue in color, were from the loam mixture 
treated with aluminum sulphate. The pink flowers came from a 
neutral soil, the blue from an acid soil. Blue flowers were produced 
also on the plants in a peat and sand soil, which was acid, although 
no aluminum sulphate was used. 
The use of druggists’ alum, placed in chunks in the pots to make 
pink hydrangeas produce blue flowers, is an old practice of florists, 
but the interpretation of the cause as a change in the soil reaction 
from neutral or alkaline to acid appears not to have been made 
until the outcome of the New Brunswick and Washington experi- 
ments in 1924.° 
NATURE OF THE ACTION OF ALUMINUM SULPHATE 
The nature of the fundamental action of aluminum sulphate on a 
neutral soil or an alkaline soil appears to be the replacement of the 
lime in the soil by aluminum, and the leaching away of the released 
>It now appears that the same conclusion was reached a year earlier, by still another 
investigator. After the present paper had been sent to the printer, a colleague directed 
my attention to a paper by W. R. G. Atkins entitled ‘‘ The hydrogen ion concentration of 
the soil in relation to the flower colour of Hydrangea hortensis W., and the availability 
of iron,” published in June, 1923, in the Scientific Proceedings of the Royal Dublin 
Society, vol. 17, new series, pp. 201 to 210. The author concluded that in acid soils the 
house hydrangea produces blue flowers, and in alkaline soils pink flowers, and that the 
cause of the blue coloration is the presence, in the flower, of an unusual amount of 
iron, dissolved from the soil by reason of the acidity of the soil solution, absorbed by 
the plant in excess of its ordinary iron requirements, and therefore present in the state 
of “inorganic” iron, the direct effect of which is to turn the pink coloring matter of the 
flower blue. 
