340 THE CULTIVATED EVERGREENS 



These experiments will be extended during the coming 

 season to larger rhododendron and other acid-soil plants in 

 the deeper soil of outdoor plantings. For such situations, it is 

 believed, amounts of aluminum sulfate up to half a pound to 

 a square yard may be applied advantageously and safely if the 

 soil is of the ordinary fertile type, the application being repeated 

 if the soil is not made acid by the first application. Outdoor 

 experiments with aluminum sulfate 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 horti- 

 culture, which thrive best in a neutral or alkaline situation, 

 are likely to be severely injured, or killed, by the aluminum 

 sulfate. For the present, the aluminum-sulfate treatment 

 should be regarded as experimental. Those desiring to try it 

 on sickly rhododendrons should apply it only to a portion of a 

 planting, always leaving another part untreated for comparison. 



Limestone water, which is alkaline in reaction, will ulti- 

 mately injure an acid-soil planting. Rainwater or some other 

 water that is neutral or even acid in reaction should be used if 

 practicable. If only alkaline water is available for sprinkling 

 purposes, it can be made neutral or slightly acid by dissolving 

 in it a suitable amount of aluminum sulfate. The proper 

 quantity can be determined by adding to a teaspoonful of the 

 treated water in a white dish a fraction of a drop of the dye 

 known as bromthymol blue. If the amount of aluminum sulfate 

 added to the water is just sufficient to make it neutral, its 

 color under this test will be green; if it has become acid, yellow; 

 if it is still alkaline, blue.* 



Evergreen species, like other plants, vary in the degree of 

 soil acidity or alkalinity to which they are best adapted. The 



*For an account of the niethod of determining the degree of soil acidity, see Edgar 

 T. Wherry, 1922, "Soil acidity — its nature, measurement, and relation to plant dis- 

 tribution," Smithsonian Rept. for 1920, pages 247-268, with 1 plate and 1 color chart. 



