BROAD-LEAVED EVERGREENS 337 



composed, much of it passing in liquid form into the underlying 

 soil, prior to the leaf-fall of the following year. Fully decom- 

 posed leaves form a true leaf-mold, black in color and neutral 

 or alkaline in reaction, in which rhododendrons and other acid- 

 soil plants will not grow. 



The continuation of acidity in upland peat is due to the 

 arrest of decomposition before it has progressed to the alkaline 

 stage, and the chief factor in the arrest is the lack of lime in 

 the soil that underlies the leaves. When an upland peat mat is 

 once established, its own acidity is fatal to the life of the organ- 

 isms that as agents of rapid decay soon destroy its acidity. 



In soils derived from granite, sandstone, sand, and gravel, 

 acid conditions are usually maintained with little difficulty by 

 the addition of upland peat, half -rotted oak leaves, or decayed 

 wood or bark. 



Sawdust and spent tanbark are acid materials useful as 

 mulch for acid-soil evergreens. They should be applied experi- 

 mentally at first, however, to test the safety and suitability 

 of the particular kind that is available. Some kinds of sawdust, 

 notably red cedar and pitch pine, contain, when fresh, sub- 

 stances that are directly injurious. Other sorts, such as bass- 

 wood, maple, and birch, are free from them. In general, it is 

 best to use sawdust that is weathered and partially decayed. 



When an attempt is to be made to grow rhododendrons or 

 other acid-soil evergreens in a place in which the soil is neutral 

 or alkaline, such as a limestone soil, the bottomland of a river 

 valley, the ordinary fertile garden, or a prairie or arid-region 

 soil, it is necessary to prepare holes or trenches and make up a 

 special soil mixture. This should consist of one part of clean 

 sand to one or two, or even four, parts of upland peat or its 

 equivalent. To keep earthworms from bringing up the under- 

 lying soil, the bottom of the hole should be lined with a two- 



