336 THE CULTIVATED EVERGREENS 



from a peaty swamp is the best material to use, with an abun- 

 dant winter mulch of forest leaves." 



ACID SOILS FOR CERTAIN BROAD-LEAVED 

 EVERGREENS.— CoviLLE 



In the course of a series of experiments with blueberry- 

 seedlings, 1906 to 1910, it was found that these plants require 

 an acid soil.* The experiments have since been extended to 

 many other plants and it has been shown conclusively that a 

 very large number of species in ornamental horticulture have 

 the same requirement. Lack of success with some of the most 

 beautiful evergreens is due to failure to provide them with the 

 acid soil they demand. This is true especially of rhododendrons 

 and nearly all other evergreens of the heather family, such as 

 mountain-laurel {Kalmia latifolia), trailing arbutus {Epigcea 

 repens), and heather {Calluna vulgaris). 



In nature, acid nourishment is provided by the accumula- 

 tion, on the surface of the ground, of a layer of half-rotted 

 leaves, twigs, and rootlets. Such an accumulation, when it 

 occurs in a sphagnum bog, is called bog peat, or simply peat; 

 on well-drained sandy or gravelly soils it is called upland peat. 

 Under good conditions upland peat is laced into a tenacious 

 mat, a few inches in thickness, by the roots of the ericaceous 

 plants that accompany it, and this mat persists year after year, 

 continually renewing itself through each year's leaf -fall and the 

 penetration of new roots into the decaying mass. Upland peat 

 is normally brown, but is often blackened by ground fires. 



On limestone soils or on soils which for any reason have an 

 alkaline chemical reaction, upland peat does not form. The 

 lime and other alkaline substances in the soil greatly hasten 

 the decomposition of the leaves. Each year's leaf -fall is de- 



*"Experiments in Blueberry Culture," 1910, published as Bull. 193, Bur. Plant 

 Industry, U. S. Dept. Agr., 100 pages of text, 18 plates, and 31 text figures. 



