380 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1926 
publications. The following restatement, however, will be useful 
here: 
In nature acid nourishment is provided by the accumulation, on the surface 
of the ground, of a layer of bhalf-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 
decomposed, much of it passing in liquid form into the underlying soil, prior 
to the leaf-fall of the following year. Fully decomposed leaves form a true 
leafmold, black in color and neutral or alkaline in reaction, in which 
rhododendrons and other acid-soil plants will not grow. 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 
plants. They should be applied experimentally at first, however, to test the 
safety and suitability of the particular kind that is available. Some kinds 
of sawdust, notably redcedar and pitch pine, contain, when fresh substances 
that are directly injurious. Other kinds, such as basswood, maple, and birch, 
are free from these substances. In general it is best to use sawdust that is 
weathered and somewhat decayed. 
When an attempt is to be made to grow rhododendrons or other acid-soil 
plants in a place in which the soil is neutral or alkaline, such as a limestone 
soil, the bottom land 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 1 part of clean sand to 1 or 2, or 
even 4 parts of upland peat or its equivalent. To keep earthworms from 
bringing up the underlying soil the bottom of the hole should be lined with a 
2-inch layer of soft-coal cinders. The depth of the peat and sand mixture 
need not be more than 8 to 12 inches. If the materials for the mixture are 
available in quantity a bed may be laid down over the whole surface of the 
ground. A permanent mulch of oak leaves will help maintain a proper degree 
of moisture and by decomposition will add to the peat supply. 
In choosing peat for the eulture of acid-soil plants two mistakes should 
be avoided. First, certain swamps contain a deposit that looks like peat but 
is neutral or alkaline in chemical reaction. 'The soil of such swamps, to. which 
the name muck should be applied, is well suited to the culture of onions, 
celery, and lettuce, but altogether unsuited to the culture of rhododendrons 
and other acid-soil plants." Second, the much decomposed peat in the sub- 
merged lower layers of deep bogs, such as is used for fuel in Wurope, or the 
lighter kinds for stable bedding, is not suitable, by itself, for acid-soil plants. 
It is many years, often centuries, old and although it may furnish the needed 
7For a further discussion of the opposing characteristics and uses of peat and muck, 
see “The agricultural use of acid peats,’’ published in January, 1925, in the Journal of 
the American Peat Society, vol. 18, pp. 5 to 7, pls. 1 to 4. 
