30 BULLETIN" 902j U. S. DEPARTMENT OF AGRICULTURE. 
or sinks and becomes buried as a layer of fibrous peat firm enough to 
support trees. 
Bog-plant associations may occur in marshy and wooded areas of 
valleys and uplands, and they may form definite sedge or "grass" 
bogs. " moss " bogs, and " heath " bogs under field conditions which 
inhibit the natural growth of plants other than the bog xerophytes. 
They are believed to be glacial relicts from a former more general dis- 
tribution of boreal plant-, some of which are represented in Europe 
and Asia by the same or closely related species. Most of these plants 
are now confined to an area extending from the north Atlantic to 
the Mackenzie basin in northern America, in which they attain 
their best development, and to locations with high atmospheric hu- 
midity. In their southern limits of distribution they maintain them- 
selves, it seems, on account of soil conditions which indicate physio- 
logical drought as distinguished from physical drought. A discus- 
sion of the selective action of the soil stratum which allows bog 
plants to outgrow others, the effects on certain cultural plants used 
experimentally, and the possible causes of physiological drought 
offered in explanation of this fact has been published elsewhere 
(5; 20). 
TThere climatic conditions, such as high atmospheric humidity, are 
especially favorable these plant associations may spread laterally 
over adjoining land surfaces or move up well-defined slopes of low 
hills. If the ground water, designated aerial as distinguished from 
telluric, ri-es in proportion, they may react upon other vegetation 
units. In northern countries of Europe they build up high moors, 
invade forests, and destroy with their accumulation a part or all 
of the tree covering. 
The peat materials of this group weather slowly if improperly 
drained, and they require fall plowing and freezing, with frequent 
packing by heavy rollers, to maintain good tilth and the upward 
movement of soil water. Of considerable practical moment is the 
fact that as a general rule the amount of mineral matter is very low 
in these types and that applications of complete mineral fertilizers 
and stable manure for inoculation with beneficial soil microorgan: 
appear to be more effective in establishing a normal balance of 
plant-food constituents and of bacterial action than in any of the 
other groups and types of peat material. The cultivation of grasses 
for hay and pasture or of potatoes as a first crop and corn, preferably 
for silage purposes, are known to be profitable in the preparation 
of these peat types for other farming practices. 
The shrinkage on dewatering is very much slower than that which 
occurs when marsh or swamp and aquatic types of peat are sub- 
jected to drainage operations. TTith irnderlying water-formed 
