30 BULLETIN 802, 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 plants, 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- 

 seh^es, 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). 



^A^iere 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^ rises 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 microorganisms 

 appear to be more effective in establishing a normal balance of 

 plant-food constituents and of bacterial action than in anj^ 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. With underlying water-formed 



