24 BULLETIN 802, U. S. DEPAKTMENT OF AGRICULTURE. 



association and the rise of the water table is favorable for their 

 growth, they become buried as extensive or fragmentary layers, essen- 

 tially intact and fibrous or partially disintegrated. On account of 

 the slight shrinkage of the plant remains, the respective layers which 

 are "built up " in this manner may constitute several feet of a deposit, 

 but further increases in thickness practically cease unless there is 

 considerable disintegration and shrinkage in the material or a rise in 

 the ground- water level from any cause. 



When an accumulation of structureless or macerated type of pet^l 

 material formed in large part by aquatic vegetation units preceded 

 and reached nearly to the water level, peat deposits of this class show 

 as a rule a fibrous matted layer, more or less sharply differentiated 

 in structure from the peat material below it. By the constantly ad- 

 vancing growth of semiaquatic and marsh plants the fibrous mate- 

 rials may gradually extend over the shoaling water, sink as their 

 thickness increases, and thus give rise to fiUed-in deposits of peat. 

 Water pockets are not uncommon as delimiting strata. 



This has an important bearing upon drainage, tillage, and other 

 operations, and upon the possible uses of the land. In planning 

 drainage improvements it is well to remember that deposits not too 

 well drained are safer than those overdrained and that in this 

 respect the distance between ditches and tiles is usually more 

 effective and important than their depth. For areas of greater 

 depths of peat or with poorly disintegrated grades of fibrous peat 

 material it is necessary to anticipate a gradual but certain sub- 

 sidence to as much as one-fourth of the original thickness. The 

 decrease in the elevation of the surface should be met, therefore, by 

 a provision for deepening the gravity outlet or for drainage by means 

 of pumps. 



The marsh types of peat are among the most profitable for agri- 

 cultural purposes, though several of them indicate in some instances 

 the presence of salts which in sufficient quantity may injure certain 

 cultivated crops. The nature of the mineral subsoil, whether clay 

 or sand, may considerablj" affect the agricultural value and need 

 for fertilizers, the former responding, it seems, more readily to phos- 

 phates, the latter requiring phosphatic and potash manures, and to 

 some extent lime. Marsh or fen types of peat are easily cleared and 

 brought under cultivation, and they are suitable for a variety of 

 crops if carefully drained or if the drainage channels are provided 

 with check gates. Some of the types when overdrained are apt to 

 become brittle and easily break down to light-colored "mull " or 

 granular peat dust. This is especially characteristic of fibrous 

 sedge and of brown-moss (Hypnum) types of peat. Moreover, 

 marked differences may arise from the preponderance of semiaquatic 



