14 BULLETIN 652, U. S. DEPARTMENT OF AGRICULTURE. 



tends in a decreasing degree as deep as the soil is drained. Tests have shown 

 that undrained Louisiana muck shrinks about 60 per cent in volume when 

 completely dry, and that it will regain only 70 per cent of its original volume 

 when saturated for a long time. Therefore long-continued dry weather and deep 

 drainage cause a shrinkage in the deeper layers of muck which is not fully 

 offset by any increase in volume which occurs after subsequent precipitation 

 or rising of the water table. Only such part of the muck as is always saturated 

 is entirely free from shrinkage due to drying. 



The vegetable material in muck soils exists in a state of partial decay. In 

 the undrained state it is so saturated with water that the process of decay is 

 relatively slow. After drainage the air enters, and decay is much more rapid. 

 The warm, humid climate of southern Louisiana is very favorable to the rapid 

 decay of vegetable material, much more so than are those sections where the 

 surface is frozen for a part of the year. Like the effect of drying, that of 

 decay is greatest in the top layer of material, but examination has shown that 

 after some years of drainage the character of the muck at a depth of 2 feet is 

 materially changed. While the effect of decay is not as rapid in action as 

 that of drying, it is practically continuous. The complete decay of the vege- 

 table matter causes some loss of weight and considerable loss in volume, thus 

 gradually reducing the surface elevation of the muck. 



As the effects of both the foregoing agencies are greatest in the top layer of 

 the muck, the density of this layer is gradually increased. After this com- 

 paratively dense material attains a thickness of a few inches it prevents free 

 circulation of the air into the muck below, and then drying and decay are much 

 slower in their action. Eventually this layer attains such a thickness that 

 further subsidence of the surface is scarcely noticeable. 



Cultivation increases the subsidence directly by the mechanical effect of 

 weight compacting the soil, and indirectly by accelerating the action of drying 

 and decay. Muck soils that are so soft after drainage that they will not permit 

 of the use of farm animals and machinery, are compacted from 4 to 6 inches by 

 the first plowing. This first plowing is usually done with some form of tractor 

 with broad wheels that cover practically all the surface plowed. Subsequently, 

 when the muck is cultivated with farm animals and machinery, the surface re- 

 ceives unit pressures far greater than exerted by the broad-wheeled 

 tractor, and a further compacting results. The underlying material turned to 

 the surface by plowing is exposed to a greater drying action than otherwise 

 would result. Decay, also, is hastened in the material thus brought to the sur- 

 face. It is the experience in cultivating newly reclaimed muck soils that for a 

 number of years after the first cultivation a uniform depth of plowing will 

 bring to the surface each year a considerable layer of muck which was undis- 

 turbed by the previous year's plowing. This layer of new material decreases in 

 thickness from year to year, until finally the cultivated layer attains such 

 density that the combined forces of drying, decay, and compacting reduce its 

 thickness very little. If the land is not plowed deeper than this layer the sub- 

 sidence of the remaining muck is very slow ; but if the land is plowed deep 

 enough to reach undisturbed muck, further subsidence results. 



LIABILITY OF BURNING. 



In the reclamation of turf lands of this character there always is more or 

 less danger that the muck will burn. On some of the newer plantations trouble 

 has been experienced in burning off the growths of weeds and grass that covered 

 the muck. This burning off can be done with safety only when the muck is wet 

 from a recent rain. In the spring of 1910, which was the driest in southern 



