38 BIG SPRING prairie; 



4th. The erosion of surface soil in the vicinity of 

 ditches. 



The above four causes are direct results of artificial 

 drainage. 



This settling of marsh soil after drainage becomes 

 of economic importance. For, if there is just enough 

 difference in level to secure proper drainage when the 

 first ditches are dug, the area in the course of years, is 

 bound to revert to the semi-marsh type, if the muck is 

 sufficiently thick. 



If, however, the fall in outlet ditch is sufficiently 

 great, the ditches must be deepened from time to time, 

 until the first two causes have approximately accom- 

 plished their work. The third force or cause does not 

 manifest itself on all drained marsh areas. But wherever 

 it is liable to occur, provision must be made both for 

 prevention, if possible; and for staying its destructive 

 progress. 



Figure 5 illustrates a method employed on Big 

 Spring Prairie to stay the ravages of prairie fires. The 

 dry prairie grass was set on fire by sparks from a pass- 

 ing engine. The sedgy or grassy hummocks project- 

 ing above the general level are quite dry, and hence are 

 set on fire by the burning grass. The heat from this 

 burning portion expels the moisture from the soil in 

 immediate contact with it and raises it above the ignit- 

 ing point and the fire spreads. Before the real value of 

 this marsh land was learned, little effort was made to 

 check its progress unless it threatened to encroach upon 

 fences or the land adjoining prairie. But, after Mr. 

 W. C. Johnson proved the value of this marsh land by 

 developing a celery and truck farm upon it, the land 

 rose in value from ^10 an acre to ;^75 or even more. 



At present if a prairie fire originates, which threat- 

 ens the burning of the soil, a trench is dug around the 

 burning area. If the trench is deep enough to reach 

 very wet soil, it effectually prevents its further ravages 



