244 AGRICULTURAL INDICATORS. 



"The sand-hills mixed association indicates conditions very similar to those 

 of the bunch-grass association, but rather less favorable, as shown by the 

 smaller amount of plant growth. 



"The short-grass vegetation represents the final stage in a succession which 

 may begin with the lichen formation and pass through the Gutierrezia- 

 Artemisia association. Or the succession may begin with the blow-out asso- 

 ciation and pass through the sand-hills mixed and the bunch-grass associa- 

 tions and (by the aid of fires and grazing) through the wire-grass association 

 to a pure short-grass vegetation. 



"When short-grass land is left without cultivation after breaking it will be 

 revegetated by either the wire-grass or the Gutierrezia-Artemisia association, 

 depending upon the physical conditions. 



"The vegetation which establishes itself after wire-grass is turned under is 

 that which is naturally characteristic of a lighter soil. 



"When the native sod of the bunch-grass or the sand-hills mixed associ- 

 ations is broken, a blow-out may result. Usually, however, the original 

 vegetation is soon reestablished. 



" When the vegetation of any of the plant associations is destroyed by break- 

 ing and the land is then abandoned the land will be reoccupied (after a weed 

 stage) by vegetation that is characteristic both of a lighter type of soil and of 

 an earlier stage in the natural succession. These successions are the result 

 of changes in the physical conditions brought about largely as a result of the 

 destruction and reestablishment of the plant cover itself. 



"The taller, deeper-rooted plants are easily shut out by the shallow-rooted 

 short grasses when the water that falls as rain is not sufficient to penetrate 

 beyond the layer of soil occupied by the roots of the short grasses before it 

 can be absorbed by them. 



"Where water can readily penetrate below the depth ordinarily reached by 

 the roots of the short grasses the conditions are favorable to the growth of 

 deeper-rooted and taller species, which shut out the short grasses by over- 

 shading them. This increased penetration of water may be due either to 

 greater rainfall or to lighter soil texture. 



"When well supplied with water short-grass land is the most productive 

 under cultivation of any in eastern Colorado. During drought, however, 

 crops suffer on this land sooner than on any other type. 



" During exceptionally dry years bunch-grass land produces the best crops 

 of any in eastern Colorado, but during wet years its production is surpassed 

 by that of all others except the land characterized by the sand-hills mixed 

 association. The soil under both of these types of vegetation is likely to 

 blow badly. 



"Wire-grass land represents a safe intermediate condition where in years of 

 ample rainfall crop production compares not unfavorably with that on short- 

 grass land and where, even during dry years, a fair crop can often be produced. 

 "One of the chief reasons for the superiority of light land over heavy land 

 in eastern Colorado is that crop growth is rapid on the latter and that the 

 total available supply of soil water lies near the plant roots, the crops, there- 

 fore, being in somewhat the same condition as potted plants. These con- 

 ditions favor a rapid exhaustion of soil moisture and, consequently, bring 

 about sudden drought. On the lighter land water is distributed to greater 

 depths, the plant growth is slower, and plants, by gradually increasing their 

 root area, can resist much longer periods of drought. 



"Investigations of soil conditions, as well as actual observations of crops 

 in the field and studies of the native plant cover, show that as we pass from 

 the prairie westward to the more arid portion of the Great Plains, the lighter 

 soils present relatively more favorable moisture conditions and, therefore, 

 conditions more favorable to plant growth than do the heavier types of land. 



