PLANT SUCCESSIOISr AND RANGE MANAGEMENT. 51 



vegetative development was appreciably greater in the wheat-grass 

 soil both in the number of leaves produced and in the total leaf 

 length. The difference in the development of the plant as a whole 

 is best expressed by the dry weight produced. In peas the propor- 

 tion of dry weight was as 1 to 8.2 in favor of the wheat-grass soil; 

 in wheat the proportion was as 1 to 2.2 in favor of the same soil 

 type. The water requirement for the production of a given unit of 

 dry weight, on the other hand, was much greater in the ruderal-weed 

 soil than in the wheat-grass soil, the proportion being approximately 

 2 to 1 in peas and 2 to 1.4 in wheat. 



The higher water requirement of plants grown in the less fertile 

 soil is particularly significant in view of the fact that impoverished 

 soils absorb and retain very much less water than do the more fertile 

 soils. This fact, coupled with the low fertility of the soil, chiefly 

 accounts for the presence of the temporary weed cover on badly- 

 impoverished, as well as newly-formed soils. 



SOIL WATER CONTENT. 



The soil moisture conditions on a first-weed-stage area during the 

 season of 1915 are summarized in figure 20. Considering the general 

 trend of the curves representing the available water content from 

 to 6 inches, 6 to 12 inches, and 12 to 24 inches in depth by 10-day 

 periods throughout the growing season, it will be observed that there 

 is a rather sharp decline in the water content of the soil from July 1 

 to September 20. Except in the last period (September 10 to 20) 

 the highest amount of available plant water was found in the 12 

 to 24 inch layer. The to 6 inch soil layer, on the other hand, con- 

 tained the lowest water content throughout the entire period. When 

 saturated the upper 6 inches of soil, except where severe washing 

 has occurred, ususflly contains several per cent more moisture than 

 the soils of greater depth. The reduction in the water content of the 

 upper soil layer is, of course, chiefly attributable to transpiration^ 

 and to a slight extent to direct evaporation from the soil.- 



The most imj)ortant physiological fact brought out in the graph 

 is the period at which the soil water content becomes unavailable 

 to the vegetation. In the case of the all-important upper 6 inches of 

 soil, from which the first-weed-stage plants procure by far the 

 greater part of their moisture supply, "the water becomes unavailable 

 to the vegetation between August 1 and August 10. In the 6 to 12 



1 Germination and growth in the case of this cover ia 1915 began on June 23. 



2 Loss of water by direct evaporation from the soil is slight as compared with that lost 

 by transpiration from the vegetation. For a discussion on this subject see Burr, W. W., 

 " Storage and Use of Soil Moisture." Research Bulletin, No. 5, Agricultural Experiment 

 Station of Nebraska : 61 : 1914. Also Romistrov, V. G., " The Nature of Drought Accord- 

 ing to the Evidence of the Odessa Experimental Field." M. L. and A. Depai-tment of 

 Agriculture, Odessa : 17 : 1913. 



