PLANT SUCCESSION 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 t 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, usually 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 1 
and to a slight extent to direct evaporation from the soil. 2 
The most important 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 in 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. Department of 
Agriculture, Odessa : 17 : 1913. 
