ENVIRONMENTAL AND HISTORICAL FACTORS. S89 
from precipitation water held in the deep surface layer of highly reten- 
tive clay loam. 
Formerly the river spread over this plain in times of flood and made 
of it a marshy area or cienega, but deep gullying of the stream-channel 
has lowered its bed several meters and rendered the present flood-plain, 
where not irrigated, a parched and barren waste in the dry seasons. In 
uncleared land occur numerous low trees of mesquite, catclaw (Acacia 
greggut), etc., but, as stated above, these may be able to obtain water 
from deeper sources. Where irrigation water is conducted to it this plain 
is exceedingly fertile, and in the rainy seasons it supports, without irri- 
gation, a luxuriant growth of those herbaceous plants which are active 
only at these times. 3 
Evaporation from this soil, while much more rapid than from the soils 
of the hill, is considerably retarded, and here also the dry mulch is notice- 
ably effective. The fact that the surface is nearly horizontal, so that 
precipitation water does not rapidly run off, gives to this soil type a some- 
what larger water-supply from precipitation than that received by the 
other types here considered. 
THE MARCH OF THE SOIL-MOISTURE CONTENT. 
The prime importance of the moisture-content of the soil, in determining 
the behavior of plants, makes it a matter of keen disappointment that no 
very satisfactory method for measuring this function is available. An 
instrument which should give data on the amount of soil-moisture at a 
given location and depth, without the necessity of disturbing the soil, would 
be as great a boon to agriculturists as to students of plant distribution, 
but, unhappily, no instrument of proved reliability is yet available. As 
far as I am aware, only two instruments have been suggested for the 
determination of the moisture of the soil 7 sztu, and with both these 
there are practical difficulties of operation, or interpretation of results, 
which prevent their being wholly satisfactory for field observation.’ In 
the absence of any better, the commonly used, but very crude method of 
sampling and drying was resorted to. 
Beginning on October 3, 1907, and continuing till April 11, 1908, a series 
of soil samples was taken, at intervals of about 10 days, from four stations 
representing the four soil types with which we have to deal. These were 
dug with pick and trowel and collected in tightly stoppered bottles of 
known weight. The data of moisture-content for these samples were 
then determined in the ordinary way, by weighing, drying in the oven, 
_ (temperature 105° to 110° C.), and reweighing. All samples were taken 

1For these two methods see N. Y. State Agric. Exp. Sta., Ann. Rep., 4: 176-179, 
1886, and Briggs, L. J., Electrical instruments for determining the moisture, tempera- 
ture, and soluble-salt content of soils, U. S. Dept. Agric., Div. Soils, Bull. 15, 1899. 
Mention should also be made of bulletins 45 and 50 of the Bureau of Soils, which are 
occupied chiefly with the determination of the moisture equivalents, or optimum 
water-contents of the soil. 
