50 
BULLETIN 791, U. S. DEPARTMENT OF AGRICULTURE. 
The wide difference in the humus content accounts chiefly for the 
higher water-holding power and available plant water in 
the wheat-grass soil, which, when it was saturated, were greater by 
10.4 per cent than in the soil which supported the ruderal-weed vege- 
tation. In order to determine what effect this difference in fertility 
and available water might have on the growth and water require- 
ments of vegetation, experiments were made on three batteries each 
of the selected soils. 1 The vegetative development and the total 
water requirements of the plants grown in the wheat-grass soil and in 
the ruderal-weed soil are shown in Table 7. 
Table 7. — Summary of vegetative growth and water requirements of peas and 
wheat developed in soils characteristic of the irheat-grass type and of the 
ruderal-weed cover. 
Peas. 
Wheat. 
Plant. 
Ruderal-I Wheat- 
weed soil, grass soil. 
Ruderal- 
weed soil. 
Wheat- 
grass soil. 
42 
791 
0.79 
667 
841 
112 
2,634 
6. 55 
3,051 
467 
22 
4,474 
5-52 
2,516 
472 
47 
Leaf length 
Dry weight 
Water used per plant 
Water requirement per unit dry matter 
mm... 
grams.. 
do.... 
do.... 
10,080 
12.09 
3/20 
34.3 
Both wheat and peas show a striking contrast in the vegetative 
growth and in the water requirements in the two soil types. The 
1 The batteries were so arranged that no water escaped from the soil except by transpi- 
ration from the plants grown. 
After being carefully sifted, the soils were moistened so as to contain approximately SO 
per cent of water. The soil was thoroughly mixed, so that the moisture content was uni- 
form throughout, and was firmly packed in heavy galvanized-iron potometers, 17 inches 
high and 14 inches in diameter. Potometers of this size, having as they do a capacity of 
about 90 pounds of air-dry soil, provided a soil mass of ample space for development and 
spread of the roots of the plants selected to be grown in the potometers. The potometers 
were fitted with lids of the same material as the cans, and five equally spaced holes 
three-fourths inch in diameter were punched in each for the plants. In the center of the 
cover a hole li inches in diameter was provided, which was used in watering and which 
was fitted with a cork stopper and capillary tube for the circulation of air. Before the lid 
was put on sufficient soil was removed from the surface center of the can for the placing 
of a granite receptacle 4 inches in height and 5 inches in diameter, perforated centrally 
in the bottom and underlaid with 11 inches of gravel. This greatly facilitated the addi- 
tion of water. After the lids were placed the space between the rim and can was closed 
by securely sealing with strips of surgeon's adhesive tape, which, when dry was coated 
with shellac. In order to have as little variation as possible in the individual plants, 
pedigreed strains of Canadian field peas (Pisum arvense) and cultivated wheat (Triticum 
durum), known as Kubanka 1440, were used. In order further to insure uniformity in 
the plants the seeds were sprouted between moist blotters and the most vigorous sprouts 
transferred to water cultures from which uniformly sized plants were subsequently 
selected for planting. In planting, a small amount of soil was removed through the per- 
forations made in the lid, the roots of the sprouts were inserted, and the soil was firmly 
pressed about the roots. A combination of melted beeswax and tallow was used to seal 
over the soil exposed by the perforations made in the lid of the potometer through which 
the plants were inserted. The methods used in sealing and in watering were essentially 
the same as those devised by Briggs, L. J., and Shantz, H. L., " Water Requirements of 
Plants." U. S. Department of Agriculture, Bureau of Plant Industry Bulletin 284 : 8-14. 
1913. 
