50 



BULLETIN" 791, U. S. DEPARTMENT OF AGRICULTURE. 



The wide difference in the humus content accounts chiefly for the 

 liighcr 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.^ 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. 



Tajble T. — Summary of vegetative g-roiisth and irater requirements of peas and 

 wheat developed in soils characteristie of the irJieat-grass type and of the 

 rvderal-iceed cover. 



Plant. 



Peas. 



Ruderal- 

 weed soil. 



Wheat- 

 grass soil. 



^^'heat. 



Ruderal- 

 weed soil. 



Wheat- 

 grass soil. 



Number of leaves 



Leaf length Tmn . 



Dry weight grams. 



Water used per plant do... 



Water requirement per unit dry matter do . . . 



42 

 791 

 0.79 

 667 

 841 



112 



2,6.34 



6.55 



3,051 



467 



22 



4,474 



5.52 



2,516 



472 



47 



10,080 



12. 09 



3, ^2fl 



343 



Both wheat and peas show a striking contrast in the vegetative 

 growth and in the water requirements in the two soil types. The 



^ 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 30 

 per cent of water. The soil was thoroughly mixed, sa 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 §rown 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 1| inches in. diameter was provided, which was used in watering- and which 

 was fitted with a cork stopper and capillary tube for the cu'culation of air. Before the lid 

 was put on sufficient soil was removed from the surfa.ce 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 li iaehes 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 surgeons 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 (Triticvm 

 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 2S4 : 8-14. 

 1913. 



