444 Mineral Nutrition of Plants 



whether or not plants are able to remove nutrients from soil that is 

 drier than the wilting percentage. When the root system of a plant 

 was divided between a moist soil and soil with moisture content below 

 the wilting percentage, Breazeale (20) noted the root system trans- 

 ported water from the moist to the dry soil, raising the moisture con- 

 tent of the latter to slightly above the wilting percentage. He also found 

 evidence that because of the moisture transfer phenomenon, the roots 

 were able to absorb potassium from the dry soil. That is, net movement 

 of potassium into the roots was in the direction opposite to the net 

 movement of water out of the roots and into the dry soil. 



A pertinent consideration in the ability of roots to remove nutrients 

 from dry soils is whether or not the roots are able to grow and pro- 

 liferate in the dry soil. Loomis and Ewan (yy) found that the roots of 

 29 different genera of seedlings completely failed to penetrate soil at a 

 moisture content of about the hygroscopic coefficient. Hendrickson and 

 Veihmeyer (58) concluded that the roots of sunflower plants would not 

 grow into soil drier than the wilting percentage. Shantz (ioy) also con- 

 cluded that the roots of crop plants lack the ability to penetrate dry soils, 

 but he believed that the roots of xerophytic plants possessed this capac- 

 ity. Lobanov (y$) found appreciable variation in the dryness of soil 

 limiting to the growth of roots of woody plants. The minimal soil 

 moisture contents (soil moisture retentive properties unknown) for root 

 growth were 6.1 1 per cent for Fraxinus excelsior, 5.1 1 per cent for Cara- 

 gana arborescens, and 2.07 per cent for Picea abies and Pinus sylvestris. 



Hunter and Kelley (62) have devised an improved technic to study 

 the entry of roots into dry soils and removal of nutrients therefrom. 

 They grew corn plants in tar-paraffin pots filled with moist soil and sur- 

 rounded with air-dry soil containing radiophosphorus (6j) . In all cases 

 the corn roots penetrated the walls of the pot and extended into the 

 air-dry soil. The moisture content of the "dry" soil increased, but values 

 as high as the wilting percentage were not obtained. The results of 

 this experiment indicated that the roots of corn are able to elongate 

 into dry soil and to build up the moisture content of that soil, but the 

 evidence obtained did not indicate the absorption of nutrients from dry 

 soil by plants. In another experiment, Hunter and Kelley (62) obtained 

 an indication that there was a movement of radiophosphorus into alfalfa 



