EXPERIMENT STATION BULLETINS. 587 



TUB SIGNIFICANCE OF THE GREAT CONCENTRATION OF SOLUTION AT THE LOW 

 MOISTURE CONTENT UPON PLANT GROWTH. 



If the trenieudous depression of the freezing point as obtained at the 

 low moisture content in many soils, really represents concentration of 

 solution, then it throws new light upon several observed phenomena in 

 plant growth. One of these phenomena is the wilting of plants. In the 

 light of the extraordinary high depressions obtained at the low moisture 

 content it would be only logical to conclude that one of the causes of 

 the wilting of plants is due to the great concentration of the soil solu- 

 tion when the moisture content has been reduced tO' the wilting coeffi- 

 cient. This conclusion appears to be amply supported by. the following 

 facts : 



(1) The depression of the freezing point of the complex types of 

 soil at the wilting coefficient was considerably greater than that of the 

 tops and roots of the wheat seedlings. Thus, the lowering of the freez- 

 ing point of many silt loams, clay loams and clays was about 1.300°C 

 while that of the tops was about i.OOO°C and of the roots about .700°C. 

 The above depression value of the soils represents an osmotic pressure 

 of 15.64 atmospheres, that of the tops 12.04 atmospheres, and that of the 

 roots S.432 atmospheres. Hence, there is an appreciable difference in 

 the osmotic pressure between the soils and the wheat seedlings, in favor 

 of the soils. Since it has already been seen that the depression increases 

 tremendously with extremely small decreases in the percentage of water 

 at the low moisture content, the concentration of the soil solution 1 or 

 2% of moisture below the wilting coefficient would be far greater than 

 the value shown above. 



(2) The recent work of Shive (17) and of McCall (18) shows that 

 plants both in tops and roots made the best growth when the initial 

 concentration of the nutrient solution was about 1.75 atmospheres. 



The high osmotic pressure of the soil solution at the wilting coefficient 

 as indicated by the freezing point method is contrary to the results ob- 

 tained by Shuil. Shull (19) attempted to measure the force with which 

 the soil moisture is held by the soil by means of Xanthium seeds. He 

 found by this method that the total force with which the moisture is 

 held by the soil or the "back pull" at the wilting coefficient of soils 

 amounts only to about four atmospheres. Shull concludes, therefore, 

 "that this critical soil moisture content must be due to the increas- 

 ing slowness of water movement from soil particles to soil particles, and 

 from these to the root hairs, the rate of movement falling below that 

 necessary to maintain turgidity of the cells of the aerial parts, even 

 under condition of low transpiration." 



As already seen, the osmotic pressure of the soil solution alone 

 amounts to about 15 asmospheres. Now as will be shown in a subse- 

 quent paper, the soil moisture at and below the wilting coefficient ap- 

 pears to exist as physically absorbed and loosely chemically combined 

 and in a solid phase. The water, therefore, in this condition must be 

 held by the soil with a considerable force. This force must be added 

 that due to the concentration of the soil solution alone. The total 

 force, therefore, with which the water is held back by the soil is many 

 times greater than that obtained by the method of Shull. 



(17) Am. Jour. Bot., 2. 157-160, 1915. 



(18) Soil Science, 2, 207-253. 1916. 



(19) Bot. Gaz., 62, No. 1, 1916. 



