Dec, 1909.] POTASH IN CLAY LOAM SOILS. 47 



seasons, and must be regarded as the lowest probable amount to 

 be found at any time. 



The average amount of soluble potassium in all the soils for 

 the three seasons was 12.16 parts per million of dry soil, which 

 would be equivalent to fifty-eight pounds of actual potash in a 

 surface foot of an acre. This quantity may be assumed to be 

 the average amount present at any time in the soil. To de- 

 termine whether it is sufficient for a maximum hay crop it is 

 necessary to compare the concentration of potash in the soil 

 water with the necessary concentration in the current of water 

 passing through the plant. It is definitely known that plants 

 must get all the water that they need through the roots, and 

 that they lose enormous quantities by transpiration through the 

 leaves. So far as can be learned, this current of water is the 

 means by which plants get their potash from the soil. 



Numerous determinations of the moisture in the soils under 

 discussion gave an average of eighteen per cent, water, which 

 would be equivalent to 720,000 pounds in a surface foot of an 

 acre, and therefore the fifty-eight pounds of potash would be 

 dissolved in this quantity of soil water, and the concentration 

 would be eighty parts of potash in one million parts of water. 



Data for the number of pounds of water actually transpired 

 by a crop during its growth are not abundant. King* has made 

 numerous determinations of the total amounts transpired from 

 the plants and evaporated from the soil in which they were 

 growing, with results ranging from 271 pounds of water for a 

 pound of dry matter in the corn crop to 576 pounds of water 

 for a pound of dry matter in the clover crop. The corn, by 

 cultivation of the soil, would have evaporation reduced to a low 

 limit, while in clover the evaporation might be a large factor. 

 In some experiments on the transpiration of wheat plants Liv- 

 ingston t and Gardner t have shown that there are from 40 grams 

 to 100 grams of water used for every gram of green weight of 

 plant. From these results it has been calculated that the trans- 

 piration of the wheat plant was approximately 300 pounds of 

 water for one pound of dry matter in the crop. This amount 



* Phy.sica of Agriculture, p. 139. 



t Bot. Gaz., 40, 178-195. 



t Bull. 48, Bureau of Soils, U. S. Dept. Agr., p. 13. 



