CONCLUSIONS. 17 



sulphuric acid. 1 This is a great secondary source of soil acidity and 

 clearly illustrates the faculty of selective absorption by the plant. 



CONCLUSIONS. 



The agricultural significance of the physiological study described 

 in this bulletin will be evident from the following discussion. When 

 a soil is in good tilth with an optimum moisture content, each grain 

 is surrounded by a film of water. The plant root comes in contact 

 with this film, and from it draws the water and the mineral salts 

 necessary for growth. Expressed in physical terms, this film is in 

 equilibrium with the soil that is, if the soil grain contains a sili- 

 cate of potassium, as feldspar, for example, the water composing the 

 film will bring into the solution about eight parts per million of 

 potash. If some of this potash is removed by the plant more will go 

 into solution, and the concentration of the film will remain fairly 

 constant as long as any feldspar remains in the soil grain. 



Although a difference of opinion exists as to how the plant foods 

 are brought into solution, whether by the action of water, by carbon 

 dioxid, or by acids which are exuded by the root tips, all agree that 

 it is absolutely necessary for them to be in solution before they can 

 be taken up by the plant. The soil may, therefore, be considered as 

 a. storehouse of reserve material, while the film of soil solution may 

 be considered as a plain nutrient solution in which the plant grows. 

 Whether this soil solution is acid or alkaline is one of the most 

 important factors in plant growth, as has been noted in the foregoing 

 physiological study. 



The acidity or alkalinity determines to a great extent not only 

 the particular kind of crop to be grown upon the soil, but also the 

 yield of the crop. No one would think of trying to grow alfalfa in 

 a peat bog or cranberries in a limestone soil. On the other hand, a 

 soil which is naturally alkaline will, under continuous cropping with 

 a rotation containing clover, for example, give smaller and smaller 

 yields as the alkalinity is diminished, until it will be found impos- 

 sible to grow this legume on account of the acidity of the soil. This 



*In this connection mention should be made of the experiments of Micheels (Action- 

 des liquides anodiques et cathodiques sur la germination. Bull. Acad. Roy. Belg., Classe 

 des Sci., 1910, 5:391), who studied the effect on wheat seedlings of an electric current 

 produced by a pile of 24 Daniell cells, when passing through various nutrient solutions ; 

 and who found that the plants grown near the anode developed roots whose length varied 

 from 10 to 25 mm, while those plants in the cathode solution possessed roots from 140 

 to 150 mm in length. Similar results were obtained in nutrient solutions containing 

 sodium chlorid, potassium chlorid, potassium nitrate, sodium nitrate, and mixtures of 

 these. A second crop being grown in each of these solutions, even after the electric 

 current was discontinued, similar results were obtained as with the first crop, due to 

 modifications brought about in the solution. From our investigations, the injurious 

 effects noted at the anode, as seen from the length of the roots, could well be attributed 

 to the formation of acid, while near the cathode, the solution being alkaline, the con- 

 ditions were reversed and therefore favorable for root development. 



