September 15, 1922] 



SCIENCE 



297 



with potassium, as is evident from the follow- 

 ing: The reaction of the sap of different 

 common agricultural plants has a range of 

 Ph 4 to 8. This range is practically the same 

 as that of the soil solution" in the humid region. 

 It thus appears that plants through adapta- 

 tion have come to have somewhat the same re- 

 action as the medium on which they grow. It 

 is well known that plants growing in solution 

 cultures of unfavorable reaction tend to change 

 the reaction of the culture to a more favorable 

 one which is usually near that of the reaction 

 of the plant itself. The plant does this^ by 

 utilizing a larger proportion of the acidic or 

 basic constituents of the nutrient medium as 

 the case may be. This again follows from the 

 law of mass action, and the composition of the 

 plant is thereby somewhat altered. Because 

 of the highly buffered condition of the soil, 

 plants can not materialy change its reaction in 

 the way a solution culture is changed. 



The unfavorable situation of a plant like 

 alfalfa with a sap reaction of pn 6 growing 

 on a soil with a soil solution reaction of pn 5 

 is thus apparent. This plant requires large 

 amounts of basic material for the second pur- 

 pose. How can it obtain this basic material 

 for this purpose from a solution or medivmi 

 which is ten times as acid as its own sap and 

 system? It can not do it advantageously and 

 hence the growth is slow and the content of 

 basic material in the plant becomes lower than 

 normal and even the reaction of the sap may 

 become more acid than is normally the case. 

 In extreme cases the plant not only grows 

 slowly but also becomes sickly in appearance 

 and easily succumbs to unfavorable weather 

 conditions or parasitic diseases. 



The buolnvheat plant also requires a large 

 amount of basic material. In fact, at the 

 blooming stage it has a higher content of cal- 

 cium than alfalfa, and yet it grows well on 

 acid soils. The explanation of this is found 

 in the high acidity of its sap, namely Ph 4 to 5. 

 It can thus utilize advantageously a soil solu- 

 tion of Ph 5 as a source of basic material for 



' Truog, E., Soil Soience, 5, (1918), pp. 169- 

 195. 



sHoagland, D. R., Science, N. S., 48, (1918), 

 422-425. 



the partial neutralization and regulation of its 

 own sap and system. 



The feeding power of a plant for calcium 

 which is to be used for the second purpose is 

 dependent largely on the normal acidity of the 

 plant sap. The more acid the plant sap the 

 more advantageously can the plant compete 

 with another system — the soil and its solution, 

 for neutralizing material which is largely lime 

 in the case of plants and soils. Plants like 

 oats and corn have a low content of calcium 

 and probably use most of it for the first pur- 

 pose. They apparently do not produce much 

 acid which needs to be neutralized. Their sap 

 is normally quite acid. They are thus well 

 able to get all the calcium they need from even 

 quite acid soils. The opposite relation of the 

 acidity of the plant sap to the feeding power 

 of many plants for calcium and potassium is 

 now apparent. A high acidity means a low 

 feeding power for potassium in dilute solution 

 and a high feeding power for calcium needed 

 for the neutralization and precipitation of 

 acids. 



The nature of the injurious or toxic action 

 of acid and alkali soils on plants is also ap- 

 parent. Theoretically the nutrient solution 

 most favorably adapted to a plant as regards 

 reaction would be one with a reaction the same 

 as that of the plant sap. In case the plant 

 needed a large amount of calcium and other 

 basic elements, a nutrient solution slightly more 

 alkaline than the plant sap would probably be 

 best. When the nutrient solution is more acid 

 than the plant sap, the plant by mass action 

 is forced to utilize acid forming elements in 

 greater proportion than is normally the case 

 and as a result the composition of the plant is 

 changed giving one with less than the normal 

 amount of basic material. If the nutrient 

 solution is much more acid than the plant sap 

 the solid material of the plant due to lack of 

 bases becomes so much more acid than is nor- 

 mally the case, that the plant sap also becomes 

 more acid. The change in reaction of the 

 whole plant system greatly interferes with the 

 normal plant processes and as a result the 

 plant grows slowly, becomes sickly and may 

 even die. If a plant growing in a nutrient 

 solution of favorable reaction were transferred 

 to one with a much more acid reaction there 



