136 AERATION AND AIR-CONTENT. 



from a condition of high acidity to one of high alkalinity, namely, 

 from pH 3.7 to pH 9.7. The conclusion is reached that soil acidity 

 is due to the presence of an excess of hydrogen-ions in the soil solu- 

 tion, direct evidence of which fact is given by hydrogen-electrode 

 measurements. This confirms the view that soil acidity is fun- 

 damentally dependent upon the equilibria of reactions yielding an 

 excess of H-ions, and is not necessarily related to the phenomena 

 known as "absorption" and "adsorption," in conformity with the 

 opinions of Loew, Hanley, Gillespie, and Truog. 



Plummer (1918 : 19) has determined the hydrogen-ion concentra- 

 tion of soil suspensions of 68 samples of soils from the southeastern 

 United States. Excessive acidity was found in the Norfolk silt 

 loam and in the mucks, reaching values of 0.1X10- 3 and 0.2 X10- 1 

 respectively. Truog and Loomis (1918) have found a range of 4.5 

 to 8.0 in the hydrogen-ion exponents for a number of the common 

 cultivated acid soils of Wisconsin. 



Hoagland (1917 : 547) has grown barley seedlings in partial nu- 

 trient solutions of like osmotic pressure, but with a considerable 

 range of H-ion and OH-ion concentrations, and found that the OH- 

 ion was relatively much more toxic. When the concentration of the 

 OH-ion was greater than 1.8X 10- 6 ,the effect was distinctly injurious, 

 and above 2.5 X10- 5 extremely toxic. With an H-ion concentra- 

 tion of approximately 0.7 X10- 5 , growth was favored, but one of 

 0.3 X 10- 3 was very toxic. These results are not in accord with those 

 of Hartwell and Pember (1907), Breazeale and LeClerc (1912), 

 Dachnowski (1914), and Miyake (1914), all of whom found the H-ion 

 to be more toxic, but these are explained as due to the fact that dilute 

 solutions of potassium or sodium hydroxid do not give the effect of 

 the OH-ion concentration on plant-growth. 



In later studies (1918 : 422), plants exhibited a strong tendency 

 to change the reaction of various potassium phosphate solutions 

 toward neutral, acid, and alkaline solutions, soon reaching a point 

 about equivalent to pH 7.0, and neutral ones remaining unchanged. 

 In extensive experiments with barley and beans, nutrient solutions 

 with acid reaction always reached a reaction approximately neutral 

 after varying periods of contact with plant roots. When barley 

 plants were grown in nutrient solutions and then transferred to solu- 

 tions of KC1, K 2 SO 4 , MgS0 4 , K 3 PO 4 , NH 4 C1, and NaN0 3 , an excessive 

 concentration of OH-ion or H-ion was nowhere found, in spite of 

 active absorption. When an acid reaction was present, it was due 

 to slightly dissociated acids, usually carbonic, to acid salts in the 

 case of NH 4 C1 solution, and possibly in some cases to organic acids. 

 In spite of the assumption that most crop plants require a slightly 

 alkaline soil solution, a reaction of pH 5.0 was found to be in nowise 

 injurious to barley seedlings or to beans, thus supporting the view 

 of Truog that acidity itself is not the limiting factor. In California 



