324 PROTOPLASM 



true, but in an organic form. In this case, alkalinity does not 

 determine the availability of the iron for the plant. 



The constancy with which soils hold their pH value is not very 

 great. Some soils are well buffered with an ammonium silicate 

 complex, and others not; the former maintain a fairly even pH 

 value. 



More uniform in its acidity is the ocean, with an average pH 

 of about 8.2, but even it is not absolutely constant in any one 

 locality, and it varies considerably between points. The coral 

 shores of Bermuda make the water there more alkaline than that 

 off the rock shores of Maine. Plant and animal life in the ocean 

 are nicely adjusted to the pH of their natural habitat. The green 

 alga Valonia, collected in Bermuda, will not grow well in the 

 waters of Massachusetts Bay (salt concentration or temperature, 

 as well as pH, may be responsible here). 



While organisms seem to be very exacting in their external pH 

 requirements, they also appear, in cases, to be rather indifferent 

 to it. Mast found that Amoeba proteus can live in a surprisingly 

 wide range of hydrogen-ion concentration. Some specimens 

 lived eight days in pH 3.8 to 4.2, and others seven days in pH 8.2 

 to 7.8. In concentrations between these, there is no pronounced 

 difference in the length of life. Beyond pH 4.2 and 8.2, the 

 length of life decreases very rapidly. 



Olof Arrhenius, son of the great Svante Arrhenius, author of 

 the dissociation theory, has aroused much interest and confidence 

 in pH as applied to agriculture in his country and in Germany — 

 so much so that a farmer in Germany will hesitate to grow neutral- 

 soil plants on an acid soil (liming being less depended upon there 

 than in the United States). For example, sugar beets have their 

 highest yield in sugar content on neutral soil (pH 7). Farmers 

 are, therefore, discouraged from risking a crop of sugar beets 

 on an acid or alkaline soil. In the main, this is a good policy, 

 but the theory does not always hold. It may be simply a ques- 

 tion of the way in which a plant wants its nitrogen, and pH may 

 determine the form in which the nitrogen occurs; but if it is 

 possible to give the plant its nitrogen in the desired form regard- 

 less of the pH value of the soil, then the plant becomes indifferent 

 to pH (within limits), and measuring the pH of the soil may then 

 tell nothing of significance. Acid-soil plants, such as members 

 of the heath family (e.g., rhododendron), prefer their nitrogen 



