BOG XEROPHYTES AND ACID SOILS. 137 



peat soils that are decidedly acid (pH 5.4 to 4.5), excellent crops of 

 barley, oats, beans, potatoes, onions, corn, asparagus, etc., were 

 produced, showing that the acidity did not interfere with growth or 

 with the formation of nitrates. 



Duggar (1920: 1) has studied the growth of seedlings of wheat, 

 corn, and field peas in relation to H-ion concentration and the com- 

 position of the nutrient solution. Theoretically, the solutions pos- 

 sessed a pH exponent of about 4.5, but solution B in particular 

 varied from pH 5.4 to pH 7.1. Under the most favorable condi- 

 tions the 3 solutions all gave excellent growth, but under extreme 

 conditions, producing high evaporation, it became important to 

 correct to the higher pH exponent. The sensitivity to high H-ion 

 concentration is in the order of wheat, corn, and peas. In general 

 it seemed that there was no single ''best" solution for the 3 plants 

 used, but there is a considerable range of salt or ion proportions 

 within the "optimum" concentration. If the initial pH of the cul- 

 ture solution is considerably less than neutral, there is a general 

 tendency for this to be shifted toward the neutral point. 



In a series of papers, Truog (1918, 1919) has given an illuminating 

 discussion of the relation of acidity to the growth of plants, which is 

 of such importance as to warrant quoting the summary in its entirety: 



"With a few exceptions agricultural plants grow best on soils well supplied 

 with readily available lime. To be readily available, lime may exist either 

 as the carbonate, as an easily hydrolyzable silicate or salt, or as a constituent 

 of easily decomposable organic matter. The classification of agricultural 

 plants as being lime-loving, lime-avoiding and indifferent, or as being acid- 

 intolerant, acid-tolerant and indifferent, leads to confusion and gives the 

 wrong impression regarding the relation of lime and soil acidity to plant 

 growth. The subject is very complex and, as has been indicated, soil acidity 

 has many indirect and general influences on soil fertility due to its effect on 

 physical, chemical, and biological conditions of the soil. 



'Tt is well known that an acid condition is unfavorable to the highest 

 development of desirable physical and biological soil conditions. An acid 

 condition usually lowers the availability of nearly all the essential elements. 

 On the other hand, soil acidity usually favors the accumulation and solubility 

 of toxic organic and inorganic substances. Among these toxic substances 

 soluble aluminum salts have been noted by a number of investigators, and 

 possibly in some cases manganese salts should also be considered. In certain 

 unusual cases of soil acidity sufficient amounts of these toxic substances 

 may be present to be very harmful to some plants. The relation of soil 

 acidity and liming to malnutrition due to a lack of iron in the plant, to plant 

 diseases and to plant competition also need to be considered in a few special 

 cases, particularly in cases where soil acidity appears to be a favorable 

 condition. 



"Besides the indirect influences which affect all plants, and some probably 

 to a considerable extent, soil acidity has a specific influence which affects 

 some plants like alfalfa and sugar beets very much more than others, like 

 cowpeas, potatoes, and oats. At various times this specific influence of soil 

 acidity has been ascribed to at least three causes: viz., (a) its effect on the 

 supply of available calcium needed by plants as direct plant-food material, 



