36 Mineral Nutrition of Plants 



tive effect of acidity or alkalinity on the root tissues. When plants are 

 grown under controlled conditions at different pH values as regards 

 the nutrient medium, it is found that a destructive action of the root 

 tissues by acidity or alkalinity usually does not take place until the pH 

 drops below 4 or rises above 9, respectively. It has been found that 

 cereals are considerably more resistant to low pH than alfalfa. (See 

 references 5, 6, and 10 for details of experiments in which these crops 

 were grown at varying pH levels.) 



Since very few soils become more acid than pH 4.5, it seems logical 

 to conclude that the injurious influence of soil acidity on plant growth 

 is seldom due to a direct toxic or destructive action on the root tissues. 

 This is further supported by the fact that the natural reaction of the root 

 sap of most agricultural plants falls in the same pH range, that is, 6 to 4, 

 as that of many acid soils. It is to be expected that plant roots will with- 

 stand an external acidity which is of the same order as their natural 

 internal acidity. 



The second and much more important direct effect of soil acidity on 

 certain plants is the unfavorable balance or excess which it creates of 

 acids over basic constituents which are available in the soil for absorp- 

 tion by the plants. In order that the metabolic processes of a particular 

 species of plant may proceed satisfactorily, it would seem to be neces- 

 sary that a certain balance exist between the acidic and basic nutrients 

 (anions and cations other than hydrogen) which are available for ab- 

 sorption. This contention is supported by findings which show that 

 each species of plant when grown under different conditions strives to 

 maintain a rather constant total equivalent base content. A deficiency 

 of one base, for example, calcium, is compensated for by the absorption 

 of an equivalent amount of other bases, such as magnesium and potas- 

 sium. Thus, in a distinctly acid soil, some of the nitrate and sulfate, 

 when present, exist in the soil solution as free nitric and sulfuric acids, 

 respectively, and are absorbed as such by plants growing thereon; more- 

 over, the more acidic the soil, the greater the proportion absorbed as 

 free acids, and the more difficult it becomes for plants to counteract all 

 this by the absorption of exchangeable calcium and other bases in the 

 form of the bicarbonate. On the other hand, when the soil reaction is 

 at about pH 6.5 or higher, practically all the nitrate and sulfate exist as 



