348 NATURE AND PROPERTIES OF SOILS 



Alabama soils indicates that it is probably of minor importance 

 as compared with aluminum. A toxic effect from magnesium 

 is possible, especially if there is not enough calcium to prevent 

 it from exerting a poisonous influence. The presence of alumi- 

 num or iron in an active form is generally accompanied by a 

 high hydrogen ion concentration due to hydrolysis, 1 which 

 takes place readily in many soils. 



186. Lack of nutrients. — Less is known regarding this 

 condition than of the two previously discussed. The lack of 

 sufficient nutritive calcium in an acid soil has often been sug- 

 gested. 2 In addition, it may be possible that some plants re- 

 quire more calcium and other bases for their metabolic proc- 

 esses when growing on a so-called acid soil, due to the gen- 

 eration of particular conditions within the cells. Plants like 

 alfalfa absorb large amounts of calcium and may find an acid 

 soil especially unfavorable on this account. 



True 3 has shown that the presence of calcium in consider- 

 able amount is necessary when certain plants are growing in 

 nutrient solution, that other nutrient ions may penetrate the 

 plant cells. Potassium, for example, was but slightly absorbed 

 even when present in large amounts, unless a certain concen- 



1 Hydrolysis is a double decomposition in which one of the inter- 

 acting substances is water. The water produces H+ and OH- ions, 

 the former uniting with the non-metallic portion of the substance and 

 the hydroxyl with the remainder. 



Active basic radicals give, with feeble acids in water, salts which 

 are alkaline. Active acids and active bases give neutral salts. Active 

 acids and less active bases yield salts which are acid in reaction. 



A feeble base and a feeble acid may produce a salt which is either 

 acid or alkaline. Ammonium sulfide (NH 4 )^ in solution is alkaline, 

 since the ammonium hydroxide which tends to form is more dissociated 

 than the hydrogen sulfide which also is present. Aluminum silicates in 

 water hydrolize readily and since aluminum hydroxide is less dissociated 

 than silicic acid, the hydrogen ions predominate over the hydroxyl ions 

 and an acid reaction results. 



a See Truog, E., Soil Acidity: Its Belation to the Growth of Plants; 

 Soil Sci., Vol. V, No. 3, pp. 169-195, 1918. 



Also, Soil Acidity: Its Relation to the Acidity of the Plant Juices; 

 Soil Sci., Vol. VII, No. 6, pp. 469-474, 1919. 



* True, R. H., The Function of Calcium in the Nutrition of Seed- 

 lings; Jour. Amer. Soc. Agron., Vol. 13, No. 3, pp. 91-107, 1921. 



