SOIL CONDITIONS AFFECTING PLANT GROWTH 81 



Substances Injurious in Small Quantities: Acids and Al- 

 kalis. H and OH ions. It has long been known that plants do 

 not grow well on acid soils, and the conclusion has been drawn 

 that the acidity is harmful to plants. As a general statement 

 this is true : additions of a strong acid, such as HC1 or H 2 SO 4 

 to a culture solution or to soil soon kills the plant. But the 

 change in reaction thus induced is vastly greater than is found 

 in soils, and from our point of view it is necessary to know the 

 effect of changes of the same magnitude as occur in nature. 

 The proper basis of comparison is the hydrogen-ion concen- 

 tration (see p. 113). Hoagland has shown (1360) that an acid 

 condition up to 07 x io~ 5 H ion (P H = 5'i6) is favourable 

 to the growth of barley seedlings, while stronger acidity is 

 harmful. Alkaline solutions stronger than I -8 x io~ 6 OH ion 

 (P H = 8-26) caused injury, and those stronger than 2 -5 x icr 5 

 (PH = 9'4) were extremely toxic. Salter and Mcllvaine 

 (243) obtained the best growth in slightly acid conditions ; 

 for wheat, soy beans, and lucerne P H = 5*94, and for maize 

 P H = 5-16; 2-96 was harmful, and 2-16 fatal. Alkalinity 

 was decidedly more harmful than acidity 1 (Fig. 13). 



It will be shown later that the P H value of soils varies 

 between 37 and 97, values beyond 4-5 and 8-5 being, how- 

 ever, unusual. The acidity is often less than that of cell sap, 

 the P H value of which varies between 4-0 and 6'O. 2 Thus, 



1 See also J. S. Joffe, Soil Sci., 1920, 10, 301. 



3 Truog and Meacham, Soil Sci., 1918, 5, 177. For further data showing 

 variation with stage of growth and conditions see A. R. C. Haas, Soil Sci., 1920, 

 9, 341. For acidity of root sap (which is generally less than that, of the aerial 

 portion), H. Kappen (Landw. Versuchs-Stat. 1918, 91, 1-40) obtained the follow- 

 ing P H values : 



Wheat . \ . 7-2 



Barley .'.... 6-85 



Oats '.,... 6-7-6-8 



Rye . . . . . . . , . . 6-6 



Mustard . . ' - 6-2 



Horse beans 6-0 



Lupins > v, . . . 5'6-5'8 



Buckwheat . . . . . . . . . 5'o-5'3 



For other determinations see C. B. Clevenger, Soil Sci., 1919, 8, 217 (values 

 5-8-6). 



6 



