354 NATURE AND PROPERTIES OP SOILS 



paint-briisli, daisy, and plantain seem especially resistant. 

 This does not mean, however, that they grow better on an 

 extremely acid soil than on one that is slightly acid or neutral. 

 Some of the common crops that are tolerant of acidity are 

 strawberry, blackberry, watermelon, red-top, Rhode Island 

 bent-grass, cowpea, soybean, rye, millet, and buckwheat. Such 

 crops as alfalfa, red clover, timothy, maize, oats, barley, cab- 

 bage and sugar-beet seem to be susceptible in various degree 

 to acid conditions. 



Reasons for the above differences are not as yet known, since 

 plants apparently alike in every other respect differ in their 

 reaction to the same acid condition. The following pairs of 

 plants may be listed as examples: watermelon and musk- 

 melon, blackberries and raspberries, apple and quince, turnip 

 and beet, beans and alfalfa, red-top and timothy, rye and 

 barley. The first of each pair mentioned mil grow well on 

 acid soils, while the second crop in each case is very detri- 

 mentally affected.^ 



190. Tests for soil acidity.^ — The great importance of 

 soil acidity to plant growth has directed much attention 

 towards methods for determining the acidity of the soil. 



^ Hartwell, B. L., Need for Lime as Indicated hy Relative Toxicity of 

 Acid Soil Conditions to Different Crops; Jour. Amer. Soc. Agron., Vol. 

 13, No. 3, pp. 108-112, 1921. 



*Some of the important methods are compared and discussed in tlie 

 following articles: 



Sehollenberger, C. J., delation Between the Indications of Several 

 Lime-requirement Methods and the Soil Content of Bases; Soil Sci., 

 Vol. Ill, No. 3, pp. 279-288, 1917. 



Christensen, H. B., Experimeyits in Methods for Determining the 

 Eeaction of Soils; Soil Sci., Yol. IV, No. 2, pp. 115-178, 1917. 



Stephenson, E. E., Soil Acidity Methods; Soil Sci., Vol. VI, No. 1, 

 pp. 33-52, 1918. 



Blair, A. W.^ and Prince, A. L., The Lime Requirement of Soils 

 According to the Veitch Method, Compared with the Eydrogen-Ion 

 Concentration of the Soil Extract; Soil Sci., Vol. IX, No. 4, pp. 253- 

 259, 1920. 



Hartwell, B. L., Pember, P. B.^ and Howard, L. P., Lime Eequire- 

 ment as Determined hy the Plant and hy the Chemist; Soil Sci., Vol 

 VII, Nc. 4, pp. 279-282, 1919. 



