86 SOIL CONDITIONS AND PLANT GRO WTH 



the plant. Such soils are called alkali soils : they may be 

 treated with gypsum, or, still better, carefully washed with 

 irrigation water, adequate provision being simultaneously made 

 for drainage. 



Calcium Carbonate is sometimes considered harmful be- 

 cause plants are liable to chlorosis on chalky soils. It is 

 equally probable, however, that the general soil conditions are 

 responsible for the disease (see p. 304). 



Magnesium Salts. — The toxicity of magnesium salts was 

 discovered by Tennant in the eighteenth century in studying 

 the alleged harmful effects of certain limestones found near 

 Doncaster (280). Modern investigations ^ on magnesian 

 limestone, however, have failed to show any harmful effect ; 

 indeed, in the Woburn experiments (290) Voelcker has ob- 

 tained an actual benefit both on wheat and on mangolds by 

 using magnesia (MgO). But the soluble salts, the sulphate 

 and especially the chloride, are harmful. Cases are reported 

 by Loew where excess of magnesia in the soil has caused 

 infertility ; none, however, have fallen under the writer's 

 observation in this country. The soil of the Greenville Ex- 

 perimental Farm, Utah, is rich in magnesia — containing over 

 6 per cent, of MgO — and is remarkably fertile. It also con- 

 tains, however, 17 per cent, of CaO and 20 per cent. COj.^ 

 As already stated, any injurious effect can be overcome by 

 treatment with lime. 



Effects of Salts on Germination. — Salts generally cause a 

 retardation in the rate of germination ; some of Guthrie and 

 Helms' (117) results are given in Table XXVII. Sigmund 

 has studied the effects of a very large number of substances 

 (267). The technical interest in the work lies in the fact that 

 seeds are sometimes treated with antiseptics before sowing in 

 order to kill any spores of disease organisms, and, moreover, 

 certain soluble salts — artificial manures — are often put into 

 the soil about the same time as the seeds are sown. 



^See, e.g., New Jersey Bull., 267, 1914, and on the other side Durham 

 Coll. Bull., 12, 1915. 



* J. E. Greaves, R. Stewart, and C. T. Hirst, jfourn. Ag. Research, 1917, 9, 

 301. 



