THE REQUIREMENTS OF PLANTS 49 



the older pot experiments were conducted in zinc vessels. In a recent 

 critical summary Ehrenberg (93#) concludes that zinc salts are always 

 toxic when the action is simply on the plant, but they may lead to 

 increased growth through some indirect action on the soil itself. 



Ferrous Salts. Ferrous salts are toxic and are commonly regarded 

 as one cause of the sterility of badly aerated soils. 



Most metallic salts appear to be toxic except those of the few 

 metals required for nutrition. No unexceptionable evidence of a 

 stimulating effect on the plant has yet been obtained, although certain 

 effects may be produced in the soil leading to increased growth (see 

 P- 130). 



Whenever infertility is traced to any of these metallic salts a good 

 dressing of lime is found to be an effective antidote. 



Various Other Substances. Sulphuretted hydrogen is extremely 

 toxic, so also is ammonium sulpho-cyanide which, in the early days, 

 used to cause trouble as an impurity in ammonium sulphate made 

 from gas liquor. It is rarely, if ever, found now. Toxic nitrogen com- 

 pounds include nitrites, which have to be removed from synthetical 

 calcium nitrate used for manure, the complex cyanides associated with 

 commercial cyanamide, and ammonium salts at too high a concentra- 

 tion. None of these, however, is for long harmful in the soil, since all 

 are fairly rapidly converted into nitrates. Perchlorates are harmful 

 and used sometimes to occur in sodium nitrate, but they are now care- 

 fully removed. Arsenates and especially arsenites are poisonous and 

 form the basis of most weed killers. 



Substances Injurious in Large Quantities. Soluble Salts. In many 

 arid districts the soil contains such large quantities of sodium and pot- 

 assium salts that the soil water is too concentrated to permit of plant 

 growth. Sodium carbonate not infrequently occurs and directly poisons 

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

 gypsum, or, still better, carefully washed with irrigation water. 



Calcium carbonate is sometimes considered harmful because 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. 142). 



Magnesium Salts. The toxicity of magnesium salts was discovered 

 by Tennant in the eighteenth century in studying the harmful effects 

 of certain limestones found near Doncaster (281). 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. 

 As already stated^ any injurious effect can be overcome by treatment 

 with lime, 



