SOIL CONDITIONS AFFECTING PLANT GROWTH 85 



io.'^ metals required for nutrition. No unexceptionable evid- 

 ence 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. 280). The literature of the sub- 

 ject is summarised by Dr. Brenchley in her monograph (53<a?). 



Whenever infertility is traced to any of these metallic 

 salts a good dressing of lime is usually found to be an 

 effective antidote. 



Various Other Substances — Sulphuretted hydrogen is ex- 

 tremely 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 compounds include nitrites, 

 which have to be removed from synthetical calcium nitrate 

 used for manure, the dicyano-diamide associated with com- 

 mercial cyanamide, and ammonium salts at too high a con- 

 centration. None of these, however, are for long harmful 

 in the soil, since all are ultimately converted into nitrates. 

 Perchlorates are harmful and used sometimes to occur in 

 sodium nitrate, but they are now carefully removed. 



Substances Injurious in Large Quantities : Carbon Dioxide. — 

 In an interesting series of investigations Kidd (147) has shown 

 that CO2 exerts a marked inhibiting effect on the germination 

 of seeds, even though all other conditions are favourable. The 

 seed is not permanently affected, and it will germinate freely, 

 though not always immediately, when the CO2 is removed. 

 He suggests that this is the cause of the remarkable dormancy 

 of certain seeds, especially weed seeds, buried below a certain 

 depth in the soil ; some of these will survive for years, and 

 will produce a copious and vigorous crop of weeds when 

 brought to the surface by deeper ploughing or breaking up of 

 grass land.^ 



Soluble Salts. — In many arid districts the soil contains 

 such large quantities of sodium and potassium salts that the 

 soil water is too concentrated to permit of plant growth. 

 Sodium carbonate not infrequently occurs and directly poisons 



^ W. E. Brenchley, Joum. Ag. Science, 1918, 9, 1-31. 



