120 Some Causes of Unproductiveness in Soils. 



^ The colour of this soil was dark grey, almost black, and yet it 

 will be seen the proportion of organic matter in it is not large. 

 The dark colour was therefore due not so much to organic 

 matter as to finely divided sulphide of iron. In such a state of 

 division, very little of it imparts a dark grey colour, and par- 

 ticularly obnoxious properties, to a large mass of soil. 



Where such dark soils occur, the air in the neighbourhood 

 at times is charged with foetid vapours, smelling faintly like 

 rotten eggs. This smell is produced by the action of the car- 

 bonic acid of the air upon the black sulphide of iron in the soil. 

 Acids, not excepting the weak carbonic acid, in the diluted state 

 in which it occurs in the atmosphere, on the point of mixture 

 disengages from sulphide of iron sulphuretted hydrogen, a gas 

 highly injurious to vegetable as well as animal life. Chemical 

 reactions are generally intensified bv elevation of temperature, 

 and thus the emission of sulphuretted hydrogen from land im- 

 pregnated with black sulphide of iron, is greater in summer than 

 in winter. In my opinion, suljihuretted hydrogen does more 

 mischief than even green vitriol, for direct experiments made by 

 I3r. Christison and others have shown that, even in a highly 

 diluted state, sulphuretted hydrogen is injurious to vegetation, 

 and, in a more concentrated state, is capable of destroying vege- 

 table life as readily as that of animals. 



All saline matters which are very soluble in water, as noticed 

 above, are injurious to vegetation when they occur in the soil in 

 too large a proportion. Tlie practical question is. What is too 

 large a proportion ? An answer has been given lately to this 

 question in the highly interesting scientific experiments on the 

 nutrition of plants by Professor Kncjp of Leipsig, who found that 

 solutions containing in all not more than 1 part of soluble mineral 

 matter to 1000 parts of water are fully as strong as liquids should 

 be from which plants are to derive food and grow luxuriantly. 

 In solutions stronger than this, plants either grow languidly or die 

 altogether, although the same mineral substances are employed 

 Avhich, in a highly diluted state, are most active promoters of 

 vegetation. If such be the case with solutions, my own experi- 

 ence leads me to infer that the soil itself should not contain more 

 than iVth per cent, of such soluble substances, and therefore that 

 soils which contain several per cent, of common salt, nitrate of 

 lime, or chloride of potassium, are unfit to maintain vegetable 

 life in a healthy state. 



I have met with several extraordinary soils, upon which nothing 

 would grow, evidently because they were overcharged with 

 soluble saline matters. An example of that kind is given in the 

 subjoined analysis : — 



