Some Causes of Unproductiveness in Soils. 115 



3. When they contain too larg^c a preponderance even of a 

 valuable ingredient, such as organic matter, sand, lime, and even 

 clay. 



4. When there is but a thin layer of soil resting on the bare 

 rock. 



5. When the land is thin and rests on an impervious and 

 very thick clay subsoil, or on subsoils containing something in- 

 jurious to vegetation. 



6. When they are badly drained. ' 



7. When they are affected by a bad climate. 



Under these seven divisions, I purpose to treat the subject of 

 the present communication. 



1. Soils are barren or unproductive when they contain something 

 injurious to vegetation. 



There is a very ready test for ascertaining whether a soil is 

 likely to contain an injurious constituent. All that is necessary is 

 to put a strip of litmus-paper in contact with wet soil ; if the blue 

 colour of the test-paper turns rapidly red, the soil is certain to 

 contain something injurious to plant-life. All good and fertile 

 soils either have no effect upon red or blue litmus-paper, or show 

 a slight alkaline reaction ; that is to say, in a wet condition they 

 restore the blue colour to reddened litmus-paper. 



The acid reaction which some barren or unproductive soils 

 exhibit, I find is caused, — 



a. Either by an excess of organic acids (humic acids), 



b. Or by the presence of small quantities of sulphate of iron 

 — green vitriol. 



Another substance which occurs in some barren soils is bi- 

 sulphide of iron or iron pyrites, a compound of 2 equivalents of 

 sulphur and 1 of iron. It is generally found in soils in which 

 the presence of green vitriol is readily detected, and, like the 

 latter, is a most undesirable soil-constituent. The simultaneous 

 occurrence of these two compounds is explained by the fact that 

 green vitriol is the result of the oxydation of iron pyrites in con- 

 tact with air and moisture, the iron of the iron pyrites becomes 

 converted into protoxide of iron, and the sulphur into sulphuric 

 acid, which, combining together, produce sulphate of iron or 

 green vitriol. Even as small a proportion as ^ per cent, of green 

 vitriol renders a soil almost barren ; and on land containing 

 little more than 1 per cent, nothing whatever can grow. 



In many unproductive soils I have found protoxide of iron 

 in considerable quantities, and scarcely any red peroxides — a 

 sure indication of poor cultivation. This, however, in many 

 instances, may be greatly improved by better drainage, sub- 

 soiling, grubbing, and other mechanical operations tending to 



I 2 



