116 Some Causes of Uiqjrodactweness in Soils. 



admit the air more freely into the soil. Protoxide of iron mani- 

 fests itself by the bluish-grey or dark-green colour which may be 

 noticed in many clay subsoils and stiff tenacious soils improperly 

 cultivated. A change of colour from blue to reddish-brown is 

 justly regarded as a sure sign of improved condition, for it indi- 

 cates the transformation of protoxide into peroxide of iron, and 

 tells of the free admission of air into the land. 



Protoxide of iron being as insoluble in water as peroxide, can 

 hardly be regarded in itself as a plant-poison ; it is rather a test 

 of the absence of atmospheric oxygen from the soil. The readi- 

 ness with which protoxide of iron unites with more oxygen and 

 produces red oxide, is seen in the rapid change of colour which 

 blue clay dug out of a drain assumes superficially within a few 

 hours on exposure to the air. Protoxide of iron, indeed, is one 

 of the most delicate tests for oxygen, and thus, though it is not 

 injurious to plants in the same sense as green vitriol, which is 

 readily soluble in water, nevertheless, its presence implies a 

 complete exclusion of the air, without which vegetation cannot 

 remain in a healthy state for any length of time. 



Chloride of sodium, or ct)mmon salt, generally occurs in an 

 injurious proportion in land recently reclaimed from the sea, or 

 in soils inundated by the sea. It is true, that some grasses and 

 maritime plants grow well enough on such spots ; but cereals, roots, 

 clover, and other forage crops, do not grow well on land con- 

 stantly kept in a very wet state, and do not ripen unless the heat 

 of the sun during the summer months has had a fair chance of 

 penetrating the surface-soil, tand expelling any superabundance of 

 moisture. Soils recently reclaimed from the sea, or land acci- 

 dentally saturated with sea-water exhibit in dry weather white 

 cfllorescences, which consist mainly of common salt, as may be 

 readily ascertained by the taste. Although the soil originally is 

 impregnated with but a dilute solution of salt, the evaporation 

 from the surface in dry weather causes the liquid brought 

 upwards by capillary attraction to become by degrees charged 

 with salt to such an extent, that portions crystallize out in the 

 shape of a white saline efflorescence which is injurious to vegeta- 

 tion. I am acquainted with land which, irrigated purposely 

 with sea-water, under the mistaken idea of improving it, has been 

 rendered unproductive for several seasons in succession. 



In some soils in India and Hungary nitrates of potash and 

 soda, two very valuable salts, occur in proportions injurious to 

 vegetation. Like solutions of common salt, nitrates give rise to 

 saline efflorescences, which invariably indicate an unhealthy 

 condition of the land, and frequently destroy vegetation altogether. 

 Common salt, nitrates of soda and potash, it is Avell known, are 

 used as manuring agents ; nevertheless they are injurious when 



