172 LECTURES ON 



hope of success, undertake to show what is this certain point or 

 define the limits which, over-passed, make the soil unproductive. 



It not unfrequently happens that the presence of noxious com- 

 pounds greatly injures an otherwise excellent soil. Soluble salts of 

 iron and alumina, especially the sulphates of these bases, are, so 

 far as we now know, the principal causes of this kind of mischief. 

 Some soils are formed from rocks that contain numerous grains and 

 larger masses of iron pyrites or sulphid of iron, which, exposed to 

 the weather, oxydize to sulphate of iron (copperas) and the solution 

 of this salt in a certain stage of concentration destroys the vegetable 

 tissues, and thereby renders the soil in which it exists unfavorable 

 to growth. In a specimen of peat from Brooklyn, Conn., the writer 

 found a not inconsiderable quantity of sulphate of iron, and likewise 

 sulphate of alumina. Both these salts have a powerful decomposing 

 effect on the rootlets of plants. 



The importance which attaches to the proper availability or solu- 

 bility of the nutriment in the soil leads at once to the inquiry, may 

 not the soluble matters be washed out and lost by rains, or may they 

 not accumulate in too great quantity ? 



There are certain influences external to the soil, which, acting re- 

 ciprocally, tend to maintain in it a nearly constant content of soluble 

 matter. On the one hand the disintegration of the soil, the decay of 

 vegetation, rain, and dew, are perpetually enriching; while vegetable 

 growth, springs, and streams, (rain that has passed through the mould,) 

 and evaporation, are as continually wasting the soil. Since the mass 

 of soil is so great, and the most rapid and exhausting of these pro- 

 cesses operate so slowly, their effect is in general to leave the soil in 

 possession of the requisite small amount of soluble matters, and only 

 in exceptional cases can positive excess or deficiency occur. 



In the soil itself we find, however, a remarkable property which 

 enables it to convert excess of soluble matters into an appropriate 

 quantity, and at the same time to store up this excess against what 

 might otherwise be a period of want. The soil has, in fact, a power 

 of regulating its supplies to vegetation, in a manner that was not 

 dreamed of but a decade since. 



The fact has been already alluded to, in treating of the physical 

 characters of the soil, that it has a power of absorbing vapor of 

 water, and in general other gaseous bodies — a power shared by the 

 soil to more or less extent with all porous bodies. 



Besides this purely physical quality, we find the soil to possess 

 another absorptive capacity, which, though not independent of phy- 

 sical conditions, appears to be chemical in its nature, that is, depends 

 upon the presence of certain lands or combinations of matter. 



Without this chemical absorption the other quality would be of 

 little avail in directly nutrifying the plant, because water alone is 

 capable of nullifying the latter, and at the same time performing any 

 office that it might appear to exercise in a much more effectual man- 

 ner. Ammonia has long been known to be taken up by the soil, and 

 to be retained in it. Previous to the year 1850 it was supposed that 

 this gas underwent absorption by surface condensation, exerted by 



