414 LIMB DECOMPOSES. SULPHATES AND SILICATK? SETS FREE 



exposure to the air, be converted into the second or red oxide, in 

 which state this metal is no longer hurtful to vegetation. 



When these salts are to be decomposed and removed from the sub- 

 soil, lime must be aided by the subsoil plough and the drain. Unless 

 an outlet beneath be provided for the surface water, by which the 

 rains may be enabled to wash away slowly the noxious substances 

 from the subsoil, even the addition of a copious dose of lime will only 

 produce a temporary improvement. 



2°. Lime decomposes also the sulphates of magnesia and of alumi- 

 na, both of which are occasionally found in the soil, and, being very so- 

 luble salts, are liable to be taken up by the roots in such quantity as to 

 be hurtful to the growing plants. When soils which contain any of 

 the three salts I have mentioned have once been limed or marled^ it 

 is in vain to add gypsum in the hope of favoring the clover crop, since 

 the lime, in decomposing the sulphates, has already formed an abun- 

 dant supply of this compound for all the purposes of .vegetation. 



3°. Among the earthy constituents of the soil, we have already seen 

 that there often exist fragments of felspar and of other minerals derived 

 from the granitic and trap rocks, which contain potash or soda in the 

 state o^ silicates. These silicates we know to be slowly decomposer 

 by the agency of the carbonic acid of the air, (Lee. X., § 1,) ana 

 their alkali set free in a soluble state. This decomposition is said to 

 be prompted by the presence of lime (p. 361.) 



Again, the stalks of the grasses and the straw of the corn-bearing 

 plants contain much silica in combination with potash and soda. In 

 farm-yard manure, therefore, much of these silicates is present, and 

 when mixed with the soil, there appears little reason to doubt that 

 they are of much benefit to the growing crops. On these silicates, 

 in the presence of carbonic acid and moisture, the lime acts as iidoes 

 upon the mineral silicates. It aids in the liberation of the potash and 

 soda, and thus promotes the performance of those important functions 

 which these alkalies are destined to exercise in reference to vegetable 

 growth (p. 328.) 



While the alkali is set free the lime itself combines with the silica, 

 and hence one source of the silicate of lime which, as I have already 

 mentioned to you, (p. 380,) usually exists in sensible quantity in our 

 cultivated soils. It has been stated by Sprengel (Lehre vom Diinger, 

 p. 310,) as one reason why the addition of lime must be repeated so 

 frequently upon some soils in which silica abounds, that an insoluble 

 silicate of lime is found, which is of no use to vegetation. But the 

 silicates of lime are slowly decomposed by the agency of the carbonic 

 acid of the air and of decaying vegetation, and to this cause in a pre- 

 vious lecture (Lee. XII., § 4,) I have ascribed much of the fertile 

 character of the trap and syenitic soils, and of their beneficial action 

 when laid on as a manure. 



4°. Potash and soda exist to some extent in clay soils in combina- 

 tion with their alumina. The presence of lime has a similar influence 

 in setting the alkalies free from this state of combination also. 



5°. Alumina has the property of combining readily with many vege- 

 table acids, and in the clay soils exercises a constant influence, similar 

 jn kind to that of lime and other alkaline substances, in persuading the 



