212 ON THE CHEMICAL CONSTITUENTS OF SOILS. 



minerals which disintegrate with facility, crops of 

 wheat and barley may be grown upon it for many 

 centuries in succession. But, in order to support an 

 uninterrupted succession, the annual disintegration 

 must be sufficiently great to render soluble a quanti- 

 ty of silicate of potash sufficient for the supply of a 

 full crop of wheat or barley. If this is not the case, 

 the soil must either be allowed to lie fallow from 

 time to time, or plants may be cultivated upon it 

 which contain little silicate of potash, or the roots 

 of which are enabled to penetrate deeper into the 

 soil than corn plants in search of this salt. During 

 this interval of repose, the materials of the soil dis- 

 integrate, and potash in a soluble state is liberated 

 on the layers exposed to the action of the atmo- 

 sphere. When this has taken place, rich crops of 

 wheat may be again expected. 



The alkaline phosphates, as well as the phosphates 

 of magnesia and lime, are necessary for the produc- 

 tion of all corn-plants. Now, bones contain the latter, 

 but none of the former salts. These must, therefore, 

 be furnished by means of night-soil, or of urine, a 

 manure which is particularly rich in them.* Wood 

 ashes have been found very useful for wheat in cal- 

 careous soils ; for these ashes contain both phos- 

 phate of lime and silicate of potash. In like man- 

 ner stable manure and night-soil render clayey soils 

 fertile, by furnishing the magnesia in which they are 

 deficient. The ashes of all kinds of herbs and de- 

 cayed straw are capable of replacing wood ashes. 



A compost manure, which is adapted to furnish all 

 the inorganic matters to wheat, oats, and barley, may 

 be made, by mixing equal parts of bone dust and a 

 solution of silicate of potash (known as soluble glass 

 in commerce), allowing this mixture to dry in the 

 air, and then adding 10 or 12 parts of gypsum, with 

 16 parts of common salt. Such a compost would 



* It has been already stated that bran contains phosphate of soda and 

 phosphate of magnesia, so that it is useful as a manure where phos- 

 phates are desired. — Ed. 



