72 GEOLOGY OF AGRICULTUEE. 



by 20 of manure ; and on the other hand, many 

 clay soils would receive more benefit from 10 loads 

 of manure and 10 of sand, than from 20 of manure. 

 The reason is, that in one case, the clay enables the 

 sandy soil to hold the manure till wanted by the 

 plants ; and in the other case, the sand renders the 

 clay soil more light, open, and porous, so that the air 

 can circulate through it. 



115. Chloride of Sodium (NaCl), or common salt, is 

 found in all good soils, in small quantities, not ex- 

 ceeding 2 or 3 tenths of one per cent. It is oftener 

 exhausted from lands remote from salt water. Lands 

 near the sea are constantly supplied with minute por- 

 tions of it, in the fogs and rains blown from the sea 

 to the land. 



116. Smphuret of Iron. — As before stated, there are 

 three sulphurets of iron, the protosulphuret (FeS), 

 the sesquisulphuret (Fe^'S^), and the bisulphuret (FeS"^). 

 The first often occurs in boggy and marshy soils. It is 

 not known to be in itself hurtful to vegetation, but when 

 exposed to the air it absorbs oxygen, which coverts the 

 sulphur into sulphuric acid, and this last, combining 

 with the iron, forms sulphate of iron, which is de- 

 cidedly injurious to vegetation. The injurious effects 

 are counteracted by the use of lime, marl, or ashes. 

 The latter should not be applied till the land is thor- 

 oughly drained, as the soluble parts (potash and soda) 

 would otherwise be lost. The bisulphuret is abund- 

 ant in nature, existing in all rock formations, and 

 probably in nearl;' all soils. When crystallized, it 



