SUPPLY OF MINERAL INGREDIENTS. 139 



205. It should be the object of the agriculturist, therefore, to 

 ascertain the chemical character of the earthy portion of the soil 

 which he cultivates, and to manure it with such substances, as he 

 finds will supply the deficiency, for the particular plants which he 

 wishes to grow. There are some soils, which contain all the 

 ingredients required for almost any kind of vegetation ; and, when 

 these cease to be productive, all that is necessary, is to allow them 

 to lie fallow during a season or two. The atmosphere then acts 

 upon the mineral particles, and causes that more complete separa- 

 tion amongst them, which is necessary to prepare them for being 

 dissolved in water, and for taking up by the roots of the plant. 

 In this manner some soils prove extremely fertile, which scarcely 

 contain a particle of vegetable mould, and have received very 

 little animal manure. Thus the land in the neighbourhood of 

 Mount Vesuvius contains clayey earths, with chalk and sand, 

 mixed in such a proportion, as to give free access to air and 

 moisture. This soil is produced by the slow decomposition and 

 separation, through the agency of the air, and of the roots of 

 a Cactus that act in the manner formerly described (. 108), 

 of the masses of lava which have at different times issued from 

 the volcano, and which contain a great admixture of mineral 

 matters, without a particle of vegetable mould. Now corn has 

 been grown on this land for thousands of years, with scarcely 

 any manure ; the method adopted being simply this. A field 

 is sown once every three years only; and is in the intervals 

 allowed to serve as a sparing pasture for cattle, which feed on the 

 weeds that spontaneously spring up. But the influence of the 

 weather sets free an additional quantity of the mineral ingre- 

 dients, which the corn requires ; the amount of nutriment con- 

 tained in the seed, is sufficient for the development of the young 

 plants ; and the soil is of a kind extremely favourable to their 

 subsequent growth. 



206. On the other hand, the very fertile land, which was 

 found by the first settlers in Virginia, has been exhausted by a 

 contrary proceeding. Harvests of wheat and tobacco were 

 obtained for a century, from one and the same field, without 

 the aid of manure ; but now whole districts are converted into- 



