138 SUPPLY OF MINERAL INGREDIENTS. 



203. The soil may be artificially supplied with the mineral 

 substances required by different kinds of Plants, as well as with 

 those which yield carbonic acid or ammonia ; and the cultivator 

 is frequently obliged to do this (though in general without 

 understanding the true benefit of the operation), especially when 

 he has forced the growth in other ways. Thus if a meadow be 

 manured only with gypsum (the use of which has been already 

 mentioned), the crops of grass will be at first greatly increased, 

 but will afterwards diminish ; for the potash which the soil con- 

 tained, is soon exhausted by the rapid growth of the grass, and 

 its further increase is checked. But if the meadow be strewed 

 from time to time with wood-ashes, which contain potash, the 

 grass will thrive as luxuriantly as before. A harvest of grain may 

 be obtained at long intervals on a sandy heath, by strewing it 

 with the ashes of the heath -plants which grow on it, and which 

 gradually collect the alkalies that are conveyed to them by water. 



204. It seems a remarkable fact, that those plants of the 

 grass tribe, the seeds of which furnish food for Man, follow him 

 like the domestic animals. The reason is, that none of the corn 

 plants can bear seeds that will yield a large quantity of flour, 

 without a good supply of phosphate of magnesia and ammonia. 

 Hence these plants grow only in a soil, which contains these 

 ingredients, in addition to the silex and potash already men- 

 tioned ; and no soil is richer in them, than those where men and 

 animals dwell together ; since these substances are largely con- 

 tained in the animal body, and are set free in their excretions 

 during life, and by their general decay after death. Again, this 

 fact explains why bone-earth is a most valuable manure to corn- 

 fields ; since it consists almost entirely of these ingredients. The 

 knowledge of it will also guide us in selecting the kind of wood, 

 of which the ashes will be most valuable ; for whilst those of 

 the oak contain but a minute proportion of the phosphate, and 

 those of the pine a quantity not exceeding the sixth part of 

 their weight, those of the beech yield the fifth part ; and thus 

 with every 100 Ibs. of the ashes of the beech, we supply a field, 

 with enough of these ingredients, to serve for the growth of more 

 than 15,000 Ibs. of corn. 



