54 THE COMPOSITION OF FERTILE AND BARREN SOILS. 



1 . Sandy soils, or soils containing above 80 per cent, of silicious sand. 



2. Calcareous soils ,,20 per cent, of lime. 



3. Clay soils 50 per cent, of clay. 



4. Vegetable moulds (humus soils) 8 per cent, of organic matter, or 



humus. 



5. Marly soils, or soils in which the proportion of lime amounts to more 



than 5 per cent., but does not exceed 20 per cent, of the whole weight 

 of the dry soil, and that of clay is more than 20, but less than 50 per 

 cent. 



6. Loamy soils, or soils in which the proportion of clay likewise varies from 



20 to 50 per cent., but which contain less than 5 per cent, of lime, 

 and all the different constituents in a finely pulverized state. 



Mineral Constituents of Plants. 



We owe a debt of gratitude to Professor Liebig for showing 

 that those mineral constituents which we find on burning the 

 grain of wheat, oats, barley, or grass, and on burning root-crops, 

 are not merely accidental, but tbat they are essential ingredients ; 

 that unless the plants you intend to grow find those materials in 

 the soil you cannot grow them with success. Indeed, if they 

 are altogether deficient, or if one of them is so if phosphoric 

 acid, for instance, is deficient altogether its deficiency prevents 

 wbeat setting its seed. Experiments have been tried repeatedly in 

 artificial soils composed of all the constituents that we find in the 

 asbcs of plants, with the exception of one which, like phosphoric 

 acid, is of great importance to the formation of the grain of wheat ; 

 the result was that the plant would grow to a certain point, and 

 even flower, but it formed no grains, or only one or two imperfect 

 ones. We must lay it down then as an established principle in 

 agricultural chemistry, that, unless those mineral constituents that 

 we find in the ashes of plants are present in the soil, or are sup- 

 plied to it, the plants cannot be grown. It is also be remembered 

 that soils remarkable for a high state of fertility contain some 

 necessary constituents in inexhaustible supplies. 



The soils that we find here are in many instances clays, and 

 contain a considerable quantity of the mineral matters that we 

 find in the ashes of plants ; and we may draw on the resources 

 of those mineral riches without fear of exhausting them whilst 

 in other instances, where the more important articles of mineral 

 food are deficient in soils, the constituents withdrawn must be 

 restored, if we do not wish permanently to detract from the cha- 

 racter of the land. In clay and loamy soils it must be an object 

 to the farmer to inquire how he can best get out of the land the 

 substances needed in plants. He need not scruple in working 

 his land well, or in taking from it without always giving back 

 the same constituent that he has taken. Truly, theoretically con- 

 sidered, nothing is clearer than that what you take out of the 

 land must be restored ; and you may lay down rules for making 



