GROWTH OF PEA AND BAKLEY COMPARED. 155 



Whilst the barley-corn should not lie lower than an 

 inch, the pea thrives best if the seed is put 2 or 3 inches 

 deep in the soil. The roots of the pea-plant do not 

 spread sideways but go deep into the earth ; hence peas 

 require a deep soil tilled down to the lower layers, and 

 a loose subsoil. Fresh manure has scarcely any in- 

 fluence upon the growth of peas. 



It results from these peculiarities of both plants, that 

 the barley derives the conditions of its growth princi- 

 pally from the arable surface soil, the pea principally 

 from the deeper layers of the soil. What the ground 

 may contain below the depth of 6 inches is a matter of 

 indifference for the barley ; the contents of these deeper 

 layers are everything to the pea. 



If we now inquire what demands are made upon the 

 soil by the two plants, we find from Mayer's investiga- 

 tions (' Results of Agricultural and Chemical Experi- 

 ments, Munich, 1857,' p. 35), that the pea-seeds contain 

 one-third more ash constituents (3.5 per cent.) than the 

 barley-corns, and that the amount of phosphoric acid is 

 pretty much the same in both (2*7 per cent.). There- 

 fore, all other conditions being equal, the subsoil from 

 which the pea derives its phosphoric acid must be as rich 

 in that ingredient as the arable surface soil which sup- 

 plies it to the barley. 



The case is different with nitrogen for the same 

 amount of phosphoric acid, peas contain nearly twice as 

 much nitrogen as barley. Assuming both plants to 

 derive their nitrogen from the soil (which is, perhaps, 

 not quite correct in the case of peas), then for every 

 milligramme of nitrogen absorbed by the roots of the 

 barley from the arable surface soil, twice as much must 

 be received by the peas from the deeper layers. 



These considerations throw some light, I think, upon 

 the cultivation of peas ; for this plant requires a very 

 peculiar condition of the soil ; and it is more easy to 

 conceive that a ground exhausted by bearing peas 

 should refuse to bear any more, than that the same soil, 

 after the lapse of some years, should again become fruit- 

 ful for this plant. 



