156 ACTION OF SOIL ON FOOD OF PLANTS IN MANURE. 



According to these considerations, and assuming an 

 equality of the absorbent root-surface in both plants, a 

 subsoil fruitful for peas must contain as much phos- 

 phoric acid, and twice as much nitrogen, as an arable 

 surface soil suited for the cultivation of barley. For 

 the phosphoric acid, the assumption is correct. 



We understand, without difficulty, the beneficial 

 effect of manure upon an exhausted barley field. Bar- 

 ley derives all the conditions of healthy growth from 

 the surface soil, which is restored to its original state of 

 productiveness by the manure applied. 



But from our acquaintance with the properties pecu- 

 liar to arable soil, we know that a layer 6 to 10 inches 

 deep will retain all the ammonia potash and phosphoric 

 acid contained in the largest quantity of manure usually 

 applied by farmers ; and this, too, so firmly that, except 

 for some accidentally favourable circumstances, hardly 

 a particle will ever reach the subsoil. 



If a field is sown with plants which require deep 

 ploughing, so that a sufficient portion of the rich sur- 

 face is mixed with the exhausted subsoil, it is easy to 

 understand that the latter may gradually become again 

 fruitful for peas. The time in which this is effected 

 depends of course upon the accidental selection of the 

 plants grown in succession on the field. 



In this view of the matter, the agriculturist has it in 

 his power, by right management of his field, to shorten 

 the time, and make the land again fit for successive 

 crops of peas. 



It is a fact, that many fields in the vicinity of towns 

 will bear year after year, or every two years, abundant 

 crops of peas, without ever becoming ' pea-sick ; ' and 

 we know that the gardener, to achieve this result, has 

 recourse to no extraordinary appliances, but merely tills 

 his land deep and very carefully, using much more 

 manure than the farmer can afford to do. 



The frequent failure of peas is therefore not so very 

 unaccountable ; and there seems no reason why the 

 farmer should despair of cultivating peas as often as 

 serves his purpose, if he employ the right means to 



