102 THE CHEMISTEY OF THE FARM 



purchase manure in its place. The sale of straw will be 

 attended with little practical loss on heavy land ; but 

 on light land both the loss of potash, and the diminu- 

 tion in the bulk of the farmyard manure, will be more 

 or less felt. The sale of hay or roots is far more 

 exhaustive and, except on the most fertile soils, must 

 demand a considerable purchase of manure or cattle 

 food to replenish the soil with plant nourishment. 



Equilibrium in Soils. — The gains of nitrogen in a 

 soil laid down to pasture proceed to a certain point and 

 then cease. The annual application of a heavy dress- 

 ing of farmyard manure to arable land will increase 

 for many years the nitrogen in the soil, but the in- 

 crease annually becomes less, and at last ceases. On 

 the other hand, if arable land in high condition is left 

 unmanured, and continuously cropped with corn, the 

 crops will at first rapidly diminish ; but after some 

 years a falling off will no longer be perceived, and the 

 small crop then obtained will become a fairly constant 

 quantity. In every case an equilibrium is established 

 between the annual supply of organic matter, and the 

 quantity annually oxidised. With an increased supply 

 of organic matter, as crop residue or manure, the 

 quantity of organic matter in the soil increases ; but 

 the agents of putrefaction and oxidation — the insects, 

 fungi, and bacteria — increase with the increased supply 

 of food, until the decrement by decomposition is equal 

 to the increment by supply. When, on the other hand, 

 a soil is falling out of condition, the living organisms 

 in the soil are necessarily starved, and gradually dim- 

 inish in numbers, till their requirements are again 

 exactly satisfied by the supply of root and stubble 



