THE WORK OF ROOTS 147 



Even the best soil becomes exhausted of certain 

 elements in time if crops are continuously removed 

 from it. The deficient food elements must then be re- 

 placed by fertilizers. The chief substances which it is 

 necessary to replace in this way are nitrogen, phos- 

 phoric acid, lime and potash. It is important to 

 understand how best to supply these elements, since 

 this knowledge often makes all the difference between 

 successful and unsuccessful farming. 



How may nitrogen be supplied to the soil ? Manure 

 piles smell strongly of ammonia gas (which contains 

 over 80 per cent nitrogen), and manure is the most 

 available source of nitrogen as fertilizer. In fresh ma- 

 nure the nitrogen is mostly insoluble, but on stand- 

 ing it decomposes (by the action of bacteria, see page 

 383) into soluble substances containing nitrogen and 

 into ammonia gas and nitrogen gas. These gases will 

 mostly escape into the air and be lost unless we mix 

 soil with the manure so as to form compost: the soil 

 absorbs the ammonia gas to a marked extent and it is 

 then changed to soluble compounds of nitrogen. 1 Air 



1 The absorption of ammonia gas by soil may be shown by filling a test- 

 tube with mercury, closing with the finger and inverting in a dish of mercury 

 so that it contains no air: now introduce ammonia gas by heating ammonia 

 water in a flask, through the cork of which passes a tube to conduct the gas 

 into the test-tube (heat at first long enough to drive out air); when the test- 

 tube is full of gas, introduce a lump of dry clay from below: it will absorb the 

 gas and the mercury will rise. In a control experiment use sand. Cottonseed 

 oil may be used in place of mercury by wedging a lump of clay in the top of 

 an ordinary tube, dipping the tube into a vessel of oil and then corking it at 

 the top with a rubber cork in such a manner that the oil fills the tube nearly 

 to the clay. Raise the tube and introduce the gas as before. In each case 

 use a control tube containing no soil. Decomposing humus also absorbs a 

 good deal of ammonia gas. 



