80 THE MANURE HEAP AND SEWAGE. 
tary. The sewage ingredients, after being incorporated into the 
soil, undergo the various types of bacterial decompositions which 
have been described in recent chapters. The organic compounds 
are decomposed by bacterial action, a part of the resulting decom- 
position products going into the air as gas, and a part being built up, 
in the soil, into nitrates, to feed the growing crops. The offensive 
waste material is thus disposed of by being converted into inoffen- 
sive and useful products. Sewage farms thus prevent the sewage 
contamination of streams, harbors, and seaside resorts. This in 
itself is sufficient to make this method of disposal a desirable one. 
But the appearance of certain practical difficulties has prevented 
a wide development of this system. It is, of course, impossible to 
expect farmers to adopt this system unless it is profitable, and, un- 
fortunately, it frequently proves that such sewage farms are run at 
a loss instead of a gain. To be sure, in some places very favorable 
returns have been yielded, and largely increased crops have been 
reported. But in other places unfavorable reports have been made, 
and at the present time sewage farming is not increasing. Since 
the objections to it are purely practical, they may in time be over- 
come. The chief objections are as follows: i. It is unhealthful to ir- 
rigate garden crops with a sewage that is sure to contain disease 
germs. This objection applies chiefly to vegetable products which 
are eaten without cooking, like lettuce, celery, etc. 2. Land near 
the large cities is usually too valuable to be used for farming proc- 
esses. If the farm is at some distance from the city the expense of 
carrying the sewage to it becomes so great as to make the under- 
taking a losing instead of a profitable one. 3. Only a fairly porous 
and partly sandy soil can absorb the quantity of sewage necessary 
for the disposal of the product. In order that the soil may absorb- 
it, the sewage must be decomposed and nitrified by bacteria. If 
this nitrification does not occur, the soil becomes clogged with the 
sewage products. Hence, if the soil is heavy and contains much 
clay it cannot be used for sewage farming. While sewage farms 
are successful and profitable in the sandy soils around Berlin, they 
are not possible in many another locality where the soil is of a dif- 
ferent texture. 4. The extreme dilution of the sewage makes it 
