298 SOILS AND MANUEES 



gredients exist, and so destroys the only possible grounds 

 upon which guanos may be considered superior to the 

 ordinary mixed or dissolved manure containing an equal 

 quantity of plant food. If dissolved or compound manures 

 are to be used, they may as well be made from the 

 cheapest materials. 



Application. It is impossible to say for what crops 

 guianos are most suitable or what quantities should be 

 used, unless their composition is known. The higher 

 grade samples, i.e., those which contain a large propor- 

 tion of nitrogen, should be applied chiefly to the cereal 

 and grass crops ; the more phosphatic varieties are better 

 adapted for roots and potatoes. The application of 

 5 or 6 cwts. per acre of strong nitrogenous guano to 

 turnips can scarcely be justified either on theoretical or 

 economic grounds, though such quantities may not be 

 too large when they contain relatively small quantities of 

 nitrogen. From 2 to 4 cwts. per acre may be advan- 

 tageously employed for grass and cereals. Guano should 

 be applied in the spring, some time before the seed, and 

 should be well harrowed in, in order to avoid risk of loss 

 by evaporation of ammonia. When used as a top dressing 

 it is advisable to mix a quantity of fine soil with it. Salt 

 has sometimes been used for this purpose, but in the 

 writer's opinion, such a practice is not to be recommended. 



The quality of guano cannot be judged from its ap- 

 pearance. The colour and smel) are fairly character- 

 istic, but they can .be easily imitated. The only real 

 safeguard is chemical analysis, and in purchasing this 

 variable and usually expensive manure ifc should never 

 be neglected. 



Sewage. Ordinary sewage consists mainly of human 

 excreta mixed with a relatively very large quantity of 

 water. It is produced at the rate of about 25 or 30 gallons 

 per head per day, and contains from 6 to 8 grains per 



