478 



Toicn Sewage. 



other fertilisers in its composition. We now proceed to consider 

 whether that composition is such as to suit the requirements of 

 our crops ; or, more precisely, whether its phosphoric acid and 

 potash — the most important incombustible constituents — are 

 relatively deficient or in excess for cereal, pulse, or root crops. 



Table V. shows the proportion of phosphoric acid and potash 

 to 100 of nitrogen in sewage, according to the mean of 10 

 analyses in the Rugby sewage. I* also shows, approximately, 

 the average proportion of phosphoric acid and potash to nitrogen 

 in various crops : — 



Table V. — Amount of Phosphoric Acid and Potassa to 100 K'itrogcn, in 

 Sewage and in various Crops. 



~ It is obvious that since the phosphoric acid of sewage, like the 

 nitrogen, will be derived almost exclusively from excreta and 

 food-refuse, its proportion to the nitrogen will be tolerably 

 uniform ; the amount of potash, on the other hand, will be con- 

 siderably greater where the streets or roads are constructed of 

 potassic minerals, such as granite. 



The Table shows that, according to the analyses referred to, 

 the Rugby sewage contained 27 parts of phosphoric acid and 42 

 parts of potash, for 100 of nitrogen. Meadow-hay contains on 

 the average almost exactly the same proportion of phosphoric 

 acid to nitrogen as the sewage, but a much greater proportion of 

 potash than the latter.* 



In the cereal grains the proportion of phosphoric acid to nitro- 



* According to Baron Liebig's estimates, hay contains 51 parts of phosphoric 

 acid to 100 of nitrogen ; but having collated and averaged the results of numerous 

 independent observers, we can see nothing to lead to the adoption of such a figure ; 

 whilst direct determinations in a number of samples of each, showed in the Rugby 

 sewaged grass 25, and in the unsewaged 32 parts. — [Note by Lawes and Gilbert.} 



