51 



found as ammonia or nitrate in the effluent being- recovered in the 

 skidg-e. There is considerable evidence that the extra nitrogen in 

 activated shidg-e, over and above that found in the old type 

 sludges, is derived from the ammonia of sewage. There is no 

 evidence of fixation of atmospheric nitrogen. The numbers of 

 protozoa in well-activated sludge approximate to 1,000,000 per 

 gram of wet sludge. The cell content of these organisms alone 

 may account for a large proportion of the extra nitrogen. There 

 is complete correlation between the numbers of active protozoa 

 and bacteria in activated sludge under varied conditions cf working. 

 Observations made in working the experimental tank at 

 Harpenden Sewage Works confirm the laboratory experiments 

 designed to find the source of the extra nitrogen content of 

 activated sludge (X)mpared with ordinary sewage sludges. They 

 afford no evidence of fixation of atmospheric nitrogen, but suggest 

 that in addition to colloidal nitrogen, ammonia is removed from 

 the sewage by physical or biological means, or both. The propor- 

 tion of total nitrogen in the Harpenden sewage recovered in 

 normal working by the acti\ ated sludge process is greater than in 

 the older methods of sewage purification, viz., 15% (^ompared with 

 10% by precipitatitMi and 4% by septic tanks. With sewage of 

 half the average strength and supplying twice the normal volume 

 of air per gallon of sewage, the recovery of nitrogen was as high 

 as 27% of the total nitrogen in the sewage. Field trials show 

 that activated sludge has a high manurial value in marked 

 (^ontrast with the old type sewage sludges tested on the Rotham- 

 sted farm in past years. 



LVI. H. J. I*A(;k. "Green Mcmurinf;." Journal of Ministrv 

 of Agriculture, 1922. Vol. XXfX. pp. 104-112'; 

 240-248. 



Green manuring is discussed as a substitute for dung, the 

 supply of which is insufficient. Variation in type of soil, climate, 

 system of cropping and the like, necessitates different systems of 

 green manuring; similarly the maintenance of productive soils in 

 good heart by green manuring is a problem distinct from that ,of 

 building up the fertility of run-down or naturally infertile land. 

 Thus such systems of green manuring as find appli(^ation in this 

 country \'ary considerably from district to district. Although the 

 beneficial effect of green manures, and of dung, depends on a 

 variety of factors (which are discussed in detail), the prime 

 function of either is to supply humic material to the soil. 

 Artificials can fulfil most of the other functions of green manures 

 or dung, but not this one. 



LVI I. H. J. Page. "Sa^nuf:^ Expense hy Green Manuring/' 

 Modern Farming, 1923. Vol. VI. No. 9. 



In seeking to develop the use of green manuring as a substitute 

 for dung, one of the greatest difficulties encountered is that of 

 fitting the green crop into the rotation, without disturbing the 

 latter. In practice this resolves itself into growing the green crop 

 (i.) during the autumn and winter before roots, (ii.) in early autumn 

 before winter corn. The first method finds application in potato 

 districts (of which instances are quoted), but its feasibility as a 

 preparation for mangolds or swedes is uncertain, and merits 



