Behaviour of Silicates. 105 



analysed to ascertain whether it contained any superphos- 

 phate. It was found not to contain any ; nor did it contain 

 nitrate, or nitrogen in any other form. It is suggested that 

 by the action of the alkaline silicate on the soil, otherwise 

 locked-up phosphoric acid was rendered available for the 

 plants. But it is possible that, when the superphosphate 

 was used, a secondary result of its action within the soil was 

 the liberation of silicates, which, without it, were not 

 available in sufficient quantity ; and hence the little effect of 

 the direct supply of silicates where the superphosphate was 

 used, and the marked effect where it was not employed. 

 Another suggested explanation is, that when the acid phos- 

 phate and the alkaline silicate are mixed together, they are 

 rendered comparatively insoluble and inactive. The result 

 may perhaps be due in part to more than one of these 

 actions. Another remarkable result was that after mineral 

 manures once applied, nitrate of soda alone, for nineteen 

 years in succession, yielded a result, in proportion to its 

 nitrogen, comparatively little inferior to ammonia-salts used 

 every year in conjunction with superphosphate, or with 

 superphosphate and salts of potash, soda, and magnesia. 



From a review of all the data brought forward in this 

 section is drawn the practical conclusion that when an increase 

 of barley is obtained by means of artificial manures, such as 

 sulphate of ammonia, or nitrate of soda, or Peruvian guano, 

 an increase of one bushel of grain (521b.), and its proportion 

 of straw (say 631b.), may, taking the average of seasons, be 

 calculated upon for every 21b. to 2Jlb. of ammonia, or its 

 equivalent of nitrogen (1'65 to l'861b.), supplied in the 

 manure provided the amount applied be not excessive, and 

 provided there be no deficiency of mineral constituents within 

 the soil. These conditions are fulfilled when barley, grown 

 after dunged roots carted off, or after another corn crop, is 

 manured by from l^cwt. to 2cwt. of sulphate of ammonia, or 

 Ifcwt. to 2Jcwt. of nitrate of soda, with 2cwt. to 3cwt. of 

 superphosphate per acre ; or, from 3cwt. to 4cwt. of Peruvian 



