ABSORPTION OF ASH-CONSTITUENTS 



93 



not nearly all of the materials thus extracted from the soil can be assimilated by 

 the plant, and also that much material that the plant might eventually absorb 

 is not thus extracted. It must also be emphasized that plant species differ 

 very greatly in their power to absorb salts from the soil. 



If the soil does not contain the essential elements in a sufficient amount and 

 in the proper form for assimilation by plants, its productiveness may be in- 

 creased by the addition of suitable fertilizers. The gain that may be obtained 

 from the use of the fertilizer depends' not only upon the properties of the latter 

 but also upon those of the soil and upon the plant species that is to be culti- 



Fig. 58. — Effect of fertilizing oats with different kinds of Thomas slag (1-3) and with 

 phosphorite (4), all showing different solubilities of their phosphates in ammonium citrate 

 solution. The relative solubilities of the phosphates are shown by the numbers below the 

 pots. Culture 5 received no addition. (After P. Wagner.) 



vated. For example, let us consider phosphatic fertilizers. Thomas slag is 

 one of the best of these. It is a by-product derived from the manufacture of 

 steel out of pig iron. The latter contains silicic acid, sulphur and phosphorus, 

 which are oxidized, through the addition of lime in the process, to calcium salts, 

 and these rise to the surface of the molten steel as slag. Such slag varies accord- 

 ing to the solubility of its phosphoric acid in an acid solution of ammonium 

 citrate. The varieties with large amounts of phosphates that are soluble in 

 ammonium citrate are good fertilizers, while other varieties are not useful in 

 this way. 



