206 GROWTH 



amount of manganese required for good growth is extremely 

 small ; an artificial manure containing but 25-5 parts of man- 

 ganese sulphate per million added to the soil being sufficient 

 to produce normal vegetative growth and fruit in the tomato.* 



Similarly minute traces of calcium are important in the 

 development of seedlings. f 



The above observations relate in the main to sand and water 

 cultures grown in laboratory conditions ; the following table, 

 results obtained at Rothamsted,^ generally illustrates the 

 action of various chemical fertilizers on field crops : — 



WHEAT— (Average yield 1852-1912). 



Grain in Straw in 



Bushels. Cwts. 



Complete minerals . . . . 32-1 32-9 



Ammonium salts -f- superphosphate . 22-9 22-3 



Ammonium salts + superphosphate + 



sodium sulphate .... 29-1 28-0 



Ammonium salts -f- superphosphate -f po- 

 tassium sulphate . . . . 31-0 31-5 



Ammonium salts + superphosphate -f- 



magnesium sulphate . . . 28-8 28 



GRASS — (Cut for hay every year). 



1918. 1919- 1920. 



Complete mineral manure . . . 45-2 3-29 46-0 cwts. 



Minerals without potash . . . 25-9 19-7 27-3 



Complete minerals + ammonium salts . 46-7 53-2 49-6 



Minerals without potash + ammonium salts 33-5 34-5 32-6 ,, 



In this connection the relative abundance of colloids in the 

 soil would appear to be of importance ; thus Jennings § finds 

 that colloids added to the culture medium promoted or de- 

 pressed the growth of wheat seedlings according to the specific 

 colloid employed and the concentration of the culture medium. 

 Agar, for instance, in the presence of low concentrations of 

 nutrient salts increases growth, but in high concentrations 

 a depression obtains. Of inorganic colloids, ferric and alu- 

 minium hydroxides depress, whilst colloidal silica increases 

 growth, doubtless because silica is much absorbed by the 

 cereal grasses and possibly in this form. In view of the 



* Schreiner and Dawson : " Journ. Ind. Eng. Chem.," 1927, 19, 400. 

 f Maquenne and Cerighelli : " Compt. rend.," 1922, 174, 1269. 

 I For these we are indebted to Dr. W. B-renchley. 

 § Jennings : " Soil Science," 1919, 7, 201. 



