46 SOIL CONDITIONS AND PLANT GROWTH 



effects which, as we have seen, can be lessened by addition of ca 

 salts ; Loew indeed considers (180) that plants require a definite 



MgO 



ratio in their food, but neither Gossel l nor Lemmermann 2 could 

 obtain evidence of any such necessity. 



Iron. For some reason difficult to explain the formation of chloro- 

 phyll is absolutely dependent on the presence of a trace of some ferric 

 salt, although iron does not enter into the composition of chlorophyll. 

 So little is wanted that iron salts never need be used as manures, ex- 

 cepting for water or sand cultures. 



Manganese is considered by Bertrand to be a constituent of oxidases, 

 and, therefore, necessary to the plant ; minute traces only are required, 

 larger quantities being harmful. A number of field experiments 3 have 

 shown that manganese salts may act as manures. Bertrand classes 

 them as "engrais complementaires " (35). 



Chlorine does not appear to be necessary to the plant, indeed Knop 

 grew even the halophytes without it. Chlorides are always present in 

 rain water in ample amount to supply any trace that might be needed. 

 In small doses iodides and fluorides have been found, according to 

 Japanese experiments, to produce beneficial results (183 and 278). 



Sulphur is probably an essential food constituent, and occurs in 

 plants, especially in cabbages and swedes, to a greater extent than is 

 usually recognised, the older analytical methods giving low results 

 (Hart and Peterson (124), Peterson (225^)). Sulphates are present in 

 rain and in soil, but further additions in manure have been found by 

 Dymond (92) to be useful for heavy crops rich in protein, although 

 they were not needed for cereals or permanent pastures. These 

 observations confirm the older work of Bogdanow. 4 



Silicon does not seem to be essential, but it occurs to so large an 

 extent in some plants that it is not likely to be wholly useless. Wolff 

 and Kreutzhage (315) found that soluble silicates increased the yield 

 of oats in water cultures and also the proportion of grain, behaving 

 in their opinion much like phosphates. Certain of the Rothamsted 

 plots are treated with sodium silicate, and marked crop increases are 

 obtained on the phosphate-starved plots (Table XXI.) Hall and 



1 Bied. Zentr., 1904, xxxiii., 226. 



2 Landw. Jahrbuch, 1911, xl., 175 and 255. 



3 Numerous Japanese experiments are recorded in the Bull. Coll. Agric., Tokyo, 1906, 

 et seq. (210), and Italian experiments in the Studi e Ricerche di Chimica Agraria, Pisa, 

 1906-8; pot experiments have also been made by J. A. Voelcker at the Woburn Experiment 

 Station. See also W. E. Brenchley (54). 



4 Expt. Stat. Record, igoo, n, 723 and 1903, 15, 565. 



