SOIL CONDITIONS AFFECTING PLANT GROWTH 75 



chloride was distinctly toxic. In Loew's experiments on algae 

 (180^) strontium salts injuriously affected the chlorophyll 

 bodies, causing loss of starch-making power and finally death. 

 Magnesium^ like phosphorus, finally moves to the seed, 

 and is thus in contrast with calcium and potassium, which re- 

 main behind in the leaf or the straw. Willstatter has shown 

 (310) chlorophyll to be a magnesium compound, an observa- 

 tion that accounts for the unhealthy condition of the chloro- 

 phyll bodies, and the final etiolation of magnesium-starved 

 plants. Further, magnesium seems to be necessary for the 

 formation of oil, the globules being absent from algae growing 

 in solutions free from magnesium salts ; oil seeds are richer 

 in magnesium than starch seeds. An excess of magnesium salts 

 produces harmful effects which, as we shall see (p. 78), can be 

 lessened by addition of calcium salts ; Loew indeed considers 



CaO 

 (i8od!) that plants require a definite ratio in their food, 



but neither Gossel ^ nor Lemmermann "^ could obtain evidence 

 of any such necessity. 



In J. A. Voelcker's experiments^ magnesium oxide, car- 

 bonate, and chloride had, like sodium hydroxide, the unusual 

 effect of causing an increase in the nitrogen content of the 

 wheat grain. The sulphate did not act in this way, although 

 in suitable small amounts it caused increases in yield of grain 

 and of straw. The chloride proved toxic at higher concentra- 

 tions. 



Aluminium compounds have been found beneficial by 

 Stoklasa.* 



Iron. — For some reason difficult to explain the formation 

 of chlorophyll 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 iror^ salts 

 never need be used as manures, excepting for water or sand 

 cultures. 



^ Bied. Zentr., 1904, xxxiii., 226. 



^Landw. Jahrbuch, igii, xl., 175 and 255. 



»y. Roy. Agric. Soc, 1915, 76, 354 ; ^9^6, 77. 260. 



* Biochem. Zeitsch., 1918, 91, 137. 



