12 SILICON ALUMINIUM CHLOKINE 



iron is divalent, and ferric compounds, in which it is trivalent ; only 

 the latter are suited to the requirements of plants. 



Iron is essential as a plant food, but a very small quantity suffices. 

 It is rarely advisable, therefore, to use iron compounds as manures. 



Silicon always occurs in combination, either with oxygen as 

 silica, SiO.>, which is found as quartz , flint, sand, etc., or with oxygen 

 and metals as the very numerous and abundant silicates, e.g., felspar, 

 mica. 



Sand, which consists of little fragments of quartz, is very per- 

 manent and is little affected by water or carbon dioxide. 



Sand forms the largest portion of most soils and, if pure quartz, is 

 devoid of plant food. It greatly affects the porosity and general texture 

 of the soil. The silica which many plants contain is not believed to 

 be essential to their growth ; it probably is taken in by the roots of the 

 plant in the form of soluble silicates or of soluble silica formed, not 

 from the sand itself, but by the decomposition of silicates. 



Silica, while not necessarily an essential constituent of plant food, 

 has been shown by Hall and Morrison to have an important influence 

 upon plant nutrition when applied as sodium silicate. In its presence, 

 barley is apparently able to obtain from a soil a larger proportion of 

 phosphoric acid and thus to give an increased yield. 



Aluminium is never found in the free state in nature. It is ex- 

 tremely abundant and is one of the most important constituents of 

 most mineral silicates. In the form of felspar and mica, it enters 

 largely into the composition of many igneous rocks. By the action of 

 water and carbon dioxide upon felspar (K 2 O.Al a O 3 .6Si0 2 ) the potash is 

 to a great extent removed and a residue of kaolin or china clay, 

 Al 2 O 3 .2SiO 2 .2H 9 O, is eventually obtained. Ordinary clay consists of 

 a mixture of kaolin with some incompletely decomposed felspar and 

 therefore is rich in potash. Clay constitutes an important ingredient 

 of soils, to which it imparts valuable properties, especially as regards 

 retentive powers for water and other substances. 



Aluminium is apparently not a plant food, though the ashes of 

 some few plants contain it in small quantities. This is notably the 

 case with certain fungi, lycopodium in particular. 



Very small dressings of aluminium salts or of alumina or kaolin 

 are also said to have a beneficial effect upon wheat, barley and flax, 

 according to experiments in Belgium and Japan. 



Chlorine is an element possessed of remarkable and well-known 

 properties. It rarely occurs in the free state in nature, but in the form 

 of metallic chlorides is very abundant. This is particularly true of 

 sodium chloride, NaCl, which is found in sea and most spring water 

 and as rock-salt. Chlorine is an essential constituent of plants, and 

 in some crops, e.g., mangolds, it occurs in large quantity. 



It is also absolutely necessary as a constituent of the food of 

 animals and is often required in larger quantities than the ordinary 

 diet of the animal can supply. 



