72 PLANT PRODUCTS 



soils contain their iron in the ferric condition, lower conditions 

 of oxidation are prejudicial to plant life. 



Aluminium. — This element occurs in combination 

 with silica. Substances like felspars are not infrequently 

 present in soils. Those felspars which contain potassium 

 are fairly resistant to weathering, but those containing much 

 sodium are more readily weathered down. Clay soils contain 

 a larger proportion of aluminium compounds than are found 

 in sands. The aluminium probably plays but a small part 

 in the chemical changes of the soil, excepting so far as it is 

 one of the constituents of complex silicates. 



Manganese. — Manganese is present in most soils to 

 a very small extent, but occasionally the amount rises 

 as high as i per cent. It is possibly an element of some 

 importance, as it is found invariably in beech trees, and is 

 a very common constituent of grass and root crops, but 

 the amounts present are small. The red colour of the red 

 beech leaf and red hair is believed to be due to manganese 

 compounds. 



Titanium is always present in soil to the extent of 

 a per cent, or so, but is commonly left mixed with silica in 

 analjrtical returns. It is not known to have any value. 



Calcium. — This element is one of the most important 

 in the soil. The most useful form is calcium carbonate, 

 which by slow solution in water containing carbon dioxide 

 becomes calcium bi-carbonate, an important agent in the 

 process of nitrification, and in the flocculation of clays. 

 Calcium sulphate is often present in small amounts. The 

 oxidation of sulphur compounds in the soil will result in 

 the production of calcium sulphate with the aid of some 

 source of lime. Complex compounds of calcium with 

 siliceous substances, and complex calcium compounds with 

 organic materials, are of only slightly less importance than 

 calcium carbonate. These compounds are respectively 

 alluded to by the vague general terms of calcium silicates 

 and calcium humates. It must not be supposed that the 

 constitution of cither of these bodies is known. These 

 names are only general terms expressing groups of compounds 



