I. INTRODUCTION 11 



Calcium is an extremely abundant element, always occurring in 

 combination. The carbonate, constituting the main ingredient in 

 limestone, chalk and marble, and the sulphate, which is found as. 

 gypsum or selenite, GaSO^E^C), and also as anhydrite, CaS0 4 , are 

 very abundant. Calcium is also found in union with phosphoric acid 

 in the various deposits of apatite, 3[Ca 3 PoO 8 ]CaCl 9 (or CaF ). Calcium 

 carbonate, which is extremely abundant, dissolves in water containing 

 carbonic acid and is therefore found in all natural waters, from which 

 it is extracted by shell-fish, of whose hard parts it forms the chief con- 

 stituent. It is essential as a plant food, but its agricultural importance 

 arises rather from its effect in altering the texture of soils and in 

 modifying the chemical changes attending the fermentation and decay 

 of their organic matter. For example, calcium carbonate, lime, and 

 other calcium compounds have a remarkable action upon clay, render- 

 ing it much less tenacious and plastic. The presence of calcium 

 carbonate or some other substance capable of acting as a weak base, 

 is essential to the important process of nitrification. Into plants, 

 calcium is probably absorbed in the form of nitrate, phosphate, sulphate, 

 or carbonate, and is found in all parts of the organism. In animals 

 the calcium compounds are usually concentrated largely in the hard 

 parts, the bones or shells. 



Owing to its abundance, calcium is rarely used as a manure in the 

 strict sense of the. word, i.e., as a plant food, but it is largely employed 

 in agriculture either as free lime for the sake of the improvements it 

 produces in the texture of the soil or, in combination with other in- 

 gredients of manurial value, e.g., phosphoric acid, as basic slag, super- 

 phosphates, etc. 



Magnesium also occurs only in a state of combination, often 

 associated with calcium. Limestone and other forms of carbonate of 

 lime invariably contain some carbonate of magnesium, which in some, 

 e.g., magnesian limestone, is present in considerable proportion. 



Magnesium is also found in many silicates, e.g., meerschaum^ 

 steatite, talc and serpentine. It is also present in sea-water and in 

 many mineral springs, to which it imparts a bitter taste. 



Large quantities of magnesium compounds are found associated 

 with potassium compounds in the Stassfurt deposits. 



Magnesium is always present in plants, but as a rule there is far 

 more present in a soil than is necessary for the crops' requirements. Con- 

 sequently it is not of much importance as a fertiliser. 



Iron is very abundant in nature, generally in the state of combina- 

 tion. Native metallic iron is occasionally found, but in insignificant 

 amount. As oxides (Fe 2 O 3 and Fe 3 4 ) and carbonate (FeCO 3 ) im- 

 mense quantities occur in "the minerals known respectively as hcematite, 

 magnetite and spathic iron ore. These compounds form the most 

 valuable ores of iron. In combination with other substances iron is 

 also abundant. It is the main cause of the red or yellow colour of 

 soils. Two series of compounds are known, ferrous salts, in which 



