INTRODUCTION 21 



Sodium is found in the ashes of most plants, but, except 

 in the case of certain species, does not appear to be essential 

 to their welfare. A striking difference between sodium and 

 potassium compounds, which are so much alike in most of 

 their properties, is in their behaviour towards the soil when 

 applied in solution. The potassium salts are retained by the 

 clay and organic matter in an insoluble form, but the sodium 

 salts are easily washed out by water and escape into the 

 drains. 



Mag*nesium is widely met with in nature as carbonate and 

 silicate. The metal itself is a bright, silvery substance, very 

 light, and capable of burning in air or oxygen with an intense 

 and dazzling white light. Magnesia, MgO, magnesium, 

 carbonate, MgCO,, and Epsom salts, MgSO^.THjO, are the 

 compounds most commonly used in the arts. 



Magnesium is found in the ash of plants, and appears to be 

 essential, but as it is rare to find a soil deficient in magnesium 

 it is of little practical importance from an agricultural 

 standpoint. 



Iron occurs in a large number of compounds. Hcematite, 

 Fe^O,, magnetite^ ^^$04, and spathic iron ore, FeCO,, are 

 abundant minerals, valued as ores of iron. The element 

 occurs in two states of oxidation, as ferrous or divalent iron, 

 and as ferric or trivalent iron. The former forms salts which 

 are white or green in colour ; the latter, compounds which are 

 red and yellow. Ferrous compounds are often present in rocks 

 or minerals deep underground, but when they are brought to 

 the surface they combine with the oxygen of the air to form 

 ferric compounds. The change of state of the iron is indicated 

 by a change in colour in the rock or mineral, often from 

 green or grey to red or yellow. Only ferric compounds should 

 occur in good Soils. Iron is essential to plants, but a small 

 quantity is all that is required, and in most soils more than 

 this is present. 



