14 Agricultural Chemistry. 



case of certain plants, does not appear to be essential to their 

 development. A striking difference between sodium and potas- 

 sium compounds, which are so much alike in most of their proper- 

 ties, is in their behavior 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 more easily washed 

 out by water and escape into the drains. Although like potassium 

 in its chemical properties it cannot take its place in agriculture. 



Magnesium (Mg) is widely met in nature as carbonate and 

 silicate. The element itself is a bright, silvery metal, and capable 

 of burning in air with an intense and dazzling white light. Mag- 

 nesium is found in the ash of plants and is required by all crops. 

 It is particularly abundant in the seeds. There is generally in 

 all soils an amount sufficient for crop purposes and it is not 

 necessary to consider this element in connection with fertilizers. 



Iron (Fe) occurs in a large number of compounds. Haematite, 

 a compound of iron and oxygen, magnetite, a similar com- 

 pound, but with a different proportion of oxygen, and spathic 

 iron ore, a compound of iron, carbon, and oxygen ; the above are 

 all abundant minerals and valued as ores of iron. The element 

 occurs in two states of combination with oxygen, one a relatively 

 small amount and called ferrous iron, the other a relatively larger 

 amount and designated ferric iron. The former yields salts which 

 are white or green in color, while those of the latter are red or 

 yellow. Ferrous compounds are often present in rocks or min- 

 erals deep under ground, but when brought to the surface they 

 combine with the oxygen of the air to form ferric compounds. 

 The change of the state of iron is indicated by a change in color, 

 often from green or gray to red or yellow. Only ferric com- 

 pounds should exist in good soils. Iron is essential to plants, but 

 a small quantity is all that is required and most soils contain 

 from one to four per cent, an abundant supply. 



Chlorine (Cl) is very abundant, especially in combination with 

 sodium, as rock salt in the sea and in spring water. Other com- 

 pounds of chlorine also occur as minerals. The element chlorine 



