74 PLANT PRODUCTS 



application of lime, but that is also true of soils which contain 

 but little magnesia. There is also plenty of evidence that 

 the general balance of fertilizing ingredients in a soil is an 

 important point, and whether the lime -magnesia ratio has 

 any specially great importance beyond other ratios, say 

 lime to iron, is a point which has not yet been satisfactorily 

 settled (see p. 8). 



Potassium. — Potassium occurs chiefly in the soil as 

 felspars, hornblende, and other minerals. A fair proportion 

 of potassium in the soil also occurs in combination with 

 organic matter, which is commonly known as potassium 

 humate. A certain quantity of soluble silicates containing 

 potassium occurs in soil water. The proportion of potash 

 extracted by weak acids is very small indeed, sometimes only 

 a fiftieth part of the total potash in a soil is capable of being 

 dissolved by a i per cent, solution of citric acid. 



Sodium. — Sodium occurs chiefly in silicates of a complex 

 type, which are not so stable as the corresponding potassium 

 compounds. The action of weathering these silicates 

 results in the production of sodium bi-carbonate, which, 

 reacting upon the fine clay particles, produces a sticky and 

 impervious mass. In some parts of the world, such as India 

 and the United States of America, salt incrustations on soils 

 are common, ruining many miles of otherwise good soil. 

 Where there is little organic matter the incrustation is 

 white, where there is much the colour is often black. The 

 terms, reh, usar, white alkali, black alkali, are the common 

 names for this type of soil. lyack of drainage is one of the 

 chief causes of the serious accumulation of sodium salts in 

 a soil. The addition of calcium sulphate in any form will 

 result in flocculating the clay, and, therefore, in improving 

 the drainage. The mere operation of cultivation will also 

 assist in improving the drainage and thereby prevent the 

 accumulation of soda. Sodium has no particular value to 

 the soil, and is, therefore, often omitted from analyses. 



Phosphoric Acid. — The only compounds of phos- 

 phoric acid that are found in the soil are derived from 

 ortho-phosphoric acid. Phosphoric acid is, of course, a 



