SOILS AND THEIR PROPERTIES 73 



having certain common properties. When such substances 

 as sulphate of ammonia come into contact with ** calcium 

 silicate or humate," the sulphuric acid part of the sulphate 

 of ammonia combines with the calcium, whilst the ammonia 

 enters into combination with the silicic or humic residue. 

 Such actions are not so beneficial to the soil as the actions 

 of the same fertilizer on calcium carbonate. It is only where 

 plant production is carried out to a low degree that calcium 

 silicate and humate can be considered as a substitute for 

 calcium carbonate. Intensive plant production necessitates 

 the presence of calcium carbonate. Water containing 

 carbon dioxide can also react with these " calcium silicates 

 or humates," producing calcium bi-carbonate. The presence 

 of calcium carbonate can be detected by the degree of 

 effervescence which is produced on the addition of hydro- 

 chloric acid. A little experience will enable one to judge 

 fairly well of this point, but sodium carbonate and magnesium 

 carbonate will give the same effervescence. For most 

 purposes a knowledge of the carbonate present is of more use 

 than a knowledge of the actual amount of calcium (see p. 75). 

 Calcium carbonate checks '* finger and toe " in turnips. 



Magnesium. — Magnesium in the soil will generalty 

 occur as magnesium carbonate, magnesium bi-carbonate, 

 complex magnesium silicates, magnesium humates, and, 

 very rarely, traces of magnesium sulphate or chloride. 

 Magnesium is certainly a necessity of plant life, and is 

 stored in the cereal seeds to an appreciable extent. Soils 

 very deficient in magnesia show beneficial results from the 

 application of magnesium carbonate, but soils containing 

 much magnesia usually show bad results from the addition 

 of magnesium carbonate. A theory has been suggested that 

 the ratio of magnesia to lime is important in plant life. 

 A soil in County Durham, for example, which has failed 

 both for agriculture and forestry shows CaO:MgO:: 1:9*2. 

 There is some evidence in support of this view, but it is so 

 much disguised by other factors that at present the subject 

 must be left open to doubt. There is no question that soils 

 containing much magnesia are generally benefited by an 



