SOILS AND THEIR PROPERTIES 79 



and mechanical analysis. Soils containing large percentages 

 of sand and insoluble silicates are of a light, sandy character, 

 those containing low amounts are of a heavy clay character, 

 unless, when we must always reconsider the results of physical 

 analyses, the soil also contains much lime or organic matter. 

 Soils containing much iron are hungry for phosphoric acid, 

 though when supplied with phosphoric acid they usually 

 become very fertile soils. The aluminium is an indication 

 of the amount of clay present. Manganese has little general 

 interest, although there is distinct evidence that small 

 quantities of manganese are useful (see p. 9). The lime 

 is a most important ingredient, and when the lime falls to 

 low figures fertility is at a low ebb. Magnesia in small 

 quantities is probably beneficial, in large quantities it appears 

 to be haimful. The ratio of lime to magnesia is sometimes 

 considered important. Where the magnesia exceeds the 

 lime there is considerable evidence that the magnesia is 

 harmful. The potash extractable by hydrochloric acid is 

 a figure of no practical value. The phosphoric acid dissolved 

 by hydrochloric acid should not fall below ci per cent. 

 Sulphuric acid may be ignored except in districts where there 

 is no coal smoke and little artificial manure used. The 

 carbonic acid evolved in the cold by dilute acids is valuable 

 as an indication of the amount of calcium carbonate in the 

 soil. It will be noticed in the figures given that the organic 

 matter and water of combination in Northumberland are 

 very high in proportion to those in the Indian soils quoted. 

 This is quite typical of the difference betw^een cold and hot 

 climates. The nitrogen is usually very low in well-cultivated 

 soils in hot countries and high in forest or pasture in cold 

 climates. The figures for nitrogen can only be taken in 

 conjunction with other evidence. The available phosphoric 

 acid and potash soluble in i per cent, citric acid form some of 

 the most useful figures in the table. Much, of course, will 

 depend upon the kind of crop grown, but for crops of no 

 great exhaustive character, O'Oi per cent, will make a good 

 dividing line between fertility and need of manure. In 

 considering the chemistry of soils one should consider rather 



