30 SYLVICULTURE. 



is unfavourable, it will either show poor growth or be unable 

 to grow at all. Hence timber-crops, like field-crops, are subject 

 to what is known as the Law of tlw Minimum, according to 

 which " the total extent of production depends upon whatever 

 essential factor is present in the lowest degree " no matter 

 how favourable may be the combination of any or all of the 

 other factors. The mineral food of trees includes potash, lime, 

 magnesia, iron, sulphur, phosphorus, and nitrogen ; while silica, 

 soda, chlorine, manganese, and occasionally alumina, are also 

 found in the ash of timber, after carbon, hydrogen, oxygen, 

 and nitrogen have been eliminated by burning. But the dif- 

 ferent physiological uses of these mineral substances are not 

 yet clearly understood. 



Experience shows that timber-crops of one kind or another 

 can be profitably grown on any kind of land that is neither too 

 dry nor too wet. Most kinds of soil contain sufficient mineral 

 food to enable tree-crops of any kind to grow ; but there is not 

 always sufficient soil-moisture to hold it in solution so as to 

 make it available for absorption by the root-system, and when 

 the land is too wet there is a deficiency in oxygen. Thus 

 depth and porosity are of more importance than chemical 

 composition. On the other hand, a rich soil furnishing copious 

 food-supplies will produce large crops of timber, but it will be 

 soft, spongy, and not so durable as timber with denser and 

 more compact annual rings, , t .,r 



Soil cannot always be classified according to its geological 

 origin ; because the same kind of rock does not always decom- 

 pose into similar soils, and its productivity depends on the 

 extent of its decomposition, while some of the lighter particles 

 of clay are more easily washed away than the heavier sand, 

 For .practical purposes the best classification is into 



Sandy Soil, containing 75 per cent or more of disintegrated sand (silica). 

 This includes sand-drifts, sand, and loamy sand. 



Loamy Soil, containing 60 to 70 per cent of fine sand, the rest being 



