STIFF SOILS 79 



and 40 years of age, of such trees as require a deep soil ; 

 whereas it would be quite impossible to grow mature timber 

 of such trees. 



4. The Demands of Trees as to the Mechanical Condition 



of the Soil. 



The soil best suited to the growth of most trees is a 

 slightly stiffish loam with a deep soil covering of decaying 

 humus. Very porous soils are usually dry, especially if 

 shallow, and hence only suited to a comparatively small 

 number of trees, whereas on very stiff soils the choice of 

 trees is still more limited. In this latter case the roots have 

 a difficulty in penetrating, and they do not obtain enough 

 air, and the soil gets sour. The natural drainage on such 

 soils is bad, and they are more liable to late and early frosts. 

 Often, especially on sandy gravels on which only heather is 

 growing, there will be a pan or a hard, thin, impenetrable 

 layer a few inches below the surface of the soil. The roots 

 of trees will never penetrate such a pan ; and if it be neces- 

 sary to plant such land, the pan must be artificially broken up. 



So also in cases where the seedling growth of trees is 

 required, it is most essential to have a few inches of the 

 humous soil, for, apart from the extra amount of moisture 

 assured on dry soil, it will enable the growth of seedling 

 roots to proceed with little hindrance. 



Now, on the stiffest soils the best trees to plant are 

 Norway Spruce, White Poplar and Pedunculate Oak, though 

 the growth of all, and especially of the latter, will be very slow. 



But it must be remembered that very stiff soils are not 

 at all suited to tree growth. Often, however, good Spanish 

 Chestnut coppice is grown on stiff clays, and it would seem 

 that many trees will succeed better on stiff soils when 

 coppiced, than when grown for timber. On soils not quite 

 so stiff, but, however, still classed as heavy land, the three 

 trees just mentioned will thrive, and also Sessile Oak, Black 

 Poplar, Black Italian Poplar, Sitka Spruce, Cupressus 

 macrocarpa, Silver Fir, Thuya gigantea, Hornbeam, Spanish 

 Chestnut, Sycamore, Norway Maple, and Beech, so also 



