36 SEEDING AND PLANTING 



forests on newly cleared sites is due to the large amount of humus 

 in the surface soil. On soils that have been free of vegetation for 

 some time or that for other reasons are deficient in humus, the less 

 exacting species should be used for the first crop. Later, as the 

 soil increases in fertility, particularly in humus content, a change 

 of species can be made. When humus is present in the surface 

 soil in favorable amount, its stimulating effect upon early growth 

 shortens the period of greatest hazard to which the young trees 

 are subjected. Its presence in favorable amount is absolutely 

 necessary in nursery practice in order to attain economic re- 

 sults. 



After a forest has been established, most soils contain sufficient 

 nutrients to grow a crop of trees provided the litter is not re- 

 moved by fire or other agencies. Although fertile soils often pro- 

 duce good crops of timber even when a part of the litter is removed, 

 its annual removal causes them to deteriorate and in time become 

 too poor for the profitable production of timber. 



25.] The Physical Condition of the Soil in Relation to the 

 Choice of Species. Artificial regeneration may fail because of 

 the unfavorable physical condition of the soil. Although all ordi- 

 nary soils contain the mineral nutrients essential for tree life, 

 growth is not determined by the relative abundance of these nutri- 

 ents but rather by the amount that the vegetation is able to ob- 

 tain. This depends very largely upon the structure of the soil 

 from a physical standpoint. The fineness of the soil particles is 

 directly correlated with water capacity, water retention, capillarity, 

 and permeability, all of which are important factors in soil fertility. 



26. SPECIES SELECTED WITH REFERENCE TO THEIR DEMANDS 

 UPON SOIL FERTILITY 



All trees grow better on a soil that is fertile, deep, porous, 

 fresh or moderately moist, warm, rich in humus and in mineral 

 nutrients. The soil which most nearly meets these conditions is a 

 sandy or a clay loam which contains from 20 to 33 per cent of clay, 

 from 50 to 70 per cent of sand, from 3 to 10 per cent of lime, and 

 from 2 to 5 per cent of organic matter. Within their climatic 

 range nearly all species will do well upon soils of this character. 

 These, however, are our most valuable agricultural soils, and it is 

 usually the case that timber must be confined to poorer or less fer- 

 tile soils. Although all species prefer a fertile soil, the different 



