SOME SILVICULTURAL ASPECTS AND PROBLEMS OF THE SOIL. 4 1 



From the same difference in the economic time factor there 

 arises another fundamental difference in the treatment of the 

 soil in the two arts. In agriculture practically the whole crop 

 is removed every year or few years from the surface of the soil 

 (except stubble, roots, grass and such like) ; hence when agri- 

 culture is continued indefinitely without the renewal of the 

 losses so incurred, the soil becomes infertile owing to the lack 

 of nutrients and of the humus "pabulum," without which such 

 plants will not grow. These losses are repaired by means of 

 the artificial addition of various manures — farmyard manure 

 and green manure provide humus and general nutrients, while 

 artificial manures provide particular salts lacking. Such losses 

 do not occur to anything like the same degree in natural forest 

 soils, provided the conditions in the forest are not abnormal : 

 the bulk of such salts as are annually removed from the soil are 

 returned regularly in the form of dead leaves, twigs, rootlets, 

 etc., which, on decomposing, make available to the roots of the 

 crop the nutrients they contain, and improve the condition of the 

 soil by the humus they form. Thus, owing to the gradual 

 liberation of such elements from the mineral soil, and their 

 circulation through the plant and return to the soil (only a 

 small annual removal occurring to be stored in the timber), it is 

 possible that the actual amount of nutrient material and humus 

 present in the soil may be greater after a first crop than it was 

 before. This phenomenon has been taken advantage of in 

 primitive methods of agriculture. When a soil becomes in- 

 fertile owing to long-continued cultivation it is allowed to go 

 back to forest conditions, or is actually planted with some tree 

 which pumps up the salts liberated from the mineral soil, and 

 deposits them along with nitrogenous leaf-mould on the surface; 

 and when this process of enrichment has gone on for some time 

 the timber is removed — cut as firewood or burnt on the spot — 

 and cultivation is recommenced. 



But the deposition and retention of these salts, and the forma- 

 tion of mild humus in the soil, depends in the main on two 

 factors, the conditions under which the leaf-litter decomposes 

 and the type of tree or trees involved. If the soil is exposed to 

 the heat of the sun, and to free play of air, decomposition may 

 take place so rapidly that the beneficial effects do not last over 

 the growing season. If it is badly aerated then semi-decom- 

 position occurs, raw humus is formed, and the salts are retained 



