5SG FOREST LITTKI;. 



a want of manure in agriculture. How greatly the soil thus 

 loses in important chemical constituents (in phosphoric acid for 

 instance) may be gathered from the fact that a soil formerly 

 stocked with broadleaved trees may, owing to the removal ot 

 litter, contain only 0*012 to 0'020 % of phosphoric acid and thus 

 become fit only for Scotch pine. For, according to Weber, at 

 least 0"0o0 % of phosphoric acid is required in soils by broad- 

 leaved trees. Another important point is the scarcity of lime in 

 most sandy soils, whilst much lime is required by most trees. 

 It is also well-known that the indispensable supply of nitrogen 

 depends on the quantity of humus in soils. When it is further 

 considered that the nourishment of plants depends chiefly on a 

 sufficiency of assimilable ash-constituents, which owing to the 

 small proportion of fine earth in many soils can hardly be 

 supplied except by humus, there can be no doubt that most 

 forest soils absolutely require the ash-constituents of humus. 

 This is especially the case with sandy soils, which are usually 

 poor in lime and potash, and with localities liable to inundations, 

 which may wash these substances out of the soil : forest litter is 

 then almost the only source of mineral matter, the only supply 

 of manure. 



In soils containing humus, according to Frank, fungi which 

 form a viycoyliiza on the roots of most forest trees are always 

 present, they are absent in soil deprived of humus [or artificially 

 sterilised by burning. — Tr.] and a long time passes before 

 mycorhizas are produced. Owing to this symbiosis of plants 

 and fungi, the former not only derive nutritive matter through 

 the humus, but are even enabled to obtain nitrogenous substanci' 

 indirectly from the atmosphere. 



Owing to the utilization of timber and other forest produce, 

 the nutritive substances obtained from humus are not sufticient 

 for the growth of trees ; fresh material must therefore be taken 

 from the sub-soil and supplied to the soil in an assimilable form. 

 Among the substances which eftect the disintegration and 

 solubility of parts of the sub-soil, besides various salts, the 

 carbon-dioxide resulting from the decomposition of humus plays 

 a very important part and without its assistance the soil cannot 

 remain contiimously active. The efficacy of carbon-dioxide 

 is not only confined to the topmost layers of soil formed by the 



