CHAP, xii.] Protection of the Soil 255 



meantime no fresh supplies had become available through the 

 continuous decomposition of the soil-particles, or if these had 

 sufficed merely to balance the annual loss due to soluble potash 

 being washed away out of the soil by the percolation of moisture. 

 And if this be the case with Silver Fir, it must a fortiori 'be true 

 of all other species of woodland trees, and for all mixed forests, 

 so far as potash alone is concerned. A similar argument 

 likewise obtains with regard to the various other mineral 

 food-supplies requisite for the growth of forest trees, either in 

 pure crops or in the preferable form of mixed woods. Hence 

 the conclusion must be arrived at that, wherever soils obviously 

 deteriorate under timber-crops, the reasons should first of all 

 be looked for in the physical conditions involved, rather than 

 in any excessive demands made by the crops with regard to 

 mineral nutrients. 



So far as concerns any direct increase in the quantity of 

 mineral food supplies contained in an available (soluble) form 

 within the soil at any given time, the nature of the woodland 

 crop, and the treatment to which it is subjected, can of course 

 exert no influence. Great, however, is the influence which may 

 be indirectly made to act upon the quantity of nutrient salts 

 available for the rootlets, by maintaining such constant density 

 of canopy overhead as will break the violence of heavy rains 

 that might otherwise wash away the surface-soil, and as will 

 also prevent the soil-moisture from being rapidly evaporated 

 by sun and wind. 



As most of the mineral food-supplies withdrawn from the 

 soil are, during the process of assimilation, concentrated in the 

 foliage, it follows that year by year the greater portion of what 

 has been withdrawn by tree-crops is restored again to the soil 

 when the decomposition of the leaves takes place. For when 

 humus is being formed from the defoliated leaves and other 

 dead parts of the tree-crops thrown down to the ground, 

 carbonic acid and ammonia are given off. The former of these 

 is essential to the solution of the mineral nutrients so as to 



