SOME SILVICULTURAL ASPECTS AND PROBLEMS OF THE SOIL. 37 



rivers, usually contains and absorbs a sufficient supply of air for 

 many plants, and is not in the same category as water retained 

 for some time in the pores of a soil.) If the air circulation is 

 too free, i.e. the soil is too light, then rapid evaporation and 

 percolation may reduce the water-supply below the requirements 

 of the least exacting species concerned, and even it will suffer 

 from drought. 



The presence of nutritive salts and their maintenance will be 

 considered later ; they are produced by the soil as a whole. 



The ability of the soil to maintain the air-water equilibrium 

 requirements of a typical plant is determined by its texture, 

 which in turn depends upon (i) the relative amount of different 

 sizes of particles of which it is composed, and (2) on the degree 

 of packing or looseness of these particles. The former point is 

 recognised in the usual classification of soils into sands, silts, 

 clays, etc.; the latter is usually neglected or is allowed for by 

 considering denser packing as equivalent to smaller grain. But 

 the actual physical quality of the mineral soil is modified, and 

 may be completely changed, by the presence of decayed vege- 

 table mould or humus in admixture with it. When incorporated 

 with the mineral soil, humus modifies in an ameliorative v^^ay 

 any extreme condition of the soil structure, decreasing the 

 porosity or lightness of the sandier types and thus increasing 

 their water-holding capacity, and opening up the texture of the 

 heavier types, and thus bringing about greater aeration and 

 better drainage. For all soils the addition of leaf-mould 

 increases the nutritive efficiency as regards food salts. 



Soils which combine in the most efficient way these structural 

 characteristics {i.e. good relative amounts of somewhat loosely 

 mixed sand, silt, clay, and humus) are called loams, and loam is 

 therefore the central ideal type of medium for plant growth. 



The nutrient salts of the soil are of two distinct types. In the 

 first place, the mineral salts are originally derived from the 

 decomposition of soil-forming rocks ; they would be rapidly 

 washed away by percolating water were it not for the absorptive 

 capacity of the finest soil particles, by which they are largely 

 retained, to be gradually liberated in the soil water and thus 

 made available to plants. Again attention is drawn to the fact 

 that this capacity to absorb nutrient salts is exhibited to the 

 most intense degree by the humus itself, while owing to its origin 

 humus contains important quantities of these essential elements 



