372 NORMAN 



be of great importance because they may give the respective host plants nu- 

 tritional advantages over other plants on the same site which lack the capabil- 

 ity of entering into these associations. But leguminous plants or normally 

 mycorrhizal tree species, if provided with an ample and complete nutrient 

 supply, will flourish and make optimum growth in the absence of the microbial 

 partner. 



Nutrient supplies and reserves. Returning to the question as to how 

 a soil may differ from an aerated solution culture in supplying nutrient 

 ions, water, and oxygen to plant roots, one has to ask in what form the 

 essential nutrients are found in soils. If they are all present in solution in the 

 soil water, then in considering the soil as an environment for plant growth 

 it would only be necessary to study a water extract of soil, or the liquid phase, 

 which is called the soil solution. For many years, indeed, the view prevailed 

 that components on the mineral fraction of soils are slightly water-soluble 

 and that the supply of nutrients is maintained by solubility replenishment. 

 Difficulties, however, arise in this viewpoint. Analyses of soil extracts or of 

 the liquid phase of soils displaced by pressure or other means revealed very 

 low concentrations of even those nutrients taken up in largest amounts. Al- 

 though it may take 120 pounds of nitrogen per acre to make a 100-bushel 

 corn crop, it is usually the case that on analysis there can only be found 

 present at any one time 1-2 pounds of nitrate per acre. The nitrate level in 

 the soil solution must therefore be renewed many times over in the course 

 of a single season. Similar and even more striking is the situation encountered 

 with phosphate, which is almost always very low in concentration in the soil 

 solution. To account for the total quantity present in a crop a rapid rate of 

 renewal has to be presumed. 



The composition of the soil solutions from soils differing greatly in fertility 

 does not fall in line with the growth behavior of plants grown in these soils. 

 Moreover, the quantities of soluble salts found seem to change greatly with 

 the moisture content of the soil, often in an inconsistent manner. In determin- 

 ing the available supply of a particular nutrient in the soil, therefore, the 

 amount present in the soil solution at any one time is not informative. The 

 crux of the situation is the rate of its renewal or replenishment in the soil 

 solution. 



Truog has spoken of the soil as a "frugal custodian" of nutrients, and 

 indeed, on reflection, it will be realized that it is well that this is so. If there 

 was a substantial supply of the necessary nutrients in soil in a soluble form, 

 it would be very susceptible to loss by leaching in humid climates. Drainage 

 waters would rapidly carry away nutrient elements as salts in areas where 

 20-50 or more inches of water annually passes through the soil mass. It is 

 true that there is some loss of plant nutrients from soil by leaching and that 

 soils in warm climates subjected to heavy rainfall become depleted, but even 

 so the composition of the mineral and organic components of soils in general 



