Oct. IS, 1919 Relation of Nutrient Medium to Plant Absorption 105 



of the growth cycle. If the concentration or supply is either sub- 

 optimum or super-optimum during this period, no subsequent favorable 

 condition is likely to produce maximum yield. An inhibitive concen- 

 tration for barley either in water, sand, or soil cultures is not extremely 

 high, possibly less than 2.5 atmospheres. The minimum concentration 

 giving a maximum yield is low, though the magnitude can not be exactly 

 stated at present. It may not be more than that represented by o.i 

 atmospheres osmotic pressure.^ 



Some recent work of Davidson and LeClerc (p) bears on certain of 

 the above statements. These investigators give evidence that the 

 greatest increase in yield of wheat is obtained when soil fertilization is 

 accomplished during the first stage of growth, a slighter effect is produced 

 during the second stage, and no effect during the third. An increased 

 percentage of N in the grain occurs when fertilization takes place during 

 the second stage. During the third stage no effect is produced. It is 

 possible that the N content of the grain may be influenced by the length 

 of time during which nitrification continues in the soil, and this latter 

 process may in part be governed by climatic conditions. Very marked 

 seasonal fluctuations in NO3 have been noted by Stewart (42) in soils 

 kept always at optimum moisture content and in an excellent state of 

 cultivation. 



Finally, in the consideration of the relation of the soil solution to 

 plant growth it should be pointed out that the interpretation of soil 

 experiments in term's of concentration of the soil solution requires recog- 

 nition of the fact that the growth of the plant is affected by the properties 

 of the solution in actual contact with the absorbing root membranes. 

 The degree to which nutrient elements are maintained at a favorable 

 concentration in this effective solution must depend upon the rate of 

 diffusion and upon the potentiality possessed by the soil for constant 

 renewal of the solution as elements are absorbed by the plant. The 

 question of diffusion has been discussed by Russell (36), while Burd (6) 

 has shown the necessity of taking into account the renewing power of 

 the soil. It follows that conclusions drawn from determinations on 

 samples of the whole mass of soil do not imply an exact knowledge of 

 conditions in the solution from which the plants are actually absorbing. 



The question of the optimum concentration of solution for barley, 

 wheat, and other grains has been discussed in a number of articles with 

 considerable disagreement in the conclusions. It may be well to define 

 the conditions necessary for the determination of the effect of concen- 

 tration on plant growth. In such a study it is essential that the con- 

 centration of the solution be maintained constant at all times. In other 

 words, as absorption takes place the solution must be renewed. Ideally 

 this can be accomplished only by a continuous flow. Such a technic is 



' Later experiments show that this concentration is somewhat less than optimtun when light and 

 temperature conditions are highly favorable. 



