Apeil 11, 1919] 



SCIENCE 



361 



variation in the solution for any given ele- 

 ment will depend upon the total quantity ab- 

 sorbed, upon the concentration in tlie original 

 solution, and also upon the volume of solution 

 provided per plant. It is essential to differen- 

 tiate between two sets of factors, the compo- 

 sition and concentration of the solution and 

 the total quantities of the various elements 

 present. The effect on the plant might be the 

 result either of the concentration as found in 

 the original solution, or of an insufficient total 

 supply of one or more elements. In order to 

 study the effects of concentration or of com- 

 position on plant growth, ideally a continu- 

 ous flow of solution should be arranged so that 

 the roots are always bathed in a solution of 

 constant composition. Such a technique is 

 ordinarily impracticable, and it is necessary 

 to approximate the desired condition by pro- 

 viding a sufficient volume of solution per 

 plant and by frequent changes. This is par- 

 ticularly true when the object of the investi- 

 gation is to determine the relative effects of a 

 series of solutions. To give a specific ex- 

 ample, certain solutions may have only one 

 tenth of their total concentration due to 

 Ca (N0„),. In such a case it is possible that 

 all of the NO3 might be absorbed before the 

 solution was changed, or at least reduced to a 

 very low level of concentration. Thus, if the 

 interpretation of the experiment is based on 

 three salt triangular diagrams, the effect, ac- 

 tually the result of insufficient NO3, might be 

 correlated with a certain calcium magnesium 

 ratio. 



In some experiments small bottles (250 to 

 400 c.c.) have been used with three to six 

 plants in each bottle, changes of solution be- 

 ing made every tliree days, or sometimes 

 only every four or five days. In the sand cul- 

 ture series the size of the jars usually permits 

 the use of only 250 to 400 c.c. of solution per 

 jar. In our experiments (to be described 

 elsewhere) from 500 to 2,200 c.c. of solution 

 per plant (barley) have been used, with 

 changes every two or three days in many 

 cases. Actual determinations of the absorp- 

 tion of each element have been made by an- 

 alyzing the solutions or the plants. It has 



been found that under favorable conditions 

 of light and temperature, more than 30 per 

 cent, of the total electrolytes may bo absorbed 

 in three days, when 500 c.c. of a favorable 

 nutrient solution of 2,500 p.p.m. concentration 

 is provided for each plant. All of the ele- 

 ments are not absorbed in equal percentages, 

 consequently not only the concentration but 

 also the relation between the elements has 

 been altered. In one experiment with solu- 

 tions containing 100 p.p.m. NO, (500 c.c. 

 solution per plant) barley plants six weeks 

 old absorbed every trace of NO3 from the solu- 

 tion in less than 72 hours. 



In several experiments in which plants have 

 been grown in solution and sand cultures the 

 yields of straw and heads are fairly compar- 

 able with those of plants produced in the field, 

 where an excellent crop is obtained. In some 

 sand and solution culture experiments re- 

 ported the yield per plant has evidently been 

 much inferior to that for similar plants grown 

 in the soil for an equal period. Some limita- 

 tion of light, temiserature, aeration or of the 

 nutrient solution must therefore have existed. 

 In many cases there is a strong presumption 

 that the supply of nutrients may have been 

 deficient, as noted above. 



We do not desire, however to criticize any 

 specific investigations. If plants are grown 

 under sub-optimal light or temperature condi- 

 tions, the total quantities of nutrients ab- 

 sorbed per plant may be much less than in our 

 experiments. Moreover, in the first few weeks 

 the plant has not reached its maximum power 

 of absorption, so that short culture periods 

 will require less quantities of nutrients. The 

 point we desire to emphasize is that plants 

 grown under the most favorable conditions 

 may absorb or require much larger quantities 

 of nutrients per plant than are ordinarily pro- 

 vided in sand and solution culture work. 

 Each set of conditions should be tested by 

 actual analysis of solutions and plants and 

 results interpreted in terms not of the original 

 solution alone, but also in terms of total 

 supply and the varying condition of the solu- 

 tion in the periods between changes. It 

 should aUo be noted that deficiencies in total 



