84 Journal of Agricultural Research voi. xviii. No. 2 



therefore of interest in themselves and will also be pertinent to the further 

 discussion of the course of absorption by the plant. 



During the growth period of this series frequent observations were 

 made on tillering and height. Except in the lowest concentration, 

 heights were practically uniform. The number of tillers per plant, how- 

 ever, seemed to increase with increasing concentration. The total 

 yield of dry matter per plant justifies the conclusion that under the con- 

 ditions of experimentation here described there is no restriction on the 

 optimum production of vegetative growth traceable to insufficient nutri- 

 ents or limited size of containers. 



SAND CULTURES, SERIES 3 



During the following summer a further set of sand-culture experiments 

 was carried out. The technic was similar to that of the preceding 

 experiment except for the improved method of distributing the solution, 

 described in the first part of the article. In this series sixteen 5-gallon 

 jars were used with three concentrations of solution of 0.95, 1.95, and 3 

 atmospheres, respectively. The concentration of solution in four of the 

 jars was decreased after 6 weeks from a concentration of 0.95 atmos- 

 pheres to one of 0.15 atmospheres, in four other jars after 9 weeks, and 

 in another set after 12 weeks. In the two highest initial concentrations 

 a decrease was made after 6 weeks to 0.95 atmospheres concentration, 

 and after 9 weeks to 0.15 atmospheres. The object of these changes in 

 the concentration of the solution during the growth of the plant was to 

 imitate certain of the conditions actually existing in the soil solution 

 during the growth of the plant, as noted in the investigations of Burd (<5) , 

 Stewart {42), and the author (17). These experiments showed very 

 clearly that the soil solution diminished in concentration after the plant 

 had grown 8 to 10 weeks. Nitrates at this period almost disappeared. 

 Nevertheless, even with this exhaustion of the soil solution after 8 to 10 

 weeks, the plants completed their cycle growth, and in most of the soils 

 the ripened crop (after 16 weeks) was characterized by a high yield of 

 both straw and grain. It must follow, therefore, that such a change in 

 the concentration and composition of the nutrient solution at the particu- 

 lar time in question is in no way unfavorable to crop production. 



The data presented in Table IV may be considered in the light of the 

 foregoing discussion. While there is considerable variabihty present, 

 the results may reasonably be accepted as indicating the general trend. 



The average yield from the four jars in which the solution was changed 

 to one of low concentration after 6 weeks was possibly inferior to that 

 of the other cultures; but the jars in which the concentration was reduced 

 after 9 weeks gave an equal yield of grain and nearly equal yield of straw, 

 as compared with the cultures in which the highest concentration was 

 maintained for 12 weeks. The initial concentration of 1.95 atmos- 

 pheres may have had an inhibitive effect, even though later the solution 



