I04 Journal of Agricultural Research voi. xviii. no. a 



would seem highly desirable that some sort of critical basis be estab- 

 lished before attempting more highly specialized experiments dealing 

 with small variations. 



It is of course quite obvious that no control of the soil is possible 

 sufficient to elucidate the fundamental points in plant nutrition. 

 Recourse to water and sand cultures is essential, without question. 

 We may, however, regard the soil as the natural habitat of agricultural 

 plants, and when a high yield is obtained we are justified in assuming 

 that the particular conditions that obtain in the soil solution are not 

 unfavorable to the development of the plant. Any definite information, 

 therefore, which may be obtained regarding the soil solution will serve 

 as a guide to at least one set of favorable factors. Very little is known 

 of the soil solution, but the method perfected by Bouyoucos and McCool 

 (j) has enabled us to ascertain with fair accuracy the total osmotic 

 values. This method has been applied in soil investigations conducted 

 by this laboratory, determinations being made at frequent intervals 

 throughout the season. The results were quite definite in showing that 

 about lo weeks after planting the barley crop had diminished very 

 significantly the concentration of the soil solution. Water extracts 

 indicated that by this particular period very little NO3 remained in the 

 soil solution. Moreoever, high yields of barley and wheat have been 

 obtained on soils whose solutions at no time during the growth 

 cycle had a concentration greater than 0.5 atmospheres. Similar rela- 

 tions have been observed for a number of other plants. We may con- 

 clude from these soil investigations that the plant does not necessarily 

 require a concentration of the nutrient medium higher than the one 

 stated above, and that it is not necessary that the concentration or 

 large supply of NO3 be maintained during the latter part of the growth 

 cycle. 



These facts were made the basis of several water- and sand-culture 

 experiments already described. It became evident from these that at 

 least for the barley plant the normal cycle of development did not 

 require that the concentration of the nutrient solution be maintained 

 after nine weeks for the climatic conditions under which the experiment 

 was conducted. A longer continued maintenance of the concentration 

 leads to no important increase in yield but does cause the plant to attain 

 a very much higher percentage of certain elements. Also when the 

 supply of NO3 and other elements is constantly replenished, the plant 

 may remain green almost indefinitely. In the soil the particular cycle 

 of development of the plant is related to the diminution in the con- 

 centration or total supply of elements in the soil solution, and this 

 diminution itself has been brought about to a large extent as a result 

 of absorption by the plant. We may infer, then, that the most important 

 condition for a high yield, in so far as the soil solution is concerned, is 

 an adequate concentration and supply of nutrients during the first half 



