Relations of Higher Plants to the Formation of Nitrates 99 



The purpose of this experiment is to ascertain the influence of certain 

 kinds of plants on the formation of nitrates in the soil after the crops have 

 been removed. The relation of the nitrate formation to the growth of 

 millet on this soil is a separate problem, probably peculiar to this particular 

 soil but possibly worth recording. The interesting condition exists that, 

 of the crops used in the experiment, each had a certain and distinct influence 

 on nitrate formation following the removal of the crops and on the growth 

 of millet also following their removal, but a high nitrate content was not 

 necessarily accompanied by a large crop of millet. It is not to be expected, 

 however, that this lack of correlation will obtain in all soils, since under 

 some circumstances the presence of an abundance of nitrates will throw the 

 influence toward a better growth of the succeeding crop. There is thus 

 presented a problem in crop rotation. If the influence of a certain kind of 

 plant on the formation of nitrates after its removal holds for soils other 

 than the soil used in this experiment, something has been learned. That 

 other influences obtain, and, under some circumstances, influences more 

 potent than nitrate formation, is also indicated by this experiment. 



Is there a mutual stimulation of plant growth through root influence? 



The stimulating influence of plant growth at certain stages on nitrate 

 formation in soils suggested a possible benefit to higher plants from other 

 and younger plants growing with them. It was decided to start a number 

 of plants of different kinds in the greenhouse, placing them far enough 

 apart to prevent interfereiice, at least while they were young, and after 

 they had attained a reasonable size to sow between th(>m seed of a different 

 kind of plant in order to ascertain whether the plants that were first started 

 received any benefit from the growth of the plants seeded later. If the 

 later-started plants stimulated nitrate formation at a time when the 

 earlier-started plants were beginning to depress it, the latter might receive 

 some benefit from the nitrates formed. 



In order to test this question, an ordinary greenhouse bench was divided 

 into compartments eighteen inches square ^.nd six inches deep. These 

 were filled with soil of good quality, prepared by rotting timothy sod which 

 had been produced on Dunkirk clay-loam soil. About twenty-five or 

 thirty of the earlier-started plants were grown at equal distances apart in 

 each square. These will be designated as the " primary " plants or crop. 

 Twice as many plants were started later, being arranged one on each side 



