108 T. Lyttleton Lyon and James A. Bizzell 



relation of the several plants to the nitrate content of the soil in the year 

 following that in which the plants were grown. 



Maize was the only crop following which the nitrates in the previously 

 planted soil were higher than in the unplanted soil. Potato soil was the 

 next highest in nitrates, and oat soil contained least nitrates. 



Millet planted on these plats on July 1 was markedly influenced by 

 the previous crops, but the luxuriance of growth was inversely propor- 

 tional to the nitrate content of the plats. 



In this case the beneficial influence of a crop on a succeeding crop was 

 not to be attributed to the favorable influence exerted on nitrification, 

 but this would doubtless differ with different soils; and if this effect of 

 certain plants on nitrate formation in the following year should be of 

 general application, the influence of a crop on nitrification may be an 

 important factor in crop rotation. 



Freezing and thawing produced a condition of soil favorable to nitrate 

 formation. The well-knowTi action of frost in producing a granular con- 

 dition in a clay soil may have operated to some extent in accomplishing 

 this. To what extent the greater activity of the nitrate-producmg bacteria 

 is due to aeration and how much it is due to the direct effect of cold in 

 readjusting the equilibrium of the bacterial flora, it is impossible to say. 

 The obvious effect is to overcome the depressmg influence of the crop 

 previously grown and to permit the resumption of nitrate formation. 



Timothy maintained a lower nitrate content in the soil than did any 

 other crop. Mixed grasses — Phleum protense, Agrostis alba, Poa pra- 

 tensis — gave much less nitrogen in the crop and the drainage water 

 combined than was contained in the drainage water from unplanted soil. 

 These facts indicate a strongly repressive influence of these plants on 

 nitrate formation, and suggest a possible cause for the injurious effect 

 of grass sod in orchards on soil in which the supply of available nitrogen 

 is deficient. 



Plants of two different kinds were grown, in combination and sepa- 

 rately, in soil and in ground quartz contaming nutrient solutions. In a 

 considerable number of cases in which the plants were harvested at the 

 blooming period or before, one or both kinds made a larger growth in 

 the combinations than in the pure cultures, although there were at least 

 twice as many plants growing in the mixed as in the pure cultures. This 

 increased growth of one plant in association with another was apparently 



