Relations of Higher Plants to the Formation of Nitrates 91 



That grass exercises a strongly depressing action on nitrate formation is 

 indicated not only by the three sets of tank experiments cited above, but 

 also by the determinations of nitrates under timothy sod previously pre- 

 sented in this paper. This characteristic of the grasses almost completely 

 prevents a loss of nitrogen in the soil drainage and thus tends to conserve 

 nitrogen, which is doubtless one of the factors in the accumulation of 

 nitrogen in grassland. 



The same properties of grass with respect to nitrate formation may be a 

 factor in the effect of grass in orchards. 



Stewart (1911) has lately called attention to the need of nitrogen in 

 orchard fertilization. Experiments indicate that under some conditions the 

 use of nitrate fertilizer has been verj^ effective in increasing the growth and 

 yield of apple trees. In sod-covered orchard soil the supply of nitrates must 

 be kept very meager by the grass, and if the other forms of nitrogen in the 

 soil are not such as to afford a suitable supply the trees must suffer from lack 

 of nitrogen. Where trees apparently thrive on sod, a part of their nitrogen 

 supply may come from some nitrogenous compounds other than nitrates. 



The plants used as cover-crops doubtless exercise an influence in this 

 respect. The practice of fallowing an orchard until July and then planting 

 a cover-crop is in accordance with the most economical management of the 

 soil nitrogen. In this way the formation of nitrates is favored during the 

 early part of the summer, and is depressed when the supply is no longer 

 needed by the trees and would otherwise be leached from the soil. This 

 is, of course, assuming that nitrates are utilized by apple trees as a source 

 of nitrogen. 



The influence of a preceding crop on nitrate formation in soil 

 While the practices of crop rotation have been fairly well worked out, 

 there yet remains much to learn regarding the underlying principles. 

 Most of the effort in experimentation with soils has been made with a 

 view to ascertain the effect of certain soil conditions on plant growth or 

 on the solubility of the mineral nutrients in various solvents. The study 

 of the principles of crop rotation necessitates the investigation of the effect 

 of plant growth on the soil. The fact that a certain plant grows better 

 when preceded by one species of plant than when preceded by another is 

 a self-evident indication that plants of different species exert different in- 

 fluences on the soil. 



