382 Journal of Agricultural Research voi. xii. no. 6 



It is strikingly evident from these data (Table III) that the freezing- 

 point depressions are not constant during the season, and that the con- 

 centrations of the soil solution are uniformly lower in the cropped soils 

 than in the same soils uncropped. This latter observation is especially 

 noteworthy, since comparisons of two samples of the same soil are pecu- 

 liarly applicable. Any errors due to unfree water, type of soil, optimum 

 moisture content, etc., would be practically constant. The same general 

 relations hold, whether the original data are considered, or the corrected 

 figures as used in the graphs. The only difference is that more exact 

 comparisons may be made when the moisture contents are reduced to a 

 uniform percentage. 



The logical conclusion from the results given in Table III is that the 

 concentration of the soil solution may vary widely at different periods of 

 the year, and that the growth of the plant has a pronounced effect in 

 lowering the concentration. Furthermore, this effect is of long duration. 

 The soils which had been cropped are still decidedly lower in the concen- 

 tration of the soil solution, as compared with the fallowed soils, eight 

 months after harvesting the crop. It does not necessarily follow that 

 the differences between the cropped and the uncropped soils are to be 

 ascribed solely to the depletion of the soil solution by the plant. The 

 evidence presented by Burd (j) is confirmatory of the idea that certain 

 biological activities are more intense in the soil without crop, as shown by 

 the increased production of nitrates and greater solubility of calcium and 

 magnesium. It is nevertheless true that the lowest concentrations in 

 the cropped soils occur at about the time when the crop has completed 

 its maximum absorption of nutrients. In certain of the fine sandy loams 

 the concentrations have been reduced to a very low point, corresponding 

 to only a few hundred parts per million of total solids. This condition 

 must be the result of withdrawals by the plant. The magnitudes of 

 absorption, as given by Burd, are quite comparable with quantities 

 present in the soil solution. After the minimum has been reached at the 

 height of the growing season, the freezing-point depressions slowly- in- 

 crease up to the last date recorded in the graphs. May i. It is highly 

 probable that several weeks after cultivation a more marked increase in 

 the concentration of the soil solution would take place. 



At this point it is desirable to compare the data presented by Stewart 

 {8) for the water extracts with the determinations made by the freezing- 

 point method. It will be noted that the same general relations obtain 

 in both cases. All the elements for which analysis was made are subject 

 to marked fluctuations, with the exception of phosphorus. This sub- 

 ject will be given further consideration later in the article. 



