338 



Journal of Agricultural Research 



Vol. XII, No. 6 



plant. It may therefore be expected that in the solution from this soil 

 more material derived from the soil minerals will be soluble than in the 

 uncropped extract. The effect of this will be to make the differences 

 which actually exist less striking and evident than is really the case. 

 The evidence given by the freezing-point method, which has greatly de-* 

 veloped the above theory, is discussed by Hoagland (27). 



The suggestion was made by Burd that it might be possible to obtain 

 some conception of this repressive effect by extracting a cropped soil 

 with a diluted extract from its uncropped duplicate. Two soils were 

 used for this purpose: Soil 5, Yolo silty clay loam, and soil 10, Tejunga 

 fine sandy loam. In each case an extract was first prepared of an un- 

 cropped reserve portion of the soil and of another portion upon which 

 a crop had been grown. A determination of total solids was immediately 

 made. The solution from the uncropped soil was then diluted with 

 distilled water, so that this solution added to that obtained from the 

 weaker cropped soil would again be equal to the strong solution. The 

 cropped soil was extracted with this diluted solution. The results of 

 this extraction are compared in Table VII with the amounts obtained 

 by the usual treatment with distilled water plus the quantities already 

 present in the diluted extract. 



Table VII. — Extraction of two cropped soils with diluted extracts from duplicate bin soils 

 [Results expressed as parts per million of dry soil] 



With the Yolo silty clay loam 5 there was seen to be a distinct de- 

 pression of phosphates and a slight depression of potassium, both of 

 which were greater than the maximum factor of error. The depression 

 of calcium was no more than the average factor of error. The effect 

 was especially seen in the amount of fixed solids which was held out 



