Feb. II. 1918 Effect of Season and Crop Growth on Soil Extract 333 



for the solution of the soil nutrients. In such a case the actual available 

 solution carrying plant nutrients would not be very different in the 

 complex silty clay loams from that which exists in the fine sandy loams. 

 It has therefore been decided to express all results on the basis of the 

 dry soil until further evidence is obtained as to the condition of the 

 moisture in the soil. 



AGREEMENT OF DUPLICATE EXTRACTIONS 



Before the results obtained by any extraction procedure can be con- 

 sidered, it is necessary to know the limits of the accuracy which it is 

 possible to achieve. The first important point in this connection is 

 the possibiUty of obtaining accurate duplicate extractions. A number 

 of sets of duplicate extracts have been made from samples of fresh 

 field soils. The results on four typical soils, two silty clay loams and two 

 fine sandy loams, are given in Table V. It will be observed that the 

 agreement is well within the limits of the mean factor of error given 

 in Table IV for all the elements except calcium. It will be remembered 

 that the factor of error for this element ranged from 4 to 10 per cent. 

 With the two fine sandy loams the variations between the two duplicates 

 is between 10 and 15 per cent. This result would readily be accounted 

 for by the solubility of the calcium compounds present in the soil and 

 indicates that it is necessary to obtain far larger changes with this 

 element than any others studied before the result can be considered 

 significant. It must be emphasized, however, that duplicate results 

 such as those given above can only be obtained if every step in the 

 procedure chosen is carefully followed. 



Table V. — Typical duplicate extractions of silty clay loams and fine sandy loams 

 [Results expressed as parts per million of dry soil. Ratio of soil to water 1:5] 



