Feb. II, 1918 Effect of Season and Crop Growth on Soil Extract 361 



very dissimilar in their nature; the silty clay loams are representative 

 of decidedly complex soils with appreciable amounts of clay and other 

 colloids, while the fine sandy loams belong to the light open soils which 

 are common in the West. The extraction graphs yielded by both types 

 are so similar that it is believed that comparisons can safely be made 

 both within the types and between the two groups. 



The extractions of the season of 191 6 are those to which the greatest 

 importance is attached and are those which will largely be considered 

 in this discussion. The previous year's work brought the soils into 

 comparable condition and also showed the probable limits of the agree- 

 ment between duplicates. It is believed that the results obtained in 

 this second season throw some light on several disputed points. The 

 method of constant comparison between the cropped and uncropped 

 soil is considered essential for the success of such a study. Only in this 

 manner can any conception of the inherent capacity of a soil be obtained. 

 When a large crop is growing on a soil, it is possible for it to affect the 

 soluble nutrients so that the extract given by it will be equal to a moder- 

 ately good or fairly poor soil. 



Such a condition is seen with three fine sandy loams: No. 7, Hanford 

 fine sandy loam, a soil of moderate production; No. 8, Fresno fine sandy 

 loam, a very good soil; and No. 9, Kimball fine sandy loam, a soil which 

 has produced poor crops. 



The nutrients extracted from each of these three soils when cropped 

 give practically duplicate graphs. But with the unplanted duplicate 

 striking differences are seen. The better soils all show greater differ- 

 ences between the cropped and the uncropped soils than do the three 

 poorest soils, No. 3, 9, 10, and 12. With these three it is possible that 

 plant food is the limiting factor, but with certain soils of intermediate 

 production, such as No. 4 and 10, it is less evident why these should 

 not fall in the most productive group. It is claimed, however, that 

 this method of study gives an expression of the inherent capacity of the 

 soil to produce water-soluble plant food. 



Between these three poor soils and the group of highest production 

 there are large differences in the range of soluble nutrients. These 

 three soils would not be classed as unfertile soils in general farming. 

 The only one which had a small crop upon it when the original sample 

 was collected was soil No. 12, Arnold fine sandy loam. The oats grow- 

 ing on this body of soil were noticeably smaller than in other portions 

 of the field. Under the controlled conditions of the experiment the 

 differences between these soils and the more productive must be con- 

 sidered significant, since they have been among the poorest soils each 

 year. It may, therefore, be stated that among the 10 soils studied the 

 3 lowest in productivity also show the lowest inherent capacity to furnish 

 soluble nutrients. 

 27809°— 18 5 



