I'^IS.] SOILS FERTILIZERS. 813 



cropped and uncropped soil, here reported, the author concludes that the evWence 

 obtained from a strictly chemical examination of the water extracts of soils 

 is sufficient " to justify the hope that we may be able to predict, within reasonable 

 limits, the relative crop-producing powers of soils by comparing their figures 

 expressing these characters with similar data derived from soils whose pro- 

 ductive power is known. Before such a method is generally applicable, however, 

 it will be necessary to study the behavior of many soils with numerous type 

 crops." 



Effect of season and crop growth in modifying the soil extract, G. R. 

 Stewart (U. S. Dcpt. Agr., Jour. Agr. Research, 12 {1918), No. 6, pp. 311-368, 

 pi. 1, figs. 24).— In the investigations at the California Experiment Station here 

 reported in detail " the water-soluble nutrients in 13 soils of 2 different types 

 were periodically determined during tn^o seasons. Throughout the second 

 season comparisons were made between the planted soil and its uncropped 

 duplicate. Notable difterences were observed between the nitrates, calcium, 

 potassium, and magnesium present in the water extracts from the cropped and 

 uncropped soils. The phosphates did not exhibit corresponding differences. 

 Great dissimilarities were observed in the phosphate content of different soils, 

 but in any one soil the amount v,-as practically constant in both the cropped 

 and uncropped plat. Striking differences occurred between the soluble nutri- 

 ents present in the various uncropped soils. While the crops were gHowing the 

 concentrations of nutrients in 8 of the 13 planted soils were practically the 

 same. These eight included both good and poor soils. The three poorest 

 soils yielded the smallest amounts of water-soluble nutrients and the smallest 

 differences between the cropped and uncropped duplicates. The comparisons 

 between the planted and unplanted duplicates furnished valuable indexes 

 of the inherent capacities of the soils to produce nutrients. . . . 



"The amounts of the water-soluble nutrients obtained by varying the ratio 

 of soil to water were studied. The relationship of the compounds extracted 

 did not change essentially in the lower concentrations. By comparison with 

 freezing-point determinations the concentration of the soil solution calculated 

 from the water extract was shown to be from two to four or five times as 

 great as the actual soil solution. 



" Variations in the water extract were correlated with variations in the 

 freezing points of the same samples of soil. From the results of the freezing- 

 point determinations it is concluded that variations in the water extract reflect 

 actual changes in the soil solution. The results of the investigation show 

 that large amounts of water-soluble nutrients are developed by cultivation, 

 fallowing, and biennial cropping, and demonstrate the soundness of tliese 

 practices." 



A list of 63 references to literature cited is given. 



The freezing-point method as an index of variations in the soil solution 

 due to season and crop growth, D. R. Hoagland {U. S. Dept. Agr., Jour. Agr. 

 Research, 12 {1918), No. 6, pp. 369-395, figs. 9).— In the investigations at the 

 California Experiment Station here reported freezing-point depressions were 

 determined on 13 soils under a variety of conditions. The concentration of the 

 soil solution was " found to vary with the season and also as a result of treat- 

 ment with carbon dioxid, leaching, incubation, etc. The growth of a crop 

 markedly diminished the concentration of the soil solution. This effect was 

 still evident at the beginning of the following season. The soil solutions under 

 conditions favorable to crop growth were found to be very dilute, particularly 

 at the height of the growing season. Certain general agreements between the 

 extraction and freezing-point methods are discussed." 



Nine references to literature bearing on the subject are cited. 



