280 NATURE AND PROPERTIES OF SOILS 



ship between the water extract of a soil and its productivity, 

 but a correlation with the strength of the soil solution as well. 

 The extract method is especially valuable in studying the 

 nitrates of the soil solution. As nitrate nitrogen does not 

 suffer as much absorption as do the nutrient bases, that which 

 appears in the extract is a fair measure of the strength of the 

 soil solution insofar as this constituent is concerned. 



The only method for measuring the concentration of the soil 

 solution in situ is that of Bouyoucos. 1 This is known as the 

 depression of the freezing point method. It is possible, when 

 dealing with a pure solution of a known salt, to calculate its 

 concentration by determining how much the freezing point is 

 lowered or depressed below 0° C. This principle is applied 

 to the soil by using a Beckman thermometer and the proper 

 control apparatus. As the soil solution carries a great num- 

 ber of different ions in unknown proportions, it is impossible 

 to calculate even the concentration with accuracy, a factor 

 of somewhat doubtful validity being utilized. The procedure 

 gives nothing regarding the presence of specific ions nor are 

 its results uniform, due to the variable dissociation of the salts 

 present. Nevertheless the method has thrown much light 

 on the many difficult problems of the soil and its solution. 



146. Qualitative composition of the soil solution. — Once 

 the dynamic character of the soil solution is conceded, three 

 points of importance immediately demand attention: (1) the 

 qualitative composition of the soil solution and its concentra- 

 tion in toto, (2) the quantitative composition, and (3) the 

 factors most important in influencing both the composition 

 and the concentration of the solution. 



It must be recognized at the outset that the soil solution 



Burd, J. S., Water Extractions of Soils as Criteria of their Crop 

 Producing Power; Jour. Agr. Ees., Vol. XII, No. 6, pp. 297-309, 1918. 



Hoagland, D. R., Martin, J. C, and Stewart, G. B., Relation of the 

 Soil Solution to the Soil Extract; Jour. Agr. Res., Vol. XX, No. 5, 

 pp. 381-395, 1920. 



1 Bouyoucos, G. J., Further Studies on the Freezing Point Lowering of 

 Soils; Mich. Agr. Exp. Sta., Tech. Bui. 31, Nov., 1916. 



