Lecture 1 — 7 — Survey 



sure. The liquid that issued from the bottom of the 

 tube was collected in fractions and on each fraction 

 a conductivity measurement was made. Those frac- 

 tions of nearly constant conductivity were assembled 

 and mixed for chemical analysis. When a reduction 

 in the conductivity value occurred, indicating a dilu- 

 tion of the soil solution, additional fractions of solution 

 were discarded. Displaced soil solutions are of course 

 composite in nature and reflect only an average com- 

 position. (See plate 2), 



To bring the discussion up to date, it may be added 

 that during the past few years, Richards (1941) and 

 his collaborators, at the Federal Salinity Laboratory, 

 Riverside, California, have devised another technique 

 for obtaining soil solutions. This is accomplished by 

 a pressure membrane apparatus, in which no dis- 

 placing liquid is used. By this technique, solutions 

 may be obtained from soils at very low moisture con- 

 tents. Also heavy soils, difficult to deal with by the 

 displacement method described above, can be more 

 conveniently subjected to displacement. 



The early experiments yielded data of much in- 

 terest at the time; concerning first, the ranges of 

 concentration of various ions and their proportions 

 in the soil solutions of these initially productive soils, 

 and second, the striking effects of seasonal biological 

 activities and of the absorption of nutrients by the 

 crop plants (barley) on the composition of the soil 

 solution. Except for sulphate and bicarbonate, cropp- 

 ing reduced the concentrations of the principal ions, 

 initially present. 



Comparisons of the soil solutions of different soils 

 at a given time and of the same soil at different periods 

 of sampling, allowed the conclusion to be drawn that 

 a soil solution is not a simple saturated system of 

 minerals and water, but instead is an ever-changing 

 system, biologically controlled by the activities of soil 

 microorganisms and of the higher plants growing in 

 the soil. The different ions did not behave alike. Their 



