342 Scientific Proceedings, lioyal Dublin Society. 



Table I. 



Effect of time of extkaction and ke-extraction of soil upon the ijB. value of the extkact. 



Soil, one pakt; water, two parts. 



Here Nos. 1 and 26 show a deci'ease in acidity, when the soil is re-extracted, 

 amounting to pH 02, quite a serious discrepancy ; but in tlie otlier samples 

 identical results were obtained. 



These results are confirmed by those shown in Table II as regards time 

 of extraction and the first re-extraction. In this series half the sample was 

 retained and examined again after a total extraction period of eighteen hours; 

 the other half was drained after three to four hours, and fresli water was 

 then added as described pi-eviously. Then an examination was made after 

 another fifteen hours, viz. eighteen in all, and the draining and re-filling were 

 repeated a second and a third time after further periods of twenty-six and 

 sixteen hours, namely, total times of forty-four and sixty hours respectively. 



Table II. 



Effect of extraction and re-extraction of soil upon the pU value of the extract. 

 Soil, one part; water, five parts by weight. 



Sample. Extraction. Ee-extraction, total time. 



3-4 hrs. 18 hrs. IS hrs. 44 hrs. 60 hrs. 

 pll. pE. pE. pE. pE. 



5 6-0 6-0 6-05 6-35 6-45 



6 5-8 5-8 5-8 6-0 6-25 



24 8-1 — 8-0 7-8 7-6 



25 7-8 — 7-8 7.65 — 

 30 7-65 — 7-65 7-55 7-5 



The second and third re-extractions have noticeable effects, tending to bring 

 the pH values nearer to neutrality. The strong buffer action of the soils 

 is well brought out by these figures. In this connexion the work of Arrhenius 

 (1922, 1) on clay as an ampholyte should be remembered. This action of 

 clay must greatly reduce the effect of the carbon dioxide dissolved in the 

 soil solution, so that the pH value is not regulated mainly by the carbonate, 

 bicarbonate, and carbon dioxide equililirium as in alkaline fresh or salt water. 

 In the writer's paper on i^lant distribution (1922) too great importance was 

 attributed to bicarbonates and too little to silicates, aluminates, and such 

 complexes. 



The preparation of a clear soil extract. 



Certain sandy soils and silts settle so rapidly that no trouble is experienced 

 owing to turbidity. "With most soils, however, three or four hours is not 

 sufficient to give a really clear solution, though, if they stand overnight, many 



