N. M. Comber 385 



presumably be liberated during the combustion of the humus and remain 

 in the residue as free base. 



These considerations seem to afford an adequate explanation of the 

 experimental facts that the base absorbing power of soils containing 

 organic matter falls during partial ignition and that the fall is very great 

 when much organic matter is present. The original peaty soil had a high 

 " hme requirement," but after 5 minutes' ignition it increased the con- 

 centration of the bicarbonate solution very considerably. 



It will be noted that normal mineral soils, after partial ignition, show 

 a lower base absorbing power but an enhanced reaction with thiocyanate. 

 The reason of this is quite obvious in view of the fact that the proportion 

 of the iron and aluminium to the calcium which present themselves for 

 dissolution in acid is increased by partial ignition. The actual power to 

 absorb lime from bicarbonate, and presumably potassium from potassium 

 thiocyanate, is less, but after the potassium has been absorbed the free 

 thiocyanic acid attacks a relatively greater proportion of iron and 

 aluminium. 



Summary. 



In support and extension of the conclusions drawn in the earlier 

 papers the following facts and deductions are submitted : 



1. The only sparingly soluble substance, from a random collection 

 examined, whose suspensions showed the same abnormal flocculation by 

 calcium hydroxide that is shown by clay, were certain phosphates of iron 

 aluminium and calcium. The abnormal flocculation of these phosphates is 

 open to an explanation quite analogous to that already advanced for the 

 flocculation of clay. 



2. Until the amount of calcium hydroxide added to a suspension of 

 clay or phosphate reaches a certain amount its abnormal flocculating 

 power is not manifested. The amount required to produce the abnormal 

 flocculation is greater for a fat clay than for a lean one. This is in agree- 

 ment with the view that the abnormal flocculation is caused by a coagula- 

 tion of emulsoid matter, for obviously such coagulation will not become 

 dominant until a sufficient amount of the precipitant has been added. 



3. The lime absorbed by a soil can be wholly and completely removed 

 by a dilute acid treatment which cannot very appreciably decompose the 

 unweathered minerals. It is therefore concluded that the absorption of 

 lime by a soil is an absorption by the soil colloids and not by the un- 

 weathered minerals. 



