Vol. XIII. No. 308. 



THE AGRICULTURAL NEWS. 



51 



SOIL INVESTIGATION. 



SETTLEMENT OF THE QUESTION OF 

 SELECTIVE ADSORPTION. 



Although the phenomenon known by the term adsorp- 

 tion has been referred to in this journal (see Agriculfnral 

 Kei's, Vol. XI, p. 281) it may be advisable, before proceed- 

 ing to discuss its relationship to the soil, to explain again 

 in a popular way its general nature. Most porous bodies, 

 like colloids, coagulated albumen, wood charcoal, and 

 starches, on account of their surface energy or 'tension' are 

 alile to take up certain dissolved substances in such a manner 

 that the material taken up cannot be removed even by most 

 thorough washing. The phenomenon is explained most 

 logically at present as a concentrating of the solute at the 

 surface of the adsorbing material. As au example of 

 adsorption, it may be noted that starch, if placed in a 

 solution of caustic soda, will take up into itself some of 

 this alkali, which, as already intimated, cannot be removed 

 by washing. 



It has been established for sometime now, that the 

 soil is also capable of adsorption*. Indeed it is the adsorp- 

 tive power of the soil which enables it to retain the soluble 

 salts necessary to plant life in spite of the leaching effect 

 of rain and the movement of the .soil .'solution towards the 

 surface of the soil in dry weather. 



But as well as possessing the power to adsorb substances 

 as a whole, porous bodies like colloids, wood charcoal and 

 the like can adsorb selectively. To enable the general reader 

 to understand what this means, a few words of explanation 

 concerning the constitution of salts are necessary. Salts are 

 considered as consisting of two parts or radicles — technically 

 known as ions. This is rendered apparent by the names 

 given to these compounds, as for example nitrate of soda or 

 sulphate of ammonia. The nitrate is one part or ion (which 

 is derived from an acid) and the soda is the other ion (which 

 is derived from a metal). Water itself, although not a salt, 

 is analogous to a substance like slaked lime, for although 

 chemical!)- its formula is represented as H.^0 it is often more 

 conveniently regarded as HOH of « hich H is one ion and 

 OH the other. 



Having explained sufficiently for present purposes what 

 an ion is, we may proceed with the subject of selective 

 adsorption. As already stated, colloids and the like possess 

 this selective power. They can not only adsorb a salt from 

 solution as a whole, but adsorb one ion faster than another. 

 Now in the adsorption of a salt like potassium chloride by 

 charcoal — to take a simple case — the following occurs: 

 KCl-f H0H = K0H + (HC1 adsorbed). .The chlorine (CI) 

 cannot leave the solution by itself, but only in com- 

 pany with one of the water ions (namely H). Now in the 

 case under consideration the HCl is adsorbed much faster 

 than the KOH: consequently there is hydrolysis or splitting 

 up of the water, and an accumulation va the solution of 

 caustic potash, the presence of which, can, in an experiment, 

 be indicated by the addition of phenolphthalein (commonly 

 employed for testing for alkalinity in sugar factories). 



We come now to the main point. Formerly the question of 

 selective adsorption in the soil has been disputed, but lately, 

 as a result of careful investigations, E. (r. Parker in America 



* Adsorption must not be confused with absorption, which 

 is merely a process of 'soaking up'. It will be observed that 

 the soil has both adsorptive and absorptive powers. 



has shown {Journal qt Agricultural Research, Vol. I, No. 3) 

 that soils possess this power of selective adsorption to a marked 

 extent. In the case of the soil, just the opposite occurs to 

 that which takes place between charcoal and potassium 

 chloride. In the case of the soil, the rate of adsorption of 

 the chlorine (CI) ions from solutions, is much less than of 

 potassium (K) ions. But something further happens besides 

 what has just been stated. The hydrochloric acid, which, as 

 v^e have shown, must accumulate as the result of the adsorp- 

 tion, reacts with, and renders soluble bases (of the Ca, 

 Mg, etc. type) otherwise unavailable. 



Considering this interesting phenomenon in more detail 

 the following results may be noted: (1) selective adsorption of 

 potassium by a soil increases in amount with the concentra- 

 tion up to a certain point and then remains practically 

 constant; (2) in general, the smaller the particles, the greater 

 the selective adsorption; (3) the presence of nitrate of soda 

 at first decreases the adsorption of potassium up to a concen- 

 tration of 37-5 grams of potassium chloride per litre, and 

 then increases it; (4) the presence of monobasic calcium 

 phosphate does not appreciably aflfect the adsorption of 

 potassium; (.5) the presence of other mineral substances may 

 or may not. 



In conclusion we see that when mineral fertilizers are 

 added to the soil and dissolved, they will be adsorbed aa 

 a whole or selectively by the vast surface of the soil 

 particles and will be held there by physical force until the 

 plant roots or subsequent leaching removes them. 



From the standpoint of the practice of manuring these 

 results are of great importance though more must be known 

 before rules based on this knowledge can be formulated for 

 specific guidance. Apparently, for instance, the retention of 

 a soluble potash manure like kainit, as a weak solution, is 

 discouraged by the presence of nitrate of soda, whereas 

 superphosphate would tend to have no effect upon the reten- 

 tion of kainit in the soil. 



Furthermore we might suppose that in a soil containing 

 but a small amount of bases, acidity might be caused by 

 selective adsorption. Again, where a predominence of 

 magnesium occurs, toxicity might be set up through the 

 action of the hydrochloric acid in converting this base into 

 a salt and thereby rendering it available for being absorbed 

 by the roots of cultivated plants. 



Bacteriological Analysis of Indian Soil.— An 



abstract of C. M. Hutchinson's work in India appears in the 

 Mont hi 1/ Bulletin, of Affi-icultural Intelligence and Plant 

 Diseases (November 1913). In an investigation of the so- 

 called weathering of soil by repeated ploughing in the hot 

 dry season preceding the spring rains, it was found that the 

 maximum temperature reached by the top \ inch was 

 60° C. Artificial weathering was therefore effected by 

 exposing soil to the heat and light of a Nernst lamp for eight 

 hours daily for a week. The result showed that the number 

 of bacteria was considerably reduced and that all the forms 

 of the suhtilis group were eliminated from the first inch of 

 soil. It was found that the nitrifying power of the soil 

 was not destroyed or altered by 'weathering', which the 

 writer explained as due to reinfection of the surface soil from 

 the lower layers. The soils were further examined for the 

 presence of protozoa by seeding into hay infusion; two types 

 of protozoa occurred (together in some cases); none were 

 found between November and .May. These two types were 

 destroyed at 60°C. but not at 5-3°C. As however they were 

 found in a soil in May, just after the 'weathering' operation 

 had taken place, it does not seem likely that the efl:'ectivenea3 

 of this operation can depend on their elimination. 



