44 



BEPOET OF CHEMICAL LAHOKATOKY 



Entire amount 

 of clay 

 removed in 

 three days 



" Correction' 

 for soluble 

 salts 



transferred to a cylindrical beaker marked at 10 centimetres, distilled water added up 

 to the mark, mixed and allowed to stand for not less than eight hours. In the case of 

 heavy clay soils so much clay is brought into suspension by this method that the first 

 sedimentation is allowed to continue over night in order to ensure that the silt may not 

 be held up by the thick liquid. The subsequent subsidence is of eight hours' duration 

 followed by one over night. In this manner the entire amount of clay may usually be 

 removed in three days. The water used for each operation should contain about one half 

 gramme of sodium carbonate per litre. It sometimes happens that clay is present is such 

 large proportion that the usual volume of water is not sufficient to hold it in suspension 

 for the necessary 8 hours. In the case of flocculation due to this cause, it is necessary 

 either to employ a more capacious beaker, or to divide the turbid liquid between two 

 or more beakers of the usual size (8 centimetres diameter). 



Flocculation, if it takes place, is more usually due to the presence of soluble salts. 

 This is especially the case with subsoils. It may, as a rule, be avoided by using a 

 smaller proportion of sodium carbonate in order to reduce the total amount of salt 

 present. Two tenths of a gramme per litre may be used for the first two decantations, 

 after which it is advisable to increase to the usual proportion. 



If flocculation has occurred, the excess of water, after decantation, should be removed 

 on the water-bath and the soil puddled with the brush. Once the floccules are 

 formed they do not readily break up again in spite of the fact that the salt may 

 have been almost entirely removed by decanting the more or less clear supernatant 

 liquid. 



Flocculation due to excess of salts occurs less frequently if the centrifugal method 

 be employed, since it usually takes place very slowly in the presence of the sodium 

 carbonate. 



After, usually, the third or fourth sedimentation and decantation, the residue is 

 puddled with the camel-hair brush as described on page 37. 



When almost the entire amount of clay has been removed, the soil is washed into 

 a weighed beaker of about 200 c.c. capacity, using distilled water (in this case without 

 sodium carbonate). After standing for the usual period (i.e. not less than eight hours) 

 or until clear, the water is poured away and the beaker and residue dried by heating 

 in the air-oven for 2 hours at 110 C. From the weight of the soil residue may be 

 calculated the percentage of clay, which is determined by difference. The figure so 

 obtained should be corrected for soluble salts if present in notable quantity. A "correction" 

 for organic matter in the mechanical analysis of a soil is, in the opinion of the writer, 

 neither necessary nor desirable. In any case the amount present in soils even 

 of semi-arid districts, rarely amounts to one per cent. The determination of humus 

 should, of course, be made, but separately, as a part of the chemical analysis. 

 If desired, however, practically the whole of the humus may be removed by using a 

 larger proportion of sodium carbonate in the shaker bottle ('5 to 1 gramme) and, 

 its proportion being determined on another sample by the usual method, the necessary 

 correction may be made. 



When it is desired to carry out the separation of the clay more rapidly, the 

 centrifugal machine is used, as already described. 



The residue in the beaker, consisting of sands and silt, is treated as detailed on 

 -page 41. The silt having been removed, the residue is washed back into the original 

 weighed beaker, dried as Viefore and weighed. The silt, like the clay, is determined by 

 difference. 



