512 



Journal of Agricultural Research 



Vol. XV, No. 9 



"Table VI. — Itiorganic colloid suspension in soil as influenced by successive decantations 

 ■with small amounts of distilled water and by single decantation with larger amounts of 

 ■water, total time on water bath being the same in all cases 



a 3 cc. of ammonium hydrate added. 



Soil mulch, plowsole, and subsoil samples taken in groves having plow- 

 sole, were rolled (not crushed), sifted through a 2-mm. sieve, and sub- 

 jected to colloid-suspension- determinations. The average percentage 

 of the inorganic colloid suspension in soil samples from seven different 

 groves was as follows : Mulch, 0.879; plowsole, 1.334; subsoil, 1.215. In 

 all groves but one the colloid content was greater in the plowsole than in 

 either the soil mulch or subsoil. The grove which proved the exception 

 was sampled a second time and subjected to colloid-suspension deter- 

 minations, which gave results similar to the first determinations. 



There is thus an accumulation of inorganic colloid matter in the plow- 

 sole layer, especially as compared with the soil-mulch layer. 



From laboratory experiments it would appear that this colloid matter 

 migrates with the soil moisture much as do soluble salts, though prob- 

 ably much more sluggishly. Soils from groves were put in flowerpots 

 in the laboratory and irrigated, the soil being allowed to dry out before 

 each irrigation and before the colloid determinations were made. In 

 every instance after one or more irrigations the surface layer of soil 

 was found to contain a greater percentage of inorganic colloid suspen- 

 sion than the subsurface soil. The increase in colloid matter in the 

 surface layer varied fro.m 11 to 70 per cent. 



The inorganic colloid condition of the soil is Jargely governed by the 

 composition of the soluble salts present. The sulphates of sodium, 

 ammonium, and calcium tend to precipitate the colloids, while the 

 nitrates, carbonates, and bicarbonates have the reverse efifect. 



It is not uncommon to find considerable accumulation of "alkali" 

 salts on the irrigation furrow slopes in Citrus groves after irrigation. 

 Breazeale (2) has analyzed some of these and found the predominant 

 salts to be sulphates and nitrates, the latter being present in the greater 

 quantity. McBeth (6) has shown that nitrates are at times present in 

 large amounts on the slopes of the irrigation furrows. The latter's 

 work shows also the relation of nitrate accumulation to the rainfall, 

 which is of importance in the present discussion. Finally the irrigation 



