EFFECT OF DRYING SOILS ON WATER-SOLUBLE CONSTITUENTS 207 



unheated samples contained considerable quantities of nitrates. No claim 

 is made that the conditions under which these had been stored are such that 

 the nitrate now recovered, together with that previously removed, should 

 equal 100 per cent of the original quantity of nitrate used. However, it is 

 notable that in all except the fine sand size the total recovery is nearly 100 

 per cent. 



These data are not in accord with those of Noyes (71) who reports all nitrate 

 recovered in one extraction nor with those of Bouyoucos (6), published since 

 the above was written, who states that the soil solution is less concentrated 

 at the immediate surface of the particles. 



Under the conditions of this experiment, not all of the nitrate added to 

 pure quartz nor to a soil containing but a faint trace of nitrate (white silt 

 loam, table 6), was recovered in one extraction nor even in two extractions. 

 WTiile the data here reported are entirely too meager to warrant definite 

 conclusions, yet it is significant that extracting the quartz itself brings the 

 nitrate recovered so near 100 per cent of the total added. 



This work corroborates in ever}amportant detail that of King (46) and though 

 these data are meager they indicate clearly that with soils containing a moder- 

 ately low percentage of capillary moisture, at, or slightly below optimum, much 

 of the soluble salts are held in this film moisture. This film adheres so strongly 

 to the particle that much of the soluble material it contains is given up to water 

 on extraction by diffusion only and this explains, in part, the effect of air- 

 drying. WTien the capillary moisture is lost the salts are left as minute crys- 

 tals on the soil particles, and it is clear that quickly washing with water will 

 bring these salts into solution so as to give a more homogeneous solution than 

 can be obtained by washing a moderately moist soil in the same way. 



When a soil is oven-dried we get all of the effect of air-drying just discussed, 

 together with several additional effects. Heating at 105°C. coagulates the 

 colloids. This the oven-dry samples showed very clearly, on working them 

 with a pestle in the mortar. With the heavier soils much difficulty was en- 

 countered in making a "thin paste," as the soil adhered so tightly to both the 

 pestle and the mortar. After heating, there was no tenacity whatever, a 

 hesLvy silt or clay loam working as easily as did fine sandy loam. This action 

 on the colloidal matter would enable the water to come into more intimate 

 contact with the material which exposes the major part of the total surface 

 of the soil. As already shown, Hulett and Allen (37), the particles of colloidal 

 size are more soluble than larger ones. This effect on colloids is undoubtedly 

 an important factor, and helps explain higher solubility in the soils with 

 smallest particles. 



Authors cited above hold that heating alters organic as well as inorganic 

 materials in the soil, rendering them more soluble. \Miile the physical factor 

 in sands and silt loam has been sho^\^l to explain in part the increased recover)' 

 of salts due to heating, it seems to the writer that the effect on colloids and the 

 well-known effect of heat in increasing the solubility of some minerals and 

 organic matter also are important factors. 



