578 SCIENCE PROGRESS 



condition than was the case in the more easily cultivated 

 soils. 



Generally speaking, sandy soils can be subjected with 

 impunity to cultivation at times when clay soils would be 

 seriously injured by similar treatment. This injury shows 

 itself in a destruction of the soil tilth : the friable and porous 

 surface, characteristic of good tilth, gives place to one which is 

 sticky and practically impervious to water or air, and which can 

 be brought back only slowly into a better condition. 



Although these two states of clay present obvious differences 

 to visual inspection, and can be readily controlled by the 

 farmer, very little is known about them. The grains in a soil 

 in good tilth are loosely attached to each other, forming small 

 porous aggregates, or compound particles, which may be 

 likened in some ways to sponges. The formation of these 

 aggregates from the individual particles is closely parallel to 

 the flocculation, by acids and salts, of clay suspended in a bulk 

 of water. Lime is used in practice to improve the physical 

 condition of soils, and also produces flocculation of a clay 

 suspension in the laboratory, and, although the proportion of 

 water in natural soils is many times less than that used in 

 flocculation experiments in the laboratory, the effects are so 

 similar that one must conclude that they are produced by the 

 same causes. 



It will, therefore, be seen that the colloidal theory is of 

 great importance in soil problems. One of the most important 

 of its immediate applications is in the investigation of the 

 jelly-like material supposed to exist on the surface of the soil 

 particles, because of the above-mentioned statement that it is 

 the forces associated with this coating which are concerned 

 in the production of good tilth. At present it is nearly 

 always necessary to examine the physical effects produced by 

 the coating as a whole, as its composition is exceedingly 

 complex, and the necessary chemical and physico-chemical 

 data relating to its constitution are not yet complete. Physical 

 measurements carried out by the writer indicate that, as far 

 as the relationship of this material to the soil moisture are 

 concerned, the inorganic colloids derived from the mineral 

 material of the soil are of more importance than the organic 

 colloids contained in the decomposed plant products, or humus. 

 In the production of good tilth, however, it is probable that 

 these humus colloids have increased importance. They will 

 effect a certain mechanical separation of adjacent soil aggre- 

 gates, and thus keep the soil more open, and they may also 

 have a weak cementing effect which would assist the formation 

 and maintenance of soil aggregates from the individual particles. 

 Recently Comber has pubHshed some interesting results bearing 



