The Soil 51 



sand, exhibit little cohesion. Clay owes its cementing power it 

 is believed, to the presence of a small quantity of a hydrated col- 

 loid (jelly-like) body, which according to Schloesing rarely ex- 

 ceeds 1.5 per cent of the clay. The remainder of the clay is com- 

 posed of extremely fine, solid particles. In the purest natural 

 clays, all the constituents have the same general chemical com- 

 position, that is, they are hydrated silicates of aluminum ; but in 

 soils the non-colloid constituents of the clay may be of a very 

 various nature. In brick clay this material is quartz sand; in 

 marl it is limestone. 



The condition of clay soils depends much on whether the clay 

 is coagulated or not. When the clay is uncoagulated, the soil is 

 -sticky, impervious to water, and cannot be reduced to a fine tilth. 

 When a clay is coagulated the soil has a granular structure, is 

 pervious to water, and can be reduced to powder. It is coagu- 

 lated by lime and by many salts, and especially by salts of cal- 

 cium. Colloid clay will remain permanently suspended in dis- 

 tilled water. It is precipitated by the addition of a small quan- 

 tity of a calcium salt. An application of lime to clay soils is 

 well known to be extremely effective in diminishing their ten- 

 acity, rendering them pervious to water, and more easy of tillage. 



In cultivated sandy soils humates are often of great value as 

 cementing materials; these, like true clay, are colloid bodies. 

 Schloesing found that 1 per cent of a humate, as calcium humate,. 

 was as effective as a cement for sand as 11 per cent of clay. 

 Humates, however, will lose their cementing power on drying, 

 while clay will not. The improvement of the texture of sandy 

 soils by the continued use of farm yard manure, or by the plow- 

 ing under of green crops, is a fact familiar to the farmer. While 

 applications of humus forming materials, as the above, increase 

 the coherence of sand, they have an opposite effect on clay, and 

 are the most effectual means at the disposal of the farmer for 

 lightening a heavy soil. Lime will also tend to increase the co- 

 herence of sand. 



