88 NATUEE AND PEOPERTIES OP SOILS 



plex structure is best developed in clay. Here the soil gran- 

 ules are made up of many particles, the colloidal material act- 

 ing as a binding agent. Such a soil may be loose, open and 

 friable, if granules of the proper size and nature are developed. 

 On the other hand, improper handling may run the complexes 

 together and an impervious and puddled condition may result. 

 The sand will obviously permit of no very great structural 

 change, while the clay can be modified very materially by 

 certain field manipulations. 



The ideal structural condition is most likely to occur in a 

 loam soil. In such a soil some of the particles are large and 

 function separately; others are medium in size and tend to 

 form the nuclei around which smaller particles, both colloidal 

 and non-colloidal, may cluster to form granules, or aggregates. 

 There are thus a few large pore spaces which facilitate drain- 

 age, and numberless small openings in which water is retained. 

 Air, therefore, finds easy movement and sanitation is pro- 

 moted. In promoting such a condition the organic matter 

 plays an important part. It usually exists as a dark, partially 

 decayed material, often colloidal in nature. It pushes apart 

 the grains and lightens the soil, and contributes much in bring- 

 ing about the loamy condition so favorable to plant develop- 

 ment. It is a valuable addition also on account of its water- 

 holding capacity and its nitrogen content. 



48. Specific gravity of soils. — The texture, as well as the 

 structure of a soil, has considerable influence on certain phys- 

 ical conditions other than those already mentioned. One of 

 these is weight. The weight of a soil is determined by two 

 factors : the weight of the individual particles and the amount 

 of the space occupied by the soil material. The former is 

 determined by the chemical and mineralogical character of 

 the particles, the latter by their structural arrangement. Thus, 

 if the soil particles are heavy and the soil is compact, the 

 weight of any given volume, a cubic foot for example, will be 

 high. 



