140 NATURE AND PROPERTIES OP SOILS 



small particles, it is only the eoarse sandy soils on the one 

 hand and very fine clayey soils on the other that ideally repre- 

 sent these two groups. Most soils, especially loams, present 

 combinations o£ the single and compound grain structures. 



Single-grain structure as found in sandy soils has certain 

 obvious advantages, such as looseness, friability, good aera- 

 tion, and drainage and easy tillage. On the other hand, such 

 soils are often too loose and open and lack the capacity to 

 absorb and hold sufficient moisture and nutrient materials. 

 They are, as a consequence, likely to be droughty and lacking 

 in fertility. There is only one method of improving in a prac- 

 tical field way^ the structure of such a soil — ^the addition of 

 organic matter. Organic material, if it undergoes favorable 

 decomposition when incorporated with the soil, will not only 

 act as a binding material for the particles but will also in- 

 crease the water capacity. Nitrogen also is added and if the 

 organic matter is properly supplemented with fertilizers and 

 lime, the soil fertility will usually be markedly improved. A 

 sandy soil high in organic matter is almost ideal from a struc- 

 tural standpoint. 



The modification of the structural condition of a heavy soil 

 is not such a simple problem as in the ease of a sandy one. 

 In the latter the plasticity and cohesion is never high even 

 after the addition of large amounts of organic materials that 

 rapidly develop into a colloidal state. In clays and similar 

 soils the potential plasticity and cohesion^ are always high 



*In the greenhouse or garden, structure may "be modified by mixing 

 different soils. This is not practicable in the field, 



* There are no satisfactory methods of determining either the plasticity 

 or the cohesion of soils. For plasticity determination, see: Atterberg, 

 A., Dis Plasti^itat der Tom; Internat. Kitt. f, Bodenkunde, Band I, 

 Heft 1, Seite 10-43, 1911. Kinnison, C. S., A Study of the Atterberg 

 Plasticity Method; Trans. Amer. Oer. Soc, Vol. 16, pp. 472-484, 1914. 



For methods of estimating cohesion: 



A good description of Sehiibler's apparatus is found on page 104 of 

 Bodenlcunde, by E. A. Mitscherlich, published by Paul Parey, Berlin, 

 in 1905. Haberlandt, H., ifher die KoMres^ens, VerTialtnisse ver~ 

 scMedener Bodenarten; Forsch. a. d. Gebeite d. Agri.-Physik., Band I, 

 Seite 148-157, 1878. Also, Wissenschafilidh prahtische UntersucJmngen 



