192 SOILS: PROPEUTIES AND MANAGEMENT 



Puddled Clay plus Muck Extract i 



Percentage 

 of pene- 

 tration 



1. Clay 100 



2. Clay plus 1 per cent of extract 85 



3. Clay plus 2 per cent of extract 76 



4. Clay plus 4 per cent of extract 69 



126. Action of plant roots and animals. — The exten- 

 sion of plant roots changes the soil structure by forcing 

 the particles apart at each growing root point, and possibly 

 also by some action yet to be explained. Crops differ 

 greatly in their effect on soil structure. Grass, millet, 

 wheat, and other plants with fine roots are more beneficial 

 to tilth than coarse or tap-rooted plants such as corn, 

 oats, and beets. Grass affects structure also by protecting 

 the surface of the ground. It is advisable to establish 

 a rotation on clay soil, that plowing may be done at fre- 

 quent intervals and that plants with different root devel- 

 opments may be given an opportunity to exert their 

 influences. The organic matter left in the soil by 

 decaying roots is always in very intimate contact 

 with the soil grains and has much to do with accelerat- 

 ing granulation. 



Animals also affect soil structure. Earthworms, by 

 carrying materials to the surface, exert a mixing effect, 

 while the lines of seepage and zones of weakness developed 

 through their burrowing proclivities are of no mean im- 

 portance. Insects, especially ants and other burrowing 

 creatures, aid in this and in other ways. 



1 Fippin, E. O. Some Causes of Soil Granulation. Trans. 

 Amer. Soc. Agron., Vol. 2, pp. 106-121. 1910. 



