PLANT ROOTS AND STRUCTURE 113 



realized when one considers the tenacity with which 

 these particles are held together in brick. This cohesive 

 attraction is inversely proportional to the square of 

 the distance between the centers of the attracting 

 bodies. Particles that can be brought so closely to- 

 gether as can clay particles are thus held with great 

 firmness. The effect of tillage, when an excess of water 

 is present, is to force the particles into large masses, 

 which become clods when dry. These masses are too 

 large to form granules, and leave the soil in a 'compact 

 condition, poorly adapted to plant growth. When the 

 soil is very dry when worked, the particles are not 

 brought close enough together to cohere, but are pow- 

 dered, forming the separate -grain structure, which 

 forms clods when wet. Tillage may thus produce a 

 granular structure when the moisture is neither excessive 

 nor deficient, and the separate -grain structure when 

 either of these conditions exists. 



47. Growth of plant roots. The growth of plant- 

 roots changes the soil structure by forcing the particles 

 apart at each growing root point, and possibly by some 

 action yet to be explained. Crops differ greatly in their 

 effect upon soil structure. Grass, millet, wheat and 

 other plants with fine roots are more beneficial to tilth 

 than coarse or tap-rooted plants as corn, oats and beets. 

 Grass also affects structure by protecting the surface 

 of the ground. (See page 119.) It is advisable to prac- 

 tice a rotation on clay soil, which requires relatively 

 infrequent plowing, and gives long periods in fine-rooted 

 grass and grain crops. 



48. Organic matter. Soils rich in humus or decom- 



H 



