TEXTURE AND STRUCTURE OF SOILS 41 



43. Effect of organic matter on structure. — The quantity 

 of organic matter in a soil is frequently the deciding factor 

 in determining its structure. Partially decomposed organic 

 matter has a loose, spongy structure and at the same time 

 a plastic quality. The latter causes the soil particles to 

 cohere, and the former gives to the organic matter the 

 property of swelling when the soil becomes wet and shrink- 

 ing when it becomes dry. These changes in volume facilitate 

 the formation of granules as previously explained. 



Large areas of land in this country have deteriorated in 

 productivity and have become compact and difficult to work 

 on account of the gradual loss of organic matter. Naturally 

 clay and heavy loam soils have suffered more in this way 

 than have lighter soils. Where marked decrease in crop 

 returns has occurred during the time that soils have been 

 under cultivation, the difficulty can generally be traced to 

 loss of organic matter more than to any other factor in plant 

 growth. Compact soil, with consequent poor tilth, is one 

 of the most common conditions in poor farming regions, 

 and is usually associated with a low content of organic 

 matter. 



44. Roots and animals. — In some way not very well 

 understood roots exert more or less influence on soil struc- 

 ture. Shallow, fibrous-rooted plants, among which are 

 the grasses, wheat, barley, millet and buckwheat, have the 

 most favorable action in granulating soil. More deeply 

 rooted, and especially tap-rooted plants, have this property 

 to a less extent. In fact, a crop of beets may help to com- 

 pact a soil already in bad condition. In establishing a rotation 

 it is desirable that some fibrous-rooted plants form one or 

 more of the courses. 



Various forms of animal life help to granulate soils. Of 

 these, earthworms are the most notable. The soil particles 

 that they excrete from the digestive tract may amount to 



