20 BULLETIN 355, U. S. DEPARTMENT OF AGRICULTURE. 



tions. If clay soil is tilled when wet, its smaller particles seem to 

 become more closely fitted into the spaces of the larger particles, 

 and in this very plastic condition the soil is said to be puddled. 



As wet soils djry out the water films surrounding the particles be- 

 come thinner, which causes a contraction of the soil mass. This 

 contraction causes separations between particles having least cohe- 

 sion, which results in irregular cracking and the formation 

 of soil masses of various sizes. Highly plastic clay soils which have 

 become puddled form into large masses upon drying, and when 

 tilled break up into clods. On the other hand, when rightly man- 

 aged, clay soUs upon drying form into small, irregular masses, which 

 by tiUing form a crumblike structure. This property is called gran- 

 ulation. The granulation of soUs has a very important influence on 

 the growth of crops, since it permits the excess of water to drain off 

 more readily than would be the case if all the soil grains were as 

 closely arranged as possible, and it offers the roots of the plants an 

 opportunity to penetrate the soil much more readily than they could 

 otherwise do. It also gives the air better access to the growing roots 

 and to the microorganisms causing changes in the soil. 



Agencies producing granulation. ^The principal agencies which 

 affect granulation in soils are: (1) Good drainage. Where land is 

 well drained any excess of water quickly passes away instead of satu- 

 rating the soil and thus- inducing puddling and the formation of solu- 

 tions which hinder granulation. (2) The use of lime. The addition 

 of lime to clay soils causes a flocculation, or gathering into aggregates 

 of materials suspended in the soil solutions, and thereby reduces 

 plasticity and promotes granulation. (3) Insects and plant roots. 

 The borings of insects and earthworms, and the penetration of plant 

 roots far into the subsurface soil, deepen the zone of granulation. 

 (4) Decaying vegetable matter in the soil. It is believed that the 

 humus in the soil becomes distributed over the surfaces of soil grains 

 and through the solutions of the soil, reducing its plasticity and per- 

 mitting better granulation. At any rate, it is a practical fact of 

 common farm experience that plowing manure, straw, and plant 

 residues deeply into the soil produces a loosening, granulating effect 

 which makes tillage easier and adds to productiveness. (5) The 

 growth of grasses. The fine, fibrous roots of grasses, completely 

 permeating the openings of the surface soil, attach themselves thor- 

 oughly to the soil particles and gradually develop a condition of 

 granulation. The good tilling properties of land which has been 

 in grass for several years are well known. (6) Tillage operations. 

 Soil must be tiUed at the right time and with the right implements to 

 secure the best granulation. 



Pore space in soils (Ref. Nos. 10, pp. 101, 102; 3, pp. 108, 109).— 

 Pore space m soils may be thought of as the space not occupied by the 



