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 fuddled. 
As wet soils dry 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 soils upon drying form into small, irregular masses, which 
by tilling form a crumblike structure. This property is called gran- 
ulation. The granulation of soils 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 ^oil 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 tilled 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 in soils may be thought of as the space not occupied by the 
