SOIL STRUCTURE AND ITS MODIFICATION 149 



83. Soil tilth. — The previous data and discussion have 

 clearly shown the very great importance of structure in the 

 successful handling of the soil in the field. Since good phy- 

 sical condition will reflect itself on crop yield it is evident 

 that structure must ultimately be considered in relation to 

 all plant growth. This relationship is usually expressed by 

 the term tilth. While structure refers to the arrangement of 

 the particles in general, and granulation to a particular aggre- 

 gate condition, tilth goes one step farther and includes the 

 plant. Tilth, then, refers to the physical condition of the soil 



Fig. 27. — A planker or drag, useful in the crushing of clods. 



as related to crop growth. It may be poor, medium, good, or 

 excellent, according to circumstances. Good tilth may de- 

 mand in many soils maximum granulation, in others only a 

 medium development. Tillage operations by influencing the 

 structure of the soil aim to develop optimum tilth. Optimum 

 tilth always implies the presence of water since the best phys- 

 ical relationships cannot be developed without such moisture 

 conditions. 



84. Summary. — The factors which control the struc- 

 tural condition of the soil to the greatest extent are plasticity 

 and cohesion, their influence intensity being due directly to 

 the presence of certain kinds of materials, especially hydrated 

 aluminum silicates, in a colloidal state. As plasticity and 

 cohesion increase the tendencies of a soil to puddle when wet 



