PHYSICAL COMPOSITION AND CLASSES OF SOILS 9! 



to changes in temperature, or to changes in moisture content. If 

 the soil is beaten or mixed when too wet, compound particles are 

 destroyed and the soil dries into a hard compact mass. 



Tilth is a condition of the soil, and only indirectly due to pro- 

 cesses of tillage. A heavy soil is not reduced to powder by the 

 mechanical force exerted through implements. Tillage stirs the 

 soil, and places it so that natural forces exert their greatest effect 

 upon it. The soil in good tilth falls into a powder 

 under the action of the various instruments. Such a con- 

 dition of the soil is very desirable. It allows the preparation of a 

 good seed bed ; it is most suitable for ensuring the best conditions 

 of moisture, temperature, and chemical action in the soil during 

 the growth of the plant. The maintenance of good tilth is 

 especially important on clay soils. 



The production of a good tilth, and the permeability of a clay 

 soil to water, depend largely upon the formation and maintenance 

 of compound particles. The conditions favorable to the forma- 

 tion of compound particles are also favorable to the coagulation 

 of clay. Any change which converts the clay from a coagulated 

 to a swelled condition is necessarily destructive to compound 

 particles, but it is quite possible to destroy compound particles 

 without affecting the coagulated condition of the clay. 



Relation of Physical Composition to Adaptation of Crops. The 



adaptation of different crops to particular kinds of soil is due to 

 different needs of the crops for moisture, for soil atmosphere, for 

 temperature, and their habits of root growth. The differences in 

 physical composition cause soils to respond differently to these 

 needs, and hence vary their adaptation. But climatic conditions of 

 rainfall, temperature and situation, modify the way which 

 the soil fulfills these conditions. Under similar conditions, and 

 in a general way, there is a relation between the physical character 

 of the soil and its adaptation to crops, and this is shown by the 

 general agricultural practice and treatment of such soils. 



The relation of physical character to adaptation to crops is 

 studied by ascertaining the kinds of crops actually grown upon 

 the various classes of soils, and how well they thrive on them. 



