42 SOILS AND FERTILIZERS 



several tons in an acre in the course of a year, while their bur- 

 rows ramify through the soil in all directions. The move- 

 ment of soil particles that results is an appreciable factor 

 in changing soil structure. Insects and other burrowing 

 creatures affect soil structure in a similar way. 



45. Tillage and structure. — The ordinary operations of 

 tillage are designed to improve soil structure, and are effective 

 if these operations are conducted at the proper time and in 

 the best way. Plowing, which is the most fundamental of all 

 tillage operations, may improve soil structure or may injure 

 it, depending on the condition of the soil at the time of 

 plowing. It is a matter of common knowledge that working 

 a soil saturated with water will cause it to puddle, or in 

 other words, to assume the separate grain structure. Plowing 

 when the soil is very dry may have the same effect, although 

 not usually to the same extent. However, when a soil is mod- 

 erately moist, plowing aids greatly in effecting a granular 

 structure. This it does by the peculiar twisting action that 

 the curved moldboard gives to the furrow slice. The soil 

 in immediate contact with the plow surface is retarded by 

 friction, and the layers above tend to slide over one another 

 much as do the leaves of a book when they are bent. The 

 soil is thus broken up into masses of aggregates correspond- 

 ing to the location of the lines of weakness. If a soil has 

 been strongly compacted, so that there are few lines of weak- 

 ness, the clods will be large when the soil is plowed. Plow- 

 ing helps to improve the tilth of the soil, but it will not over- 

 come entirely a bad physical condition. 



46. Structure as affected by lime. — One of the properties 

 possessed by lime is that of flocculating clay. This may be 

 readily observed by stirring a spoonful of clay in a tumbler 

 of water and then adding a quarter of a spoonful of burnt 

 lime. It will be noticed that the soil settles much more 

 quickly after the lime has been added than before. Sandy 



