. Soils and fertilizers . ecting a granularstructure. This it does by the peculiar twisting action thatthe curved moldboard gives to the furrow slice. The soilin immediate contact with the plow surface is retarded byfriction, and the layers above tend to slide over one anothermuch as do the leaves of a book when they are bent. Thesoil is thus broken up into masses of aggregates correspond-ing to the location of the lines of weakness. If a soil hasbeen 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 til
. Soils and fertilizers . ecting a granularstructure. This it does by the peculiar twisting action thatthe curved moldboard gives to the furrow slice. The soilin immediate contact with the plow surface is retarded byfriction, and the layers above tend to slide over one anothermuch as do the leaves of a book when they are bent. Thesoil is thus broken up into masses of aggregates correspond-ing to the location of the lines of weakness. If a soil hasbeen 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 propertiespossessed by lime is that of flocculating clay. This may bereadily observed by stirring a spoonful of clay in a tumblerof water and then adding a quarter of a spoonful of burntlime. It will be noticed that the soil settles much morequickly after the lime has been added than before. Sandy. Plate VII. Tillage. — Good tilth is a response to good soil manage-ment. The upper figure is an illustration of poor, the lower of good, tilth. TEXTURE AND STRUCTURE OF SOILS 43 soils are not flocculated to the same extent by lime, but are thusaffected in proportion to the quantity of clay they possess. Of the different forms of lime, quick-lime and water-slacked lime are more active in producing a granulated struc-ture of soil than is ground limestone, marl or air-slackedlime. This is one reason why the burned lime is superiorto ground limestone for use on heavy clay soils, on whichthere may be a pronounced difference in the effect of the twokinds of lime on crop production. Warington reports astatement of an English farmer to the effect that by theuse of large quantities of lime on heavy clay soil he wasenabled to plow with two horses, while three were necessarybefore applying lime. 47. The soil survey. — The purpose of a soil survey is toclassify and m
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