TiiK I'KKriLnv OF Tin-; son. 19 



The percentages of moisture, humus and carbonate of hme 

 are not inckided in the mechanical analyses of soils. 



Surface Area of Soil Grains. — The surface area of soil trains 

 varies with the size of the particles. The smaller the grains 

 the more surface area is exposed to the action of water and soil 

 organisms, and the more quickly is plant food rendered available. '■' 



Diameter of grain. Square feet of siirface in a pound. 



Coarse sand i mm. 11 .05 



Fine sand, o. i mm. 1 10.54 



Silt, o.oi mm. 1,105.38 



Clay, o. 001 mm. 11,053.81 



_Fine clay, o.oooi nmi i, 100,538.16 



Lumpy Soils. — The mechanical composition of a soil is import- 

 ant, for the farmer to consider, in order to keep the soil re- 

 ceptive for growing crops. The clustering or lumping of soils 

 is brought about by the adhering of the particles due to the sur- 

 face tension of the films of water surrounding the grains. As 

 the water dries out the grains are held together with the aid of 

 the salts in solution. Fine soils, like clay, contain manv more 

 particles than sandy soils, so it is apparent that clay soils are 

 more apt to form lumps than the coarser soils. 



Cracking of Soils. — When soils become dry the films of water 

 around the soil particles become thinner and the soil contracts, 

 breaking in the weakest point, causing cracks. 



Puddling of Soils. — If soils are worked when in a very wet 

 condition the soil particles run together and a puddling soil is 

 formed. After such a soil, especially a clay soil, dries out it 

 becames very hard and most difficult to restore to good condition. 

 A farmer should never work a clay soil when it is too n-et. 



Freezing and Thawing. — When soils are plowed deepl>' in the 

 fall and allowed to be acted upon by the frosts a helpful crumb 

 like condition results. The action of frosts is more ap])arent 

 when northern and .southern soils are compared. The northern 

 soils treated as above are usually in better tilth than the south- 

 ern soils in sections of little or no frost. 



Plants are Benefited by Open Soils. — A good tilth of the soil 

 helps the plant a great deal in securing its food, and is therefore 

 an important factor in the production of crops. A soil should 



