176 SOIL PHYSICS AND MANAGEMENT 



Under different systems of tillage of the same soil type, the 

 apparent specific gravity is an approximate measure of the tilth 

 of the soil when determined under field conditions. In order to 

 do this take a tube with a cutting edge and force it into the soil 

 to a certain depth marked on the tube, thus securing a definite 

 volume of the soil. Dry, weigh, and compare with an equal volume 

 of water, or, in other words, determine its apparent specific gravity. 

 The soil having the lowest apparent specific gravity is in best tilth. 

 As an illustration, the apparent specific gravity of brown silt loam 

 from a heavily cropped field was 1.36, while that of a well treated 

 field was 1.10, indicating that the latter was in much better tilth 

 than the former. The apparent specific gravity of soils varies from 

 1.7, that of sand, to 0.5, that of peat. 



3. Weight of the Soil. The weight of any quantity of soil 

 may be determined by multiplying the weight of an equal volume 

 of water by the apparent specific gravity of the soil. A cubic foot 

 of soil varies from 106 pounds to 31 pounds per cubic foot, the 

 former being sand, the latter peak Knowing the weight of an 

 acre-inch of water to be 226,000 pounds, it is easy to obtain the 

 weight of an acre-inch or any number of acre-inches of soil. (See 

 the table page 120 for weight of soil strata.) 



4. Color of Soils. The color of soils is one of the most notice- 

 able or striking characteristics and always appeals to practical 

 farmers as one of the best means for indicating soil differences. 

 Its importance in estimating the character of the soil must depend 

 upon the material producing it. Color is due almost entirely to 

 the presence of two substances, organic matter and iron in some 

 form. 



The color imparted by organic matter varies with the 'amount 

 present, the stage of its humification, the moisture content of the 

 soil, and the amount of limestone present. The color imparted 

 varies from black through brown to gray. The least decomposed 

 imparts a brownish color, while the organic matter that is thor- 

 oughly humified gives a very dark brown or black color to the soil. 



The presence of limestone imparts a darker color to the organic 

 matter and hence to the soil. It further aids by preventing the 

 leaching out of the black humus by forming insoluble compounds 

 with it. Soils fairly well drained but deficient in limestone are 

 usually light in color. The acid of soils bleaches the organic matter 

 so that its effect in coloring soils is not so striking as in those 

 containing limestone. In areas of acid soils, the presence of lime- 



