EXERCISE 14 



THE TEXTURE AND STRUCTURE OF SOILS 



Statement. The size of the soil grains and their arrangement, together with the amount of organic 

 matter present, determine the power of the soil to absorb and to hold water. These factors, to a large 

 extent, determine also the mellowness or tilth of the soil. 



Object. To determine the size, shape, color, and arrangement of the soil grains and to form an 

 idea of the amount of organic matter present in a soil. 



Materials. Samples of air-dry soil ; sources of heat ; a microscope. 



Directions, i. Take lumps of dry soil from each sample collected and crumble them in the hand, 

 noting the ease with which the soil mass breaks. This gives an impression of the manner in which 



abed 

 Fig. 18. Soil particles as they appear under the microscope 



Magnified 130 times; a, coarse sand; b, medium sand; c, very fine sand; d, silt. The particles of clay are so small that they are not visible 



under the magnification used. 



the soil will act when plowed, A soil naturally cloddy and hard to crush usually contains a large 

 amount of clay and a small amount of organic matter. 



2. Place a few soil grains representing the average of the soil on a microscopic slide, moisten with 

 a drop of water, and examine them under the microscope. Classify the soil grains according to their 

 color. Notice the presence of a black material clinging to the soil grains. This is organic matter. 



3. Pour a little water upon a lump of soil from each sample and observe the rapidity with which 

 the water is absorbed. This gives an indication of the readiness with which the water may be taken up 

 by soils. Soils having a gray tint when dry become almost black when wet. When there is only a slight 

 change in color upon wetting, the absence of, or a deficiency in, organic matter usually may be inferred. 



4. By noting the effect of alternate freezing and thawing upon the different types of soil, their 

 texture and structure may be determined. Use heavy clay soil, garden loam, and sandy loam. From 

 each sample make two well-puddled mud balls, about the consistency of putty. Place one ball of each 

 soil where it will dry at room temperature and place the others where they will freeze and thaw daily. 

 When the samples are dry, pulverize each with the hands, noting the difference in the hardness as 

 influenced by the action of freezing and thawing. 



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