THE MATERIALS OF WHICH SOIL IS COMPOSED (XXIII-1)

The problem. Soils differ so widely that even an amateur student can readily detect certain differences, and those who culti vate the land know qualities which may make a soil worth several hundreds of dollars per acre, or other qualities which may make it worthless. It is not hard to discover some of the qualities of soils, but others can be determined only by highly trained scientists. What to use. Three or four pieces

of glass tubing one-half inch in diameter and two feet or more in length (appendix, p. 178), cork stoppers, samples of three or four soils from the vicinity, and magnifying glass.

What to do. 1. Close one end of each tube with a stopper (Fig. 41). Mix the soil sample with water so as to make a very thin mud, and pour into each tube enough of the mixture to make a layer six inches deep in the bottom of the tube. Then fill the tube with water. Close the upper end of the tube with a stopper. vert the tube and watch the soil as it settles through the water

In

2. Examine the soil samples with a magnifying glass (Fig. 42) while the particles are settling in the water.

FIG. 41

Questions. Which parts of the soil settle first and which last? How many and what kinds of materials can you detect in each soil? Does all the soil settle during one class period? in one day? Why? Do all soils contain rock and organic materials?

FIG. 42. Samples of the particles which compose soils

The size of the particles and the magnification used in these illustrations are given. A, clay, .0050 mm. X 28; B, silt, .05.005 mm. X 28; C, very fine sand, .1-.05 mm. X 28; D, fine sand, .25.1 mm. X 28; E, medium sand, .5.25 mm. X 28

Suggestions for report. Prepare a written description of the appearance and composition of the samples of soil examined.

Reference work. Read Chapter XXIII.

Optional problems. Does a fertile soil necessarily contain more or fewer of the finer particles than the poor soil? If a microscope is available, small pieces of soil should be studied under magnification. Prepare a set of soil tubes five feet in length for use in this experiment and set up the apparatus for a more prolonged experiment.