EXTENSION COURSE IN SOILS. 39 



the excess of soluble salts to percolate through the soil and pass 

 away in the drains. Where alkali spots occur in arid lands the 

 most effective and certain relief is usually afforded by under- 

 drainage combined with surface flooding. The few rains which 

 occur in these places, or a flooding of the land where possible, will 

 finally carry the excess of harmful soluble salts from the surface 

 soils into the drains. Calcium sulphate (gypsum) is very advan- 

 tageously applied to black alkali lands before flooding. This results 

 in a chemical reaction yielding calcium carbonate and sodium sul- 

 phate, which is much less harmful than sodium carbonate and is 

 readily removed by drainage. 



The methods of underdrainage of irrigated and alkali lands differ 

 from those used in humid sections. 



EXERCISES, LESSON V. 



Materials required. — Four boxes 1 foot square and 4 inches deep ; a sufficient quantity 

 of clay or silt loam to fill these boxes; a few small thermometers; three 3-inch unglazed 

 tile; two tight wooden boxes 10 by 12 by 10 inches; a small quantity of paraffin or 

 paint; paper, pencil, and ruler. 



Influence of slope (Ref. Nos. 2, pp. 228, 229; 3, pp. 458, 459).— Fill two boxes, each 

 1 foot sqiiare and 4 inches deep, level full of the same kind of soil. Have the soil 

 equally compact in both boxes. Place both boxes in the sunlight, so that the surface 

 of the soil in one box will be at right angles to the rays, and in the other nearly parallel 

 with the rays. After an hour or two compare the temperature of the two soils. 

 Explain the differences. What are the advantages of a north slope as a site for an 

 orchard? 



Influence of water content. — Fill two boxes as in the preceding exercise with almost 

 dry clay or silt loam. Compact soil equally in both boxes. Wet the soil in one box 

 almost to saturation with water'. The water used should be of the same temperature 

 as the soil in the other box. Take the temperature of the soil in both boxes, then 

 place them in the sun. After two or thrge hours compare the temperature of the soil 

 in the two boxes. Which requires more heat to raise 100 pounds 1°, water or dry 

 soil? Why should a low, wet soil be called a cold soil? 



How tile works (Ref. No. 8, p. 28). — Secure three 3-inch unglazed tile and two 

 tight wooden boxes about 10 by 12 by 10 inches. Cut two holes in opposite sides 

 near the bottom of one and on opposite ends near the bottom of the other box large 

 enough to allow the tile to enter. Place one tile in the first box so that the two ends 

 will project from either side. Place the other two tile end to end with the joint in 

 the middle of the box and the ends of the tile projecting from either end of the box. 

 Make both boxes water-tight by means of paraffin or paint (dp not seal the joint of the 

 tile in the box containing the two tile), and fill each box with sandy soil. Saturate 

 the soil with water and note results. Explain fully how tile works under field con- 

 ditions. Are there any objections to glazed tile? 



Two drainage systems. — A level field 80 rods long and 20 rods wide has a ditch 6 feet 

 deep across one end. . Draw out to scale of 5 rods to 1 inch two systems for laying out 

 drains, namely, one with a long main and short laterals, and the other with a short 

 main and long laterals Place the laterals in each case 4 rods apart. 



Compare the number of rods of tile required for the two systems. Make computa- 

 tions from the drawings. 



Principles of tile laying (Ref. No. 8, pp. 63-68). — An outlet ditch 6 feet wide at the 

 bottom, 7 feet deep, and 20 feet wide at the top has a line of tile emptying into it 4 feet 



