EXTEITSIOlSr COUESE IN SOILS, 23 



(b) Let the original 2-iiich cube represent a soil grain of a coarse-textured soil. Cut 

 it in three directions, and cut each resulting cube again in three directions. Into 

 how many cubical particles is the original soil grain now divided? The total surface 

 area of these particles is now how many times that of the original soil grain? 



A soil made up of these resulting particles is how much finer textured than soil 

 made up of particles like the original soil grain? WTiich soil will hold more film 

 water? Why? 



Texture and pore space. — Secure two 1-pound baking-powder cans and make them 

 water-tight by use of a little paraffin. Fill one within half an inch of the top with 

 dry sand, and the other with dry clay loam and silt loam. Measure carefully the 

 cubic inches of water required to saturate the soil in each can. Apply the water to 

 one edge of the can only as fast as it is absorbed by the soil. This will allow the air 

 in the soil pores, or spaces, to escape. Determine the number of ctibic inches of soil 

 in each can (3-|- times radius squared times height of soil column equals volume) and 

 compare each volume of soil with the volume of water required for saturation. 



What percentage of the volume of band is water? 



Where is this water in the soil? Draw a diagram showing the relation of the sand 

 grains to the water. What, then, is the approximate percentage of pore space in the 

 sand? In the clay or silt loam? Explain fully why clay or silt loam is more porous 

 than sand. When is a soil said to be saturated? What is the relation between the 

 porosity of a soil and its texture? Between porosity and weight of dry soil? 



Plasticity. — Place small quantities of a clay soil and some other class of soil upon 

 two different pie tins or saucers. Add water slowly to each and continue to stir 

 until thje samples can be molded like dough. Which soil shows the greatest degree 

 of plasticity? Add water and mix each sample until the soils have become puddled. 

 Set aside to dry. "V^Tiat happens to the two samples when entirely air dried? 



Granulation. — Put a small sample of dry clay soil upon a dish and add water with- 

 out stirring so slowly that the soil absorbs it as fast as added. Do not add water 

 enough to saturate the soil, only add what would be held by a well-drained soil. 

 Set the soil aside to dry and try to stir at just the time when best granulation can be 

 effected. How does the structure of this soil now compare with the dried-clay sample 

 that was puddled? 



Field trips. — If possible, field trips should be made to study soil classes. With a 

 spade dig down through the surface soil and partly into the subsurface. Make a 

 smooth, perpendicular edge. Now note line between surface and subsurface and 

 measure the exact depth of the surface soil. Observations as to differences of color, 

 texture, and structure should also be made. What has caused these variations? 



REVIEW QUESTIONS, LESSON m. 



1. What is meant by texture of soils? By structure? 



2. How do you distinguish between heavy and Light soils? 



3. By what process is the texture of the soil determined? 



4. Compare the sand, silt, and clay content of a fine sandy loam with that of a clay 

 soil. 



5. Mention some ways in which the fertility of a soil is influenced by its texture. 



6. Explain fully the influence of the area of surface of the soil grains on the water- 

 holding capacity of soils. 



7. What is meant by soil granulation? Soil tilth? 



8. Mention several agencies which develop granulation in soil. 



9. How is puddling of soils produced? 



10. What is meant by pore space in soils? Draw diagram to illustrate. 



11. About what fraction of clay loam soils under the ordinary field conditions is 

 pore space? 



