38 AGRICULTURE. 



plowing and harrowing, thereby exposing the 

 soil to the action of the wind and rain ; by bor- 

 ing wells, and excavating mines and quarries ; 

 by controlling or directing the water of rivers 

 and streams, and by irrigating dry or desert 

 regions (Fig. 12), thus changing the natural 

 environment very greatly if not altogether 



4. Field Exercise No. 2. A Study of Organic^ife as a 

 Factor in Soil Formation. 



PART i. Mechanical Action. (a) Note the disinte- 

 grating processes of plant life. Pull off the moss, or 

 lichens, growing upon a solid rock, and see how far be- 

 neath the surface the root-like extensions have crept. 

 Measure and calculate the length of some great root- 

 system which is exposed along the bank of a stream, or 

 find rocks burst asunder by root action ; note examples 

 of retarded evaporation. 



(b) Note the protection of soil by plants. 



(c) Note vegetable accumulations. In the woods, 

 notice the formation of humus from the decayed leaves, 

 twigs, and bark, and contrast the soil with that in the 

 meadows, roads, and lawns. Account for these varia- 

 tions, and discuss all factors concerned, as sunlight, air 

 currents, depth of feeding roots, and kinds of material 

 obtained by them at the different strata. What is the 

 relative value of the soil from each place ? T v ake a sam- 

 ple of each of these soils back to the laboratory, and try 

 to grow a plant of the same kind and size in each soil, 

 and record and compare your results. 



PART 2. Work of Disintegrating and Pulverizing the 

 Soil. (a) Describe the work of as many different kinds 

 of animals as it is possible to find in your trip ; dig up 

 a block of soil containing the burrows of earthworms, 



