EXERCISE 20 



THE EFFECTS OF A SOIL MULCH 



Statement. A mulch on the surface of the soil increases the water available to growing crops in 

 three important ways : (i) it helps the moisture which falls on the surface to get into the soil ; (2) 

 it helps to prevent the waste of this moisture through the growth of weeds ; (3) it reduces evapo- 

 ration of moisture from the surface of the soil. 



Object. To compare the different forms of mulches as to their value in conserving soil moisture. 



Materials. Twelve flowerpots or tin cans ; loam ; clay ; sand ; balances. 



Directions. 1. Label each flowerpot, weigh it, and record the weight. Fill four of them with fine 

 clay, four with loam, and four with sandy soil. Use air-dry soil in each case. Reweigh and record the 

 weight of the soil. In one pot of each soil plant weed seeds, watering as necessary, and allow them to 

 get well started before continuing the exercise. Saturate each soil with water; weigh and determine the 

 percentage of water retained by each. When the surface of the soil is dry enough to work, stir the top 

 inch of soil sufficient to make a soil mulch in one pot of each kind of soil. Remove an inch of the surface 

 soil from one pot of each kind of soil and replace with an equal weight of dry dust. Leave the other 

 pots untreated. Weigh each pot and record the weight. Reweigh at intervals of twenty-four hours for 

 fourteen days and compute the amount of moisture lost daily by each. 



2. That capillary action may be interrupted by plowing under weeds and rubbish late in the season 

 may be shown as follows : Fill two lamp chimneys half full of a common type of soil and compact 

 each. Place a layer of finely chopped straw or chaff about one inch thick in one of the chimneys and 

 fill it with soil. Fill the other one with the soil, compact both, and set them in shallow pans of water. 

 Observe the rise of the water through the soil. Explain the results. 



3. Test the efficiency of different kinds of mulches by filling boxes with moist soil and covering the 

 surface of each with a different kind of mulch. Use road-dust, chopped straw, sand, and cloddy earth. 

 Cover the surface to the same depth with each kind of mulch and weigh the boxes from day to day 

 to determine the loss of water. Compare the results and explain the reasons for the difference shown. 



4. If capillary action is permitted to bring moisture to the surface rapidly, the consequent evapora- 

 tion decreases the soil temperature. The effect of evaporation upon the temperature may be shown as 

 follows : Cover the back of the hand with cottonseed oil and note the apparent effect upon the tem- 

 perature of the hand. Remove the oil and wet the back of the hand with water. Note the feeling. Wipe 

 dry and apply alcohol. What is the effect upon the apparent temperature ? 



5. Compare the moisture in the soil under a board lying on the ground with that of the uncovered 

 soil near by, and explain. Remove the cultivated layer of soil in a cornfield or garden and compare 

 its moisture content with that of the underlying soil and explain. Compare the dryness of a soil 

 which has supported a rank growth of weeds with that of one near by which has been cultivated and 

 kept clean, and explain the difference. 



Questions. By what means is water removed at the surface of the soil ? Explain the meaning of a 

 soil mulch. Explain how the mulch checks the loss of water. Why should soil be cultivated after a 

 rain? Which is the cause of greater loss of moisture, surface evaporation or weed growth? Name 

 substances, other than soil, that may be used to produce a mulch. Is the layer of finely pulverized 

 soil which serves as a mulch dry or is it as moist as the soil underneath, and why? What happens 

 to the mulch when it rains enough to wet the surface soil, and why? When a crust forms on the 

 surface what should be done? 



References. Waters, H. J. Essentials of Agriculture, pp. 66-67. Ginn and Company. Mosier and 

 Gustafson. Soil Physics and Management, pp. 232-236. J. B. Lippincott Company. King, F. H. Physics 

 of Agriculture, pp. 185-189. Mrs. F. H. King. King, F. H. The Soil, pp. 194-202. The Macmillan Company. 



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