236 SOIL PHYSICS AND MANAGEMENT 



it contains is unavailable for the use of the crop. If the mulch is 

 three inches deep it means that about one-half of the plowed soil, 

 the most fertile part, has little value except for the conservation 

 of moisture, and in humid climates this layer is of much greater 

 value to the crop for the plant food it contains than for the moisture 

 it conserves. 



Maintenance of the Mulch. Under certain conditions the 

 soil mulch may be entirely destroyed or rendered much less effec- 

 tive by a shower, so that it becomes necessary to renew it. Tillage 

 of some kind must be resorted to in order to reproduce it. After a 

 light shower a harrow or weeder may be effective in renewing it. 



The ease with which a mulch may be maintained depends to a 

 large extent upon the kind of soil. Sands and sandy loams respond 

 readily to tillage and the mulch is easy to produce. Soils contain- 

 ing large amounts of organic matter are granular, and a loose, 

 mellow surface mulch is maintained without difficulty. Heavy 

 soils, low in organic matter, present the greatest difficulty, since 

 they are likely to be cloddy and deficient in granulation. To pro- 

 duce a good mulch in these soils by mechanical means alone is 

 almost impossible. Anything that encourages flocculation will 

 materially aid in the formation of mulches. 



The maintenance of a mulch is especially important in arid 

 and semi-arid sections where so much depends upon the conserva- 

 tion of moisture. Even with small grain the mulch is maintained 

 by means of a light spike-tooth harrow or weeder until the grain 

 by shading the soil prevents excessive evaporation. 



QUESTIONS 



1. What effect does compacting have? 



2. What form of tillage increases the moisture capacity of soils to the 



greatest degree? 



3. TTow does organic matter affect the water-holding capacity of soils? 



4. Explain the effect of deep rooting crops on water capacity of soils. 



5. How may water be removed by tillage? 



6. How may excessive percolation be overcome or prevented ? 



7. Explain how transpiration may be reduced. 



8. How much moisture is lost by interstitial evaporation and diffusion 



through the mulch ? 



9. Why is it impossible to have a perfect mulch under field conditions? 



10. How does the humid soil air of the mulch prevent evaporation? 



11. Define an artificial mulch. 



12. Give advantages and disadvantages of its use. 



13. What is a soil mulch? 



14. How is it effective in retaining moisture? 





