Mulching 109 



by actual low temperature. Both soil and plants are 

 expanded and forced upward, since water expands in 

 freezing. When the field thaws the soil settles back into 

 place, but the plants do not; their roots are torn loose, 

 and they may be left upon the surface or with a very in- 

 secure hold on the soil. If they are not killed outright, 

 the first severe drought finishes them. On clay soils, 

 unmulched plants have been lifted six to eight inches 

 during a single winter. Heaving is most serious upon 

 heavy soils, especially if they are flat and poorly drained ; 

 sandy soils and well drained slopes heave very little. 

 The more level and clayey the land, the greater the danger 

 from heaving. 



A mulch prevents heaving by preserving more equable 

 conditions of soil temperature, and by preventing rapid 

 thawing. Frozen plants that are thawed slowly, in the 

 shade, are less likely to be injured than those that are 

 thawed quickly, in the sun. The object of the mulch 

 is not to keep the plants from freezing at all, but to 

 prevent them from being affected by the frequent 

 changes of temperature. In the North, snow is na- 

 ture's mulch; if the ground is covered continuously 

 with snow until spring there is no heaving. The ne- 

 cessity for mulching increases as the permanence of the 

 snow blanket lessens. 



Prevents freezing. 



In the prairies of South Dakota, North Dakota, Minne- 

 sota and Manitoba, where the winters are very cold and 

 dry, with little snow, a winter mulch is necessary to prevent 

 the plants from being injured by low temperature. Even 

 a dormant plant transpires a small amount of moisture 

 every day during the winter. If the tissue of the plant 



