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MULCHES AND PLASTIC SOIL COVERS AFFECT SOIL MOISTURE, 

 SOIL TEMPERATURE AND PLANT GROWTH 



Martin Weeks 

 Department of Agronomy 

 and 

 William J. Lord 



Mulches include any protective material or covering that rests on the 

 soil. They have been used for centuries by farmers, gardeners and fruit 

 growers to protect plants from drought and weed competition and to prevent 

 rapid changes in temperature from injuring root systems. Mulching materials 

 in these early times have included dust, gravel, a variety of organic materials 

 such as straw, old hay, manure or leaves. It has always been difficult to ob- 

 tain sufficient quantities of the natural materials that are free of weeds or 

 other objectionable constituents. For this reason, in recent years paper and 

 plastic mulching materials have been produced together with machinery that 

 aids in applying them uniformly to the soil. 



Mulches are used mostly in accordance with rule-of-thumb experience either 

 of the grower himself or of his neighbors. Since some of these mulches and 

 plastic covers do have a pronounced effect on soil climate the principles 

 governing their effects should be of interest. 



Porous mulches permit more-or-less evaporation of soil moisture depend- 

 ing on the material. Whether they conserve any moisture may depend on the 

 length of time between rains and the degree to which they prevent runoff. Also, 

 their effect on transpiration plays some part in conservation. In a controlled 

 experiment, workers at the Iowa Agricultural Experiment Station found recently 

 that unmulched soil evaporated 1.25 to 5 times more water than mulched soils 

 in a given time. In this work a gravel mulch was most effective, a corn cob 

 mulch less so and a dust mulch least effective in retarding evaporation from 

 the soil surface. Orchardists in some areas have traditionally used a straw, 

 hay or manure mulch in combination with sod cover. One effect of this practice 

 was to supply nitrogen and some potassium to the trees. It has been especially 

 beneficial at times on shallow soils. One disadvantage is the fact they form 

 conditions favorable for mice. 



During the last two or three years, grower interest has developed concern- 

 ing the use of plastic as a means of weed control under non-bearing trees. The 

 question has been asked as to the effect of plastic on soil temperature and 

 moisture. 



Waggoner, £t al. (Conn. Agr. Exp. Sta. Bui. 63A) have made an extensive 

 study of plastic mulching: its principles and benefits. Their results show 

 that in the spring, black plastic has little ability to warm the soil. On the 

 other hand, clear plastic has great ability to warm the soil, and their data 

 show that the depth of soil thawing on March 17, 1958 was double that of under 

 black plastic. Aluminum film and hay mulch both retarded the warming of soil. 



The data obtained by Waggoner, et al. show that during the summer months 

 soil temperatures were 4 to 10 degrees higher under clear plastic than under 

 a hay mulch. Black plastic sheets had little, if any, effect on soil temperature, 



