Mulches to Conserve Moisture 145 



of the pore spaces is smallest, but which observation proves not 

 to be true. The only explanation which now occurs to the writer 

 for the law is, that even in the air-dry condition of soil, the film 

 of moisture still investing the soil grains, although so extremely 

 thin, is subject to the same disturbance by evaporation at the 

 exposed surface that it is when that film is much thicker, as in the 

 case of soils containing the right amount of moisture for plant 

 growth, and when evaporation from the surface takes place 

 rapidly. 



Earth mulches lose in effectiveness with age. When a good 

 earth mulch has been developed, it does not remain equally effec- 

 tive for an indefinite period, even if no rain falls upon it. This is 

 particularly true early in the season, when the amount of soil 

 moisture is high, and when it tends to creep into the lower part 

 of the mulch, saturating it and causing the open texture to 

 disappear by breaking down the crumb structure, and thus restor- 

 ing the original and normal capillary power. A soil mulch devel- 

 oped to a depth of two or three inches thus grows gradually 

 thinner with age by reverting to the original condition. This be- 

 ing true, it is necessary, when the greatest protection is desired, 

 to repeat the stirring of the soil as often as observation shows that 

 its effectiveness has been impaired. 



Mulches that are not made from soil. By far the largest part 

 of the protection offered against the loss of water by surface 

 evaporation from the soil is and must be furnished by mulches 

 developed from the soil itself. But it should be understood that 

 all vegetation growing upon the surface of a field, whether it 

 completely covers the ground or not, exerts a protective influence, 

 tending to diminish the loss of water from the surface of the 

 ground. This protection comes partly from shading the ground, 

 partly from a reduction of the wind velocity close to the surface, 

 and partly from the tendency of vegetation, by the transpiration 

 from its foliage, to saturate the air with moisture, and so reduce 

 the rate of evaporation which otherwise would be possible. 



Even in pastures where the grass is short, if it is only close 

 and completely covers the ground with its foliage, the mulching 

 influence is marked. Hence, in order to get the largest returns 



