SOIL IMPROVEMENT 159 



air and water can enter the soil. Darwin believed that the 

 common earthworms brought soil up from their burrows and 

 deposited it on the surface in such large quantities that side- 

 walks have been buried. 



The degree of acidity may influence the texture of the 

 soil, but acidity is more commonly considered in relation to 

 the requirements of various plants. It is spoken of as the 

 hydrogen (H) ion or pH of the soil and refers to the propor- 

 tional number of free H-ions (which are responsible for the 

 acid reaction). For general purposes we might make three 

 groups of plants: those requiring strongly acid soil (azalea) ; 

 those requiring nearly neutral soil (legumes); and the third 

 group, between the extremes, but usually less exacting, with 

 a slightly acid requirement (grass and most garden plants) . 



Soil can be readily tested for these three degrees of 

 acidity by inexpensive devices to indicate acidity by color. 

 With some study and experience it is quite possible to judge 

 the acidity by the kinds of plants that grow best; for example 

 where moss and sheep sorrel grow in the lawn, lime should be 

 applied. Iron is insoluble in an alkaline soil, which may 

 restrict the plant's absorption of that element. Liming an 

 acid soil may set free a certain amount of potassium, ex- 

 changing it for the calcium, so that, liming may supply the 

 37lant indirectly with potassium as well as calcium. 



Soil texture might include the dust mulch on the surface. 

 The value of any mulch (dust, straw, or sand) is largely the 

 retardation of evaporation and retention of a loose soil sur- 

 face, as an aid to absorption of air and rainfall. A good 

 mulch will prevent at least half the water loss from the soil 

 by direct evaporation, which is often enough to save a crop. 

 A dust mulch is valuable because it breaks the films of capil- 

 lary water which tend to move to the soil surface. The more 

 nearly all the films are broken the better the mulch. Shallow 

 frequent cultivation both maintains a good mulch and de- 

 stroys weeds. A black paper has been put on the market to 



