THE- CONTROL OF SOIL MOISTUBE 273 



of the essential elements, and bacterial activity is en- 

 couraged. As a general rule, more soluble plant-food is 

 likely to be found under a mulched soil, other conditions 

 being equal, than under a soil not so treated. 



191. Kinds of mulches. — Mulches are of two general 

 sorts, artificial and natural. In the former case, foreign 

 material is merely spread over the soil surface and evapora- 

 tion is obstructed thereby. Manure, straw, leaves, and 

 the like, may be used successfully. Such mulches, while 

 very effective, are not generally applicable to field crops 

 where intertillage is practiced, since they would make 

 cultivation absolutely impossible by cumbering the soil 

 surface with a large amount of inert material. Their 

 use is therefore limited to intensive crops such as are 

 found in trucking operations. Leaves, including pine 

 needles, and sawdust are very effective as a mulch, but 

 some precautions should be observed in their application. 

 For example, the oak is rich in tannic acid, which may 

 be washed out of the mulch into the soil and by its effect 

 on the growing plant may cause a lowering of productivity. 

 In some European countries, as well as in a few localities 

 in America, stones have been drawn on the soil to serve 

 as a mulch, particularly in orchard and vineyard culture, 

 with markedly beneficial effects. Particularly is this 

 true on such lands as are too steep to permit cultivation. 

 As further evidence of the utility of this practice, it has 

 been observed in the fruit-growing section of the Ozark 

 Mountains, and doubtless in other regions, that the 

 removal of stones from the land not only results in the 

 soil's becoming harder, but also reduces crop yield by 

 increasing loss of moisture. It is therefore necessary 

 for the farmer to decide whether the inconvenience 

 to tillage or other operations due to the presence of 



