264 SOILS. 



course, greatly diminish and sometimes suppress evaporation 

 from the soil-surface ; thus very nearly fulfilling the same con- 

 ditions referred to above (chap. 7, page in) in discussing 

 the effect of natural vegetation in rendering tillage unneces- 

 sary ; the beating of rains, and the formation of surface crusts, 

 being alike prevented. This fact is of essential importance in 

 contributing to the welfare of crops sown broadcast, where 

 subsequent cultivation is impracticable. 



Weeds Waste Moisture. The injurious effects of weedy 

 growth among culture plants are in most cases due quite as 

 much to the appropriation of moisture that should have gone 

 to the crop, as to the abstraction of plant-food, to which the 

 injury is generally attributed. This is much more obvious in 

 the arid region, where during the dry summers every pound of 

 moisture counts, than where summer rains obscure this in- 

 fluence. It has led orchardists in California almost to an ex- 

 cess of clean culture, resulting in the burning-out of the humus 

 from the bare surface-soil during the long, hot summers, and 

 an injurious compacting impossible to remedy by the most 

 careful tillage. It thus happens that greenmanuring, the 

 natural remedy for this evil, cannot safely be done there with 

 summer crops, but must be accomplished with winter crops, 

 such as can be turned under before the dry season begins. The 

 same objection holds against the growing of summer crops 

 between the orchard-rows. 



DISTRIBUTION OF MOISTURE IN THE SOIL AS AFFECTED BY 

 VEGETATION. 



The investigations of Wollny and others have long shown 

 quantitatively what common experience has taught the farmer, 

 viz., that a field in crops or grass is always drier within the soil- 

 mass penetrated by the roots than is a cultivated field bare of 

 crops, unless perhaps when heavily crusted on the surface. The 

 depletion of moisture caused by grass sward is the most easily 

 observed because of the shallowness of the root-system ; and 

 this is one cause at least why grass sward does not occur natur- 

 ally in the arid region, and when planted cannot be maintained 

 without irrigation repeated at short intervals. Deeper-rooted 

 plants of course deplete the soil at different and varying levels ; 



