76 



SOILS AND FERTILIZERS 



when weeds are removed by scraping the surface of the 

 ground as when the soil mulch is maintained. It seems 

 possible that the latter result may occur only in those 

 regions in which conditions are such that a natural mulch is 

 formed by the rapid drying of the surface soil, in which 

 process moisture is removed so quickly that the capillary 

 column is broken and further loss of moisture is stopped. 

 This would confine it to semi-arid and arid regions of high 

 summer temperatures. 



The failure of the soil mulch to conserve moisture in corn 

 land has been explained on the supposition that the corn 

 roots ramifying through the upper soil absorb so much 

 water that they cut off the upward movement as effectually 

 as does a mulch. The results of some experiments in 

 semi-arid Montana indicate a high degree of usefulness for 

 the mulch. 



Table 16. — Moisture Content op Mulched and Unmulched 

 Eastern Montana Soils. Average of Three Years 



The investigator comments on these results as follows : 

 " If the wilting point of this soil is 6 percent, the mulched 

 area contains more than twice as much available moisture. 

 This 3.8 percent of available moisture by which the mulched 

 soil excels the unmulched is equivalent in a five-foot depth 

 to about 250 tons of water, enough to increase the crop by 

 a ton of dry matter." 



