144 Irrigation and Drainage 



The character of the mulch, also, has an important influence 

 on the amount of water which is permitted to escape through it. 

 Thus, it was found that when the same soil was covered to a depth 



Fig. 25. Difference in growth of corn where there is a difference of 

 3 per cent of soil moisture in the surface foot. 



of 2 inches with mulches of different kinds, the observed loss of 

 water per 100 days was as stated below : 



INCHES 



Through 2-inch mulch of coarse sand 



' ' black marsh soils 



fine clay loam 



dry peat 2 



clay loam, crumb-form 2.8 



From these results it is seen that a coarse-grained texture 

 produces a better mulch than one extremely fine ; that is, the loss 

 of water by evaporation through the coarsest sand was less rapid 

 than it was through the fine sand, and it was more rapid through 

 the finely powdered clay loam than it was through the same soil 

 left in the crumbled condition in which we usually find it when 

 the soil is in good tilth. The small loss from the peat mulch, too, 

 was due largely to the fact that it did not rub down to a ne 

 texture. 



Just why this law holds for soil mulches cannot now be stated, 

 except that it seems evident that the water is not lost by direct 

 evaporation at the surface of the damp soil, for in that case we 

 should expect the largest losses to take place from the mulches 

 having the most open structure, and the least when the diameter 



