DEPTH OF MULCHES 207 



be effective. One inch of sand will permit loss by diffu- 

 sion of less than three inches of water per year, under the 

 most favorable conditions. In practice, however, it is 

 found that two or three inches are usually most effective 

 because of capillary action. And Fortier has concluded 

 from experiments on irrigated soil in California that a 

 ten-inch mulch conserves more moisture than one of less 

 depth. But the efficiency of the ten-inch mulch as com- 

 pared with the four-inch is very much less in proportion 

 to depth, and the latter conserves 75 per cent of the 

 water lost where no mulch was used. Sand mulches may 

 be thinner than clay mulches. King found in Wisconsin 

 that, for corn, cultivation with a small toothed culti- 

 vator to a depth of three inches saved more moisture 

 in fifteen cases out of twenty than did more shallow 

 tillage, but that increase in depth resulted in no corre- 

 sponding increase in efficiency. The sweep or blade type 

 of cultivator (Fig. 137) may be used more shallow than 

 an implement producing ridges. The mulch should be no 

 deeper than is necessary to prevent loss of water, since 

 this top layer is usually most rich in available plant-food, 

 particularly nitrates, and the roots are excluded from 

 it by tillage. Unnecessary depth reduces the root range. 

 Some results from an experiment conducted at Cornell 

 University serve to illustrate the relation of mulches 

 and weeds to soil moisture and crop production in a 

 humid region in a season of good rainfall. The crop 

 grown was maize. Every third plot was a check, and 

 was given normal treatment. The figures show the in- 

 crease or decrease in yield as compared with the nearest 

 check plots. Moisture determinations were made on 



