EXPERIMENT STATION BULLETINS. 



649 



perature was below that of the uncultivated sand. The moisture content 

 of these series of sands was determined at the beginning and at the 

 end of the experiment and it was found to be as follows: Initial percent 

 moisture of both sands 8% ; at end of experiment, cultivated sand 1.4%, 

 uncultivated sand 1.30%. The cultivated sand would show a greater 

 moisture content if the dry mulch had been excluded from both series of 

 sands. 



The dry mulch does not only prevent the soil from attaining a high 

 temperature during the day but also prevents the soil from attaining a 

 low temperature during the night. Its effect here is threefold: (1) it 

 acts as a blanket and greatly reduces the rapid loss of heat; (2) it 

 reduces the loss of heat by cutting down the radiation, as it has already 

 been shown; and (3) it tends to conserve moisture in the lower strata 

 and thereby increases the specific heat of the soil and consequently re- 

 duces the rate of cooling. It is partly on account of these effects, there- 

 fore, that the cultivated soil seems to be accumulating more heat during 

 the early spring than the uncultivated soil, and to cool less rapidly 

 during a severe cold weather. 



The effectiveness of the dry mulch to conserve moisture is shown in 

 the following table. This table contains the percent moisture content 

 of all the different plots as determined during the different periods. 



TABLE 76.— PERCENT MOISTUREIOF THE DIFFERENT PLOTS. 



Date. 



April 4 



July 27 



September 9 

 November 4. 



Unculti- 

 vated T 



21.52 

 11.53 

 14.56 

 13.81 



Culti- 

 vated 7* 



23.61 

 13.85 

 15.47 

 13.82 



Sod 7' 



17.37 

 12.02 

 10.83 

 13.97 



That the better heat conducting power of the uncultivated or compact 

 soil over the cultivated or loose soil is not the greatest factor which 

 causes the difference in temperature between these soils is further proved 

 by the fact that the temperature begins to rise at the depth of 7 inches 

 in the cultivated soil only about half an hour later, as has already been 

 shown, than it does in the uncultivated soil. This is a strong evidence, 

 that the loose soil below the dry mulch is not so poor a heat propagator 

 as is commonly believed, and that it is not the main cause of the differ- 

 ence in temperature between these differently managed soils. 



Summarizing the foregoing data, then, it has been seen that the un- 

 cultivated soil does not always have a higher temperature than the cul- 

 tivated soil, during the warm part of the year, but only at the warmest 

 part of this period and especially when the temperature of its lower 

 depths is much higher thau that of the corresponding depths on the cul- 

 tivated soil, and that the factors which bring about the difference in 

 temperature on these soils at the upper depths are (1) the dry mulch 

 on the cultivated soil, (2) different rates of evaporation, (3) the differ- 

 ent rate of heat propagation and (4) the higher temperature at the 

 lower depths of the uncultivated over the cultivated soil. 



