302 



SOILS. 



condition of moisture known to be most favorable to plants, 

 viz., one-half of the maximum water capacity, the influence of 

 the water-content upon the temperature will still be as great 

 as that of the entire soil mass. This consideration emphasizes 

 the importance of such control. 



Cold and Warm Rains. It is not surprising then that the 

 occurrence of cold or warm rains or the use of cold or warm 

 irrigation water at critical periods, may largely determine the 

 success or failure of the crop. It is well known that the oc- 

 currence of a cold rain after vegetation has started actively in 

 early spring, may not only destroy the season's fruit crop by 

 preventing the setting, or thereafter causing the dropping, of 

 the fruit, but may even, if the suppression of vegetative action 

 be continued for some length of time, result in serious injury 

 to, or death of trees. Widely extended disastrous experience 

 of the kind was had in California in February and March, 

 1887, resulting in the death of tens of thousands of fruit trees 

 and vines during that and the following season. It is obvious 

 that in such a case as this the rapid draining-off of the cold 

 water through underdrains would have materially mitigated, 

 if not wholly prevented, such injury. 



Solar Radiation. Aside, however, from such overwhelming 

 influences as the above, the soil temperatures are measurably 

 controlled by the extent to which they receive and absorb the 

 sun's heat rays, whether directly or through the mediation of 

 the air. The direct effect of the sun's rays upon the surface 

 is, upon the whole, the most generally potent, although warm 

 winds may occasionally exert a very strong influence. The 

 varying influence of the sun's rays depends primarily upon 

 the change of seasons, which themselves result from the vary- 

 ing angles at which the sun's rays strike the surface; as well 

 as upon the duration of the day. The greater or less cloud- 

 iness or fogginess of the sky, of course, exerts a decided effect 

 in this connection. 



TJic Penetration of the Sun's Heat into the Soil. In the 

 temperate regions of the earth the daily variations of temper- 

 ature cease to be felt at depths ranging from two to three feet, 

 according to the nature of the soil material and its more or less 

 compacted condition. The monthly variations, of course, 



