SOIL WATER 117 



perature one degree as would be required by an equal weight of soil. An 

 excess of water in the soil, therefore, greatly lessens its rate of warming. 

 In wet soils much evaporation of water takes place at the surface. It 

 requires more than five times as much heat to transform one pound of 

 water from liquid to vapor as it does to raise the temperature of an equal 

 weight of water from the freezing to the boiling point. In other words, 

 the heat consumed in the process of evaporation is sufficient to cause a 

 change of 900 degrees in temperature in an equal volume of water. This 

 fact emphasizes the importance of removing surplus water by means of 

 drainage, instead of allowing it to evaporate from the surface of the soil. 

 An amount of evaporation sufficient to maintain a proper soil tempera- 

 ture in prolonged heat periods may be desirable, but excessive evaporation 

 is undesirable in temperate latitudes, especially during the early grow- 

 ing season. Reduced temperature as the resul ; of such evaporation often 

 causes disaster during the seeding cr planting seascn and retards the 

 early growth of crops. 



Water Storage Capacity of Soils. Since the rains of summer are 

 rarely fully adequate to meet the needs of growing plants, it is essential 

 to increase the storage capacity of the soil as far as possible. For this 

 purpose, the chief agencies are plowing, methods of tillage and the use 

 of organic manures. Deep plowing and the incorporation of organic 

 matter to the full depth of plowing will increase very materially the 

 capacity of the soil for water. In conjunction with this, the soil should 

 be so cultivated that it will receive the rainfall and thus have an oppor- 

 tunity for holding it. This means the maintenance of a porous surface 

 so that rainfall will not escape over the surface until the soil has become 

 filled with water. 



Those crops endowed with the power of deep-root penetration, such 

 as alfalfa, can draw their moisture from greater depths in the soil than 

 shallow-rooted crops. In regions of low rainfall this amounts to the 

 same thing as increasing the storage capacity of the surface portion of 

 the soil. 



Moisture Conservation. The practical conservation of soil moisture 

 is effected chiefly by preventing direct evaporation from the surface of 

 the soil, and also by exterminating all foreign plants in the nature of 

 weeds that tend to rob the crops of their moisture supply. Evaporation 

 is most economically reduced to the minimum by surface tillage and the 

 establishment of an earth mulch. The earth mulch to the depth of two 

 or three inches is formed by periodic cultivation or a stirring of the surface 

 of the soil so as to break the capillary action with the soil immediately 

 beneath. The efficiency of such mulches depends largely on the perfec- 

 tion with which they are made. A surface mulch to be effective should 

 consist of rather finely pulverized loose soil. This becomes dry to such 

 an extent that the soil moisture film is discontinuous and water ceases to 

 rise to the immediate surface. In this condition, any loss that takes place 



