LOSS BY EVAPORATION 195 



their reach. Therefore, on soils subject to excessive 

 leaching, water may be conserved by use of the roller 

 or other compacting implement, and by such manage- 

 ment as permits the deep subsoil to become more dense. 

 94. Evaporation. The second form of soil-moisture 

 loss is by surface evaporation. It has been shown that, 

 in the process of growth, a large volume of water is 

 evaporated directly from the tissues of the plants. In 

 this process it performs useful functions. But a large 

 amount of water is also lost by direct evaporation from 

 the surface of the soil. If the plants which evaporate 

 the soil water are those of the desired crop, the loss is 

 proper and not to be avoided. But it frequently happens 

 that, either before the regular crop is on the land or 

 mixed with it, are large numbers of worthless plants 

 through which this same moisture loss occurs. This is 

 of course a waste of moisture, and is to be avoided by 

 preventing their growth. It may happen in the spring 

 that the late plowing of land bearing a heavy growth 

 of vegetation permits so great a loss in this way that, 

 unless the subsequent season is one of abundant rain- 

 fall, the regular crop may suffer from the lack of moisture 

 which was stored in the soil, and by timely plowing and 

 preparation could have readily been utilized. In this 

 connection, it should be kept in mind that green manure 

 crops may be directly injurious the first season if they 

 are permitted to grow so late before being turned under 

 as to unduly deplete the soil moisture. In the manage- 

 ment of green manure crops, that optimum point when 

 the excess of water due to heavy spring rain and winter 

 snow has been removed, but the capillary supply not 



