28 tllE CtiEMlSTRY OF THE F^tlM 



Water thus absorbed is too firmly held to be of use 

 to crops, but in hot climates the evaporation of this 

 hygroscopic water during the day lowers the tempera- 

 ture of the soil, and saves the crop from scorching 

 (Hilgard). Soils may condense considerable amounts 

 of water from the air when the temperature of the 

 surface falls below the dew point. 



A characteristically dry soil is one of coarse, non- 

 porous particles, and loose texture — a gravel or coarse 

 sand for example. Such a soil holds little water, and 

 evaporates it freely. If the subsoil is one of free drain- 

 age the evil is at its worst. With dry soils the farmer 

 should aim at increasing the amount of humus ; crops 

 should be sown early, and the land kept clean and 

 solid ; very shallow summer cultivation should be 

 resorted to. Such land has a few distinct advantages. 

 It furnishes the earliest crops to market gardeners, the 

 soil being easily warmed. A little rain will also wet 

 it to a considerable depth, and the whole of the water 

 it contains is available for plants. 



A soil is seldom too wet because it has too great a 

 power of holding water when drained, the mischief is 

 generally owing to want of drainage ; the cure is there- 

 fore to be found in deep tillage and draining. Clay 

 burning, applications of lime and chalk, or an increase 

 ia the proportion of humus, may, in special cases, be 

 effective means for rendering the surface soil more 

 pervious to water. 



The wettest soil does not always supply the largest 

 amount of water to a crop. A peaty soil holds most 

 water, but much is not available to plants, being com- 

 bined with colloid matter. A stiff clay fails in drought, 

 the water being firmly held and moving with difficulty. 



