Plants as Affected by Insufficient Water. 139 



232. Crumbling of the Surface Soil (cultivation) tends 

 to Prevent Drought, since it greatly lessens the points of 

 contact in the soil particles, and thus interferes with the 

 rise of the soil water by capillary attraction to the sur- 

 face where evaporation chiefly occurs. An air- dry sur- 

 face layer of crumbled soil also tends to prevent evapor- 

 ation by keeping the soil cooler beneath. A puddled 

 crust on the surface of the soil, as is formed by rain on 

 soils containing clay, tends, on the other hand, to restore 

 capillary action and thus to promote evaporation. Some 

 gardeners cultivate their hoed crops as soon as possible 

 after rains for the main purpose of breaking this crust 

 and thus stopping the capillary action. 



Cultivation is also beneficial by aerating the soil (94). 

 The roots of plants should never be forgotten nor 

 ignored in cultivating crops (110). 



233. Mulching tends to Prevent Drought by interposing 

 a layer of poor- conducting material between the ground 

 and the sun's rays. This keeps the surface soil cooler 

 and so checks evaporation. 



The best mulching material is the one that conducts 

 both heat and moisture slowest. Straw, marsh hay, 

 leaves, manure, shavings, sawdust, spent tan and sand 

 are all useful for mulching, but the first four named are 

 generally preferable to the others, especially if free from 

 weed seeds. 



Growing plants tend to dry the soil because the root- 

 hairs continually draw in soil water and force it into the 

 leaves (102) where it passes, off by transpiration (75). 

 Weeds, therefore, rob crops of moisture (336). 



234. Irrigation, i. e., the extensive watering of out- 

 door plants, is the final remedy for drought. It is neces- 



