>72 SOILS AND MANURES 



is noticeable in the amount of water evaporated from year 

 to year notwithstanding considerable variations in the rain- 

 fall. The lowest rainfall 20'49 inches was recorded 

 in 1898 and the highest 38'69 inches in 1903. The 

 latter is nearly double the former, the difference between 

 them being 18.2 inches, but the difference in the amount 

 evaporated is only about 2 inches. 



As the soil dries at the surface, water rises from below 

 by capillary action and the soil dries up to a consider- 

 able depth. Anything which interferes with capillary 

 action, therefore, must tend to check evaporation, and so 

 to conserve the moisture in the soil in periods of drought 

 when it is most required. The most obvious and one of the 

 most effective methods is to cover the soil over with a mulch 

 of straw or other material into which the water cannot rise 

 by capillarity. This is not always practicable, and it 

 interferes with cultivation. But light surface cultivation 

 itself hoeing, scuffling, etc. has a similar effect. It 

 breaks the capillary connection; the loose soil at the sur- 

 face becomes very dry and hot, but forms a protective 

 covering which, like the mulch, checks evaporation. Many 

 gardeners believe that, as a means of keeping the soil 

 moist, the Dutch hoe is better than the ^watering can. 

 Deeper cultivation promotes evaporation by opening up the 

 soil to the circulation of dry air which carries off the mois- 

 ture. Consolidation of the surface soil also tends to pro- 

 mote evaporation by increasing the capillary action. 



The loss of water by evaporation from soils under crop is 

 enormously greater than from bare soils. The latter alone 

 has, so far, been under consideration. The amount of 

 water absorbed from the soil by large trees is so great that 

 the ground underneath them is often quite bare, notwith- 

 standing the reduced evaporation from the surface due to 

 the shade they afford. A tree planted in a hedgerow 

 almost invariably ruins the hedge for some distance on 



