12 SOILS 



As soon as the dropping has ceased, weigh the whole ; the differ- 

 ence in weight will approximately show the proportion of water 

 held by the soil. 



Capillary action of soils. The power of soils for drawing 

 up- water from the lower strata towards the surface is of the 

 greatest importance from the cultivator's point of view. It is 

 possessed in a greater or less degree by all soils, being greater in 

 proportion as their pores are hner. Sand possesses this power to 

 the least extent, and fine clay to the greatest; thus a clay soil is 

 always wetter than sanely soil placed under the same influences. 

 There is, however, a limit to the fineness cf particles which 

 favours capillarity, and when this limit is passed, fineness is dis- 

 advantageous, since the minute particles ot earth are then apt to 

 cohere together so closely that few, if any, spaces are left between 

 them for the admission of water. Therefore too open a tilth 

 is not desirable, especially for light soils ; hence the necessity for 

 rolling such soils after ploughing and harrowing- From the sub- 

 soil, moisture is gradually and steadily sucked up and transmitted 

 to the upper cultivated surface. When the surface soil is left undis- 

 turbed and becomes hard, much of its moisture passes off by eva- 

 poration. One of the chief objects of tillage or good cultivation is 

 to cut off the column conducting the underground water at a point 

 below the actual surface of the soil, so that the supply of moisture 

 shall be brought to the roots of the plants instead of escaping freely 

 into the open air. A mulch or any dry covering that can be 

 placed between the atmosphere and the soil also has the effect of 

 checking the evaporation of moisture. When soil is nearly dust 

 dry, nitrification ceases ; thus soil-stirring operations in dry weather 

 not only preserve the soil moisture, but keep the nitrifying 

 organisms employed. The process of soil capillarity may be illus- 

 trated by taking a lump of loaf-sugar to represent the hard-caked 

 soil. Hold one end of the lump so that it touches the surface of a 

 cup of water, when instantly the whole lump is moistened ; then 

 take about the same quantity of granulated sugar (equalling in 

 height the thickness of the loaf sugar), and hold on a screen so 

 that the lower surface of the sugar just touches the water ; in this 

 it will be found that the water rises very slowly, and not until 

 some of the sugar is dissolved will it reach the top. KING calculated 

 that 6'24 tons of water a day evaporated from 1 acre unstirred 

 .soil, while only 4'52 tons evaporated when the surface was raked 

 or harrowed. 



