io8 THE MOVEMENTS OF SOIL WATER [chap. 



penetrates with difficulty and largely runs off to the 

 ditches. Not only is more water collected, but the 

 exposure of the plough slice to the weather, to the 

 pulverising action of freezings and thawings, wettings 

 and dryings, tends to flocculate the clay particles on 

 heavy soils, so that later they are easily worked down 

 to a fine tilth. This weathering action has even some 

 effect on the chemical constituents of the soil ; the potash 

 compounds in particular are found to become more 

 soluble after a winter's exposure of the soil to frost 

 and rain. The autumn ploughing is particularly valuable 

 on really heavy soils ; they are rarely dry enough during 

 the winter or early spring to be fit for cultivation ; 

 in the process they become somewhat tempered and 

 smeared by the plough, kneaded also by the treading 

 of horses and man, so that they would dry into tough 

 and difficult clods were they not afterwards exposed to a 

 succession of spring frosts to bring them into condition 

 again. One cultivation of a clay soil in a wet state in 

 the spring may easily spoil its working and therefore its 

 productivity for the rest of the year, because there are 

 no longer any frosts to restore the tilth, whereas a 

 mistake in the autumn becomes repaired by the action 

 of the later frosts. 



Assuming that the land has been ploughed in the 

 autumn, it is then desirable to plough once more in 

 the spring as soon as the land will carry horses safely — 

 the object of this second ploughing being to conserve 

 the main supply of water in the subsoil, and also to 

 enable the layer on the surface to become dry and 

 warm so as to be ready for the formation of a seed 

 bed. After the winter rains the plough slices will have 

 become somewhat closely beaten down and joined up 

 once more into a compact mass with the earth forming 

 the subsoil below ; thus the film of water on the particles 



