iv.] DRAINAGE 95 



and deep worm tracks, are all brought into the soil by its 

 aeration, and result in more rapid percolation. Again, 

 the washing through the soil of soluble salts derived 

 from the surface, especially the bicarbonate of lime 

 which is so characteristic a constituent of drainage 

 water, also induces flocculation of the fine clay particles. 

 Lastly, there is a steady removal by the drains of the 

 finest clay stuff, for whenever tile drains are running 

 freely the water will be found slightly turbid with clay 

 matter. All these causes contribute to establish a better 

 texture in the drained soil, beginning at the tiles and 

 spreading slowly outwards. The other result of drainage 

 which may be noted here is the greatly increased warmth 

 and earliness of a drained soil ; the high specific heat of 

 water, and the cooling produced by evaporation when 

 the water table is near the surface, combine to hinder 

 a water-logged soil from warming up under the sun's 

 heat in the spring, so that undrained land is notoriously 

 cold and late. 



Effect of Autumn Cultivation upon the Water Content 



of the Soil. 



In regions where the annual rainfall is not very high 

 and occurs chiefly during the early winter months, it is 

 important to get as much of it as possible into the soil 

 for the use of the subsequent crop. Breaking up the 

 stubbles after harvest is an important factor in catching 

 the winter rain ; all land which is to lie idle through the 

 winter, previous to the sowing of roots or spring corn 

 should be early turned over with the plough and left 

 rough through the rainy season. On the old stubble 

 which has been made solid by the weather and the 

 trampling during harvest, the rain lies for some time and 

 evaporates, and if the land be at all on a slope the water 

 shoots off into the ditches. But the broken surface of a 



