Soils and fertilizers . large, andparticularly where it falls in torrential showers, more thanhalf of the precipitation may escape in this way. Thewater so removed is, of course, entirely lost so far as itsutiUzation by plants is concerned. The proportion lost byrun-off is greater on slopes than on level land, and on com-pact soil than on sandy soil or on soil in good tilth. The removal of excess water by means of open ditches is,to some extent, a utilization of run-off to drain land, butit is not so desirable a method as tile drainage. It is better \ U \^I^--^-^k ^- • ^ ^jJL ^^ \f ^ffl^ ?\ KL
Soils and fertilizers . large, andparticularly where it falls in torrential showers, more thanhalf of the precipitation may escape in this way. Thewater so removed is, of course, entirely lost so far as itsutiUzation by plants is concerned. The proportion lost byrun-off is greater on slopes than on level land, and on com-pact soil than on sandy soil or on soil in good tilth. The removal of excess water by means of open ditches is,to some extent, a utilization of run-off to drain land, butit is not so desirable a method as tile drainage. It is better \ U \^I^--^-^k ^- • ^ ^jJL ^^ \f ^ffl^ ?\ KL-Ji^i^^mm {B^^^S^ iWn^n^B^Kt ^H^HBeiBr^^^ I^B^^^^^*^ ^^^BS^^^BA /.^^JllMWMl ISP* -^v^ ^m^^^^ v^S^I^H^I ^SnKtfjT 1^*^ ,...,? - 1 ^^?^;;«^ *->f? ? ?????.-. :^ ?*- .r-t^te^ ^ixsy^^ ^fe^^ls^ ,-m^-:^K,.:-:m B^^wswJRI^^vS ^^TS^^^^m^~f^ w^^B^BR mK^^EV^^^u^HE3m^S mi^|^K|MyJ^^B^^tf^^^ IBiSEaMiifflBiilgBiilB ?•«^;«)(^ ?? V ;5l?^J;.:\H;. , , 1 .. // ,?[. ,?..;:.™S-f. Plate VIII. Forms of Erosion. — Erosion of soil by water in upperfigure. Erosion by wind in lower. SOIL WATER 73 to have the moisture pass into the soil and this is encouragedby any of the operations and conditions that favor the main-tenance of good tilth. Fall plowing and early spring plow-ing also serve this end. In arid and semi-arid regions run-off is usually not of any moment. Terracing a hillside isoften resorted to as a preventive of run-off, especially inthe south Atlantic states where the rainfall is often tor-rential. 85. Percolation. — Water that enters a soil is eitherretained by the capillary spaces or eventually percolatesinto the subsoil. The percolate is lost to crops, except thatpart which remains in the subsoil and is later raised bycapillarity to within reach of roots. The chief considerationis to maintain the soil in good tilth, which gives a largecapillary capacity, thus storing within easy reach of theroots a maximum quantity of the descending water. Themore
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