PHYSICAL, CHEMICAL, BIOLOGICAL 53 



will absorb from two to four times its own weight of water. The sponge 

 best illustrates the capacity of organic matter to absorb and hold water. 



Water Movement in Soil. — The movement of water in the soil is 

 influenced chiefly by soil texture. In soils of coarse texture the water 

 moves very freely. Drainage is rapid and the soils dry soon after rains 

 so that tillage operations may soon be resumed. On such soils there is 

 generally little loss of time during the period when they need tillage. 

 On very heavy soils, that is, on those consisting chiefly of clay and silt 

 particles, the movement cf water within the body of the soil is exceedingly 

 slow. Drainage is difficult, and where the land is level and the sub- 

 stratum is dense, underdrainage is often required in order to make the 

 soils productive. In sandy soils the rainfall penetrates and descends 

 rapidly through the soil body. In this kind of soil leaching is rather 

 rapid. Rain penetrates heavy soils very slowly, and if the rainfall is rapid, 

 its passing from the surface of the soil causes severe erosion. Further- 

 more, a large proportion of the rainfall is thus lost and in no way benefits 

 the growing plants. On the part of the farmer it therefore becomes 

 essential so to plow and cultivate the fine-textured, heavy soil as to in- 

 crease its penetrability and facilitate the movement of air and water and 

 the penetration of roots as much as possible. In case of the very sandy 

 soil it is often advisable to do just the reverse. Applications of lime, 

 which tend to cement the particles together, and of organic matter to fill 

 up the interspaces, and compacting the soil by rolling to reduce the spaces, 

 are often resorted to. Where land has a high value it may even pay to add 

 clay to a sandy soil in order to improve its physical properties. On the 

 other hand, it may sometimes pay to add sand to a very heavy, clay soil. 

 Such practice, however, is justifiable only in case of land of high value 

 when used for intensive cropping. 



Absorption of Fertilizers. — The absorptive power of the soil is also 

 proportional to the surface area of the particles within a unit volume. 

 Soils of fine texture are, therefore, capable of absorbing and holding much 

 larger amounts of fertilizers than those that are sandy. This is very 

 important in connection with the application of fertilizers. It is also 

 true that the soil absorption is much stronger for some substances than it 

 is for others, and this will often determine the time of application of fertil- 

 izers. The absorptive power of the soil is less marked for nitrogen, either 

 as ammonia or nitrates, than it is for either potash or phosphorus. Con- 

 sequently, nitrogenous fertilizers should be used in quantities just suffi- 

 cient to meet the needs of the crop, and applied just preceding the time 

 at which the crop most needs it. In view of this fact, surface applications 

 of nitrogen are often effective, since the downward movement of the 

 material in the soil soon brings it into the region of root activity. 



Potash and phosphorus are, however, absorbed and held much more 

 tenaciously by the soil particles, and are not subject to severe loss by 

 leaching. Liberal applications of potash applied to the surface of the 



