THE I KB 1G ATI ON AGE. 



175 



of water in the soil is BO much greater than is required 

 to supply that which is held by surface tension that the 

 remaining space is filled the soil is said to be saturated. 

 If we provide an outlet for the water the surplus will 

 pass off by force of gravity, leaving only the films which 

 are held by surface tension and which furnish the de- 

 sired moisture to plants. Thus from 15 to 20 per cent 

 of all the water which a soil will hold will not pass off 

 as drainage, but will remain as capillary water to con- 

 tribute to the growth of plants, and to aid further in 

 the preparation of additional plant food. This neces- 

 sary moisture moves through the soil independently of 

 gravity by the force of capillary attraction or surface 

 tension as illustrated in the rise of liquids in small 

 tubes and between surfaces of solids which are close to- 

 gether which tends to distribute and equalize moisture 

 in the soil. Where the principal supply is above or in 

 the surface layers, it is drawn downward; where it is 

 below it is drawn upward. 



As before stated, about 50 per cent of the volume 

 of ordinary soils is space which is always filled with 

 water or air. The individual spaces are larger or small- 

 er according as the soil grains are more or less minute. 

 A close clay soil and a very coarse sandy poil will illus- 

 trate the extremes of difference. The fine grains pre- 

 sent more surface in a given volume of soil and hence 

 will retain the greater quantity of moisture. The 

 coarser soils will permit a much freer percolation of 

 water and hence quicker drainage than the finer ones, 

 since the closeness of the particles offers an additional 

 resistance to the passage of water by gravity through 

 the soil. 



Another mechanical condition of soils has more to 

 do with their drainage properties than the differences 

 already noted. It is the massing of particles of different 

 character to form compound soil grains which lie con- 

 tiguous and have spaces between them. Those who have 

 examined the physical structure of soils minutely will 



FIG. 3. Soil grains 

 and spaces. 



FIG. '. Soil particles 

 in masses. 



FIG. 5. Joint clay 

 structure. 



have observed the granular structure of soils containing 

 a mixture of humus and clay. Some subsoils are com- 

 monly known as "joint clays" from the fact that they 

 show natural cleavage or fractures which mark them 

 as soils easily drained. Others of a marly, sandy, or 

 gravelly nature mass their particles into irregular forms 

 which can not lie close together, and are known as light 

 subsoils. Still other clays appear to be wanting in the 

 characteristics named, their individual particles lying 

 compactly together and forming a dense mass capable 

 of retaining a larger percentage of water than any other 

 variety, and are commonly known as retentive or imper- 

 vious soils. Some of these mechanical characteristics 

 are illustrated in Figs. 3, 4 and 5. 



The varieties of soils and the numberless combina- 

 tions which their particles assume in such a way as to 

 materially affect their drainage properties can not be 

 described here. Indeed, the structure of a given soil 



*From Bulletin 65, Minnesota Experiment Station. 



can not be known until a personal examination and test 

 have been made. The reading of soils for drainage op- 

 erations requires a skill which can not be imparted with- 

 out special field lessons; yet any close observer may 

 soon acquire a fair degree of proficiency in judging 

 soils within the limits of the area with which he is con- 

 cerned. 



NATURAL AND ARTIFICIAL DRAINAGE. 



Some of our best soils have perfect natural drain- 

 age. Thev are underlaid with strata of material which 

 give free passage to surplus water and are composed 

 of elements which respond readily to the efforts of the 

 cultivator. On the other hand, there are soils just as 

 rich in natural fertility which are unproductive because 

 under all ordinary circumstances they contain too much 

 water. Whether the excess of soil water is caused by 

 rainfall direct or by seepage from lands which occupy 

 a higher level, the surplus water must be removed be- 

 fore the soil will be in proper condition for plant 

 growth. 



The process of drainage, either natural or artificial,, 

 only regulates the quantity of water in the soil by pro- 

 viding the means by which its surplus may pass off by 

 gravity, and in no case removes the moisture required 

 by plants, since such moisture is retained by the par- 

 ticles of soil. 



SURFACE DRAINAGE AND UNDERDRAINAGE. 



Surface drainage, as commonly understood, is ac- 

 complished by open ditches which, in addition to receiv- 

 ing and removing water from the surface of land con- 

 tiguous to them, may, if sufficiently deep, act as re- 

 ceiving drains for water which percolates through a 

 porous substratum through which the ditches are ex- 

 cavated, and under such conditions facilitate under- 

 drainage as well as carry off water from the surface. 

 The advantages of removing water downward through 

 the soil instead of over the surface may be briefly stated 

 as follows : 



The surface soil is retained entire instead of the 

 finest and most fertile parts being carried off with every 

 considerable rainfall. 



Any plant food in manure or other fertilizer de- 

 posited upon the soil is carried into it with the water 

 as it percolates downward from the surface, and so be- 

 comes thoroughly incorporated with the soil. 



Eain water as it passes through the soil serves a 

 most useful purpose by dissolving and preparing crude 

 soil material for the nutrition of plants. 



The soil, having been well prepared, is at all times 

 during the growing season in readiness for the growth 

 of plants, such growth not being hindered by stagnant 

 water or saturation. 



The fros't goes out earlier in the spring, so that 

 the planting season opens one or two weeks earlier than 

 in the case of soils affected by surface drains only. 



Where stiff clays are found the soil is made more 

 porous, open and friable, and roots penetrate more 

 deeply than they do into surface-drained soils. 



The effects of drought are diminished, as has been 

 found by experience, owing to the enlarged and deep- 

 ened soil bed and to the more favorable condition of the 

 surface for preventing excessive evaporation of mois- 

 ture. 



It aids in making new soil out of the unprepared 

 elements, since it permits a freer entrance of air and 

 atmospheric heat which disintegrate soil material 

 hitherto unavailable for use of plants. 



