WATER-HOLDING POWER OF SOILS 87 



which breaks up the compound particles, pulverizes the clods, 

 and breaks up the solid or packed masses, will allow the soil 

 to hold more moisture. The modern deep-tillage machines which 

 stir the soil to a depth of 12 or perhaps 24 inches are among the 

 most successful machines for the deepening of the "water reservoir." 

 Heavy clay soils become very compact below the level of ordinary 

 plowing. The use of subsoil plows will loosen the soil and subsoil 

 to a greater depth and if done when the soil is not too wet will 

 deepen the root bed and otherwise increase the moisture capacity 

 of the field. 



2. Humus in soils increases their water-holding capacity. We 

 have seen that clay soils will hold much more capillary moisture 

 than sandy soils. A rich, black soil containing an abundance of 

 well-rotted organic matter will hold four, or five times as much 

 moisture as a heavy clay soil, if both are in well-drained vessels 

 where the free water may fall away. The amount of humus in 

 any soil is largely within the control of the owner. It may be 

 readily increased by plowing under barnyard manure, straw, vines, 

 weeds and other farm wastes, green manure and special crops 

 grown for the purpose. The best systems of farming are 

 those in which suitable rotations are established whereby certain 

 crops are grown during the rotation to be plowed under for this 

 purpose. Sods, resulting from the growth of grasses for hay or 

 pasture, will produce much humus in the soil when plowed under. 



3. Underdrainage has its effect upon the amount of water 

 available to plants. If the free water table be near the surface 

 roots can feed to only a shallow depth. If the water table be 

 lowered by underdrainage the soil space above the deep water 

 table will be suitable for the growth of roots. The amount of 

 capillary water which the roots may obtain is thus greatly in- 

 creased. If by any natural or artificial means the water table is 

 lowered to a very great depth the free water will not readily re- 

 plenish the capillary moisture as fast as it is used up by roots. 



4. In the soils of arid regions perhaps no free water table can 

 be found at certain seasons of the year. The soil a few feet below 

 the surface may be drier than that above it. The problem of in- 

 creasing the capacity of the soil in such cases is chiefly that men- 

 tioned in paragraphs 1 and 2; but if the soil be very open and 

 porous the moisture during the rainy season will penetrate to 

 great depths and be lost from use of the crops which are soon to 

 be planted. If such loose soils are firmly packed by the use of 



