218 SOIL PHYSICS AND MANAGEMENT 



size of the particles and the pore spaces, but inversely as the total 

 pore space or the porosity. Since these factors depend upon the 

 size of the particles, the physical composition is the controlling 

 factor in percolation. If a fine-grained soil has 50 per cent of pore 

 space and a coarse-grained one has 33, it must follow that the pore 

 spaces in the former must be infinitely more numerous and smaller 

 than in the latter. If the average diameter of the particles of the 

 fine-grained soil is 0.01 mm. and of the other 1 mm., the number 

 of pores for equal areas will be approximately 10,000 times more 

 numerous in the fine than in the coarse, and consequently the resist- 

 ance to the movement of the water would be much greater in the 

 fine-grained soil or through the smaller pores. The nearer the par- 

 ticles approach uniformity in size the more favorable the conditions 

 for percolation. If various sized particles are present and of sim- 

 ilar shapes the smaller ones may tend to clog the interspaces between 

 the larger and may render the soil impervious. If the particles 

 are very irregular in shape, regardless of size, the permeability of 

 the soil may be increased. This is true of volcanic ash soils. The 

 structure of the soil is an important factor in percolation. 



2. Granulation. In the case of clays and other fine-grained 

 soils the cementing of the soil particles into granules aids percola- 

 tion. The large interspaces existing between the granules allow 

 free movement. Even in soils with considerable amounts of sand 

 percolation may be aided by granulation. Heavy soils devoid of 

 granules are almost absolutely impervious. Such soils are puddled. 

 They may be so naturally, or they may become so by some mechani- 

 cal operation, such as plowing or tramping of stock when wet. 

 This condition may be only temporary. 



Any substance that causes or aids granulation will increase per- 

 meability and consequently percolation. The application of lime, 

 chalk, marl, or limestone to clay soils is a well-known practice for 

 producing better tilth. Clay soils are readily permeable to water 

 only when their colloids are in a flocculated condition. 



3. Organic Matter. A very favorable effect is produced upon 

 the permeability of medium- and fine-grained soils by the incorpo- 

 ration of organic matter, but in coarse-grained, sandy soils the effect 

 of organic matter is to retard percolation, a thing very desirable in 

 such soils. In silt and clay soils the irregular fragments of unde- 

 composed parts of plants impart a porosity helpful to the downward 

 movement of water, while the humified material aids in the pro- 



