40 EGYPTIAN AGRICULTURE. 



bad conductors of air and water on account of the fineness 

 of the channels between the particles, and hence a "porous" 

 soil may be nearly impervious to water. Again, from the 

 examples in the table given above, it may be noticed that 

 a porous soil has often a small water capacity. The 

 " Tenacity " of a soil is due to the cohesive attraction 

 between the surfaces of the soil particles, and accordingly 

 clay soils, which possess small particles and therefore 

 much internal surface, show great tenacity, whereas coarse 

 sand show very little. The tenacity varies with the 

 amount of water in the soil, and thus wet sand is slightly 

 tenacious, dry sand not at all. Further, the tenacity of 

 a soil is partly due to the presence of colloid or gluey 

 matter. This sticky matter is chiefly found in the finest 

 clay particles, but humus has also a binding effect on mineral 

 particles. In the table given above it will be noticed that 

 to break a column of the Facus soil of 1 sq. cm. section 

 requires a force equal to the weight of 546 grams. Tena- 

 cious soils are called " heavy," a term having no relation 

 to their weight but referring to the difficulty experienced 

 in their cultivation. Besides being difficult to cultivate, 

 tenacious soils are poor croppers as the soil is badly drained 

 and badly ventilated, and the growth of the plant roots is 

 thereby restricted. When the soil is just moist, neither 

 wet nor dry, the tenacity is reduced and cultivation is 

 possible. Lime and certain soluble salts coagulate and 

 flocculate the colloid clay, thereby decreasing its tenacity 

 and increasing "its permeability. 



