IX, A, 1 Cox and Argiielles: Soils of Luzon 3 



The different sizes of sand and silt generally constitute the 

 bulk of a given soil mass, and the relative proportions of these 

 largely determine the cultural qualities. To be productive, a 

 soil must have sand, silt, clay, and humus in the proper relations 

 and conditions in both the surface and the substratum to provide 

 drainage with the given topography, aeration, movement, and* 

 distribution of the moisture and soluble constituents of the soil ; 

 and water-holding and -absorbing power, required by a given 

 crop and water supply; as well as the proper proportion of 

 the different essential inorganic constituents. These factors are 

 incapable of alteration as a whole, but it has been found practi- 

 cable to modify and adapt some of them to local conditions and 

 desirable crops. Generally, it is not difficult to increase or 

 diminish the water supply and the tenacity of the soil. Un- 

 satisfactory water-holding capacity and cohesiveness sometimes 

 have parallel causes and possible remedies. Both may be eco- 

 nomically increased in the case of some sandy soils by the admix- 

 ture of a small amount of clay, but only in case the clay forms 

 the subsoil or is near at hand. This has been practiced effec- 

 tively in some places in England. Humus, aside from being 

 a source of plant food, plays an important role in the physical 

 behavior of a soil. The humus substances are gelatinous when 

 moist, do not possess marked plasticity nor adhesiveness, but 

 have the power of retaining both gases and moisture in large 

 quantities. For the latter reason, another way to increase the 

 moisture-retaining power and the tenacity of sandy soils is to 

 increase the humus content. For limited areas barnyard ma- 

 nure may be used effectively to increase the content of organic 

 matter, but for large tracts it is more practicable and economical 

 to grow legumes and plow them under as green manure. The 

 coherence of sand, silt, lime, and humus is greatest when they 

 hold sufficient water to fill all interstices between their particles, 

 and as they dry the coherence decreases. On the other hand, 

 clay increases in_ coherence as it dries, and at last becomes a 

 hard solid mass.* Therefore, it is essential in the control of 

 the water-holding capacity and coherence of a soil that the con- 

 stituents be properly proportioned. The chemical composition 

 of a soil materially influences its physical properties, but other 

 factors being equal clay probably has the greatest individual 

 effect upon the mechanical structure in that the maintenance of 

 floccules and tilth depends thereupon. The heaviness of a soil is 



' Warington, Physical Properties of Soils. London (1900), 25. 



