4 The Philippine Journal of Science wu 



greatly influenced by the colloidality of the clay, and depends 

 largely on the total amount of the constituents finer than 0.05 

 millimeter average diameter; it is usually difficult to work soils 

 containing over 75 per cent of these, even though the loss on 

 ignition is high, and still more difficult the greater the proportion 

 of clay becomes. A large percentage of silt and a small per- 

 centage of clay is the easiest to work of any fine soil. A little too 

 much water in a soil of this type makes it very sticky, and 

 the percentage of water may be so high as to make it impossible 

 to work it. If it could be plowed, its condition would be even 

 worse, for it would turn over, dry out, and harden in chunks 

 without being pulverized. There is an optimum percentage of 

 water in any soil when it works best, but especially in such 

 a soil, and the only plow that will scour well in the latter is 

 one with both mold board and share of the right shape and 

 made of the finest soft center highly polished steel. When 

 such a soil dries out, a very strong plow and a great deal of 

 power are necessary to plow it, and even then it is very hard 

 to put the ground in tillable shape owing to the hard lumps. 

 Colloidal clay renders a soil comparatively impervious to water, 

 and imparts to it a maximum plasticity. Up to a certain point, 

 there is coincidence of plasticity and bonding power, and several 

 of the other physical properties stand in close relation to the 

 former, so that tillage of a veiy plastic clay soil is almost im- 

 possible. On the other hand, the same soil becomes pervious 

 to water, and a good tilth may be maintained when the clay 

 is coagulated. These facts are of great importance in actual 

 tillage operations, for excessive tenacity prevents proper root 

 development and fosters the retention of an excessive amount 

 of moisture in the rainy season and the formation of a hard 

 compact mass in the dry season. 

 Roland ' gives the following : 



The ways of decreasing the plasticity, as: First, by the addition of 

 hydroxyl ions, lime water being the cheapest reagent for this purpose. 

 If the concentration of the hydroxyl ions of the lime water is too low 

 for some clays, it may be increased by the addition of sodium hydroxide 

 solution or any strong base combined with a weak acid. The latter class 

 is represented by phosphates, silicates, etc. 



This method of coagulating colloids has found wide applica- 

 tion in practical farming. The influence of clay in a clayey 

 soil may be modified and the physical composition changed by 

 the addition of humus, sand, burnt clay, and lime, all of which 



'Sprechsaal. Coburg (1906), 42, 1371. 



