IX, A, 1 Cox and Argilelles: Soils of Luzon 5 



reduce the tenacity of clay. In clayey soils the presence of 

 humus establishes conditions favorable to aeration, bacterial 

 activities, increased permeability to water and circulation of 

 the soil moisture, and better tilth, and probably renders the 

 mineral plant nutriment more available. The addition of sand 

 to improve clays is uneconomical on account of the large amount 

 of sand necessary materially to change the texture. The ap- 

 plication of burnt clay has been found of occasional practical 

 value. The application of limestone or burnt lime has found 

 the widest application in correcting most clayey soils. Burnt 

 lime is uneconomical and undesirable as it tends to dissipate 

 the organic matter. Air-slaked lime, pulverized limestone, or 

 ground oyster shells which contain the lime largely in the form 

 of carbonate have been found least injurious to the organic 

 matter and more beneficial to the crops than any other form. 

 It is usually unnecessary to apply more than from 25 to 30 

 tons of limestone or from 10 to 15 tons of lime per hectare. 

 In some cases it is better to supplement this by an addition 

 of organic matter, in which case the amount of lime may be 

 decreased and new applications made at intervals, as indicated 

 by field observations. 



Lowering the temperature, which has also to do with the 

 colloids of clay, increases the plasticity of clays, and raising 

 the temperature decreases the plasticity. The effect is a very 

 slow one, but may explain what seems to be apparent — that it 

 is easier to cultivate a very clayey soil in a tropical country 

 than in a cold climate. 



The effects of plowing, fallowing, draining, rotation of crops, 

 and manuring are all associated in one way or another with 

 the conservation of the plant food and bacterial activity in the 

 soil; with changes in the organic matter, physical composition, 

 and water supply of the soil necessary to maintain productivity ; 

 and with the root development of the plants. The above con- 

 siderations have largely to do with the improvement of the 

 surface soil, but under certain circumstances heavy soils may 

 be effectively and permanently remedied by proper treatment 

 of the soil substratum. Cases occur where the physical texture 

 of the surface soil is satisfactory, but the subsoil is sufficiently 

 clayey to hinder proper drainage and to cause the water to 

 collect on top of the subsoil and rise to such height as to be 

 detrimental to the growing crop. Depending on the circum- 

 stances, drainage, subsoil plowing, or similar treatment is the 

 only recourse. The beneficial effect is usually immediate. 



The use which can be made of the soils of Luzon and the 



