IX, A. 1 Cox a7id Argiielles: Soils of Luzon 13 



pulverized. Quartz is practically inert as a source of plant 

 food, and barrenness is commonly associated with sandy land. 

 This is not necessarily true in arid regions where kaolinization 

 and disintegration take place very slowly. In such cases the 

 larger grains may contain potentially available plant food, but 

 in the case of Philippine soils under tropical conditions there 

 has been rapid decomposition of the particles of sand and gravel 

 and the detritus on a 1-millimeter sieve is usually very small, 

 and we believe the portion passing a sieve of this size contains 

 practically all the constituents from which the plant derives 

 its food and includes all that should be termed "fine earth" 

 or soil.*^ 



From the following analyses of the chlorhydric soil extracts 

 one can deduce the requirements as to mineral plant food of all 

 the soils we have investigated. Soils that show a very low 

 percentage of any one element necessary to plant nutrition will 

 yield a low crop production, if not at once, at least within a 

 few years after cultivation has begun unless remedied by 

 supplying that element in the form of fertilizer. On the other 

 hand, any essential element shown by analysis to be present in 

 abundant amount, especially in virgin soils, may generally be 

 assumed to be the last to become deficient in the course of crop 

 production. 



THE PHYSICAL ANALYSIS 



The physical analyses were made with a Schone apparatus 

 according to a method outlined by one of us.*^ The air-dried 

 soils were disintegrated by shaking in water, and special effort 

 was made to secure complete disintegration of the aggregates, 

 without which there is no constant means of comparison, before 

 they were separated into the individual fractions." 



Hilgard ^* has proposed to determine the mechanical struc- 

 tures of soils by photographing the various fractions in glass 

 tubes of uniform bore, and adds that a series of such tubes 

 would describe a curve of the soil composition. We have carried 

 on analyses to test the accuracy of this method. It is very 

 difficult to photograph glass tubes because they reflect light, and 



" If there were a large detritus on a 1-millimeter sieve, the results with 

 respect to certain constituents might appear high and would not fairly 

 represent the composition of the soil. 



''Cox, This Journal, Sec. A (1911), 6, 313. 



"^ Oven-dried soils are apt to form hard aggregates which cannot be 

 disintegrated by shaking, and the percentages of the coarser grains may 

 be greatly increased. 



" Loc. cit., Soils, 94. 



