IX, A, 1 Cox and Arguelles: Soils of Luzon H 



In general, the concern of the chemist with reference to plant 

 food supply is the phosphorus, potassium, and nitrogen content, 

 for, if the percentage of any one of these existing In the soil 

 falls below that necessary to yield a nutrient solution of the 

 concentration demanded during the period of most active as- 

 similation, the productive capacity of the soil may be ques- 

 tioned.^* The quantity of organic matter and calcium present 

 are also of special interest in that they affect the availability 

 of plant nutrients, the bacterial activity, and the physical text- 

 ure of the soil. 



Two aspects to be determined by chemical soil analysis are 

 clearly distinguishable: First, the permanent productive value, 

 the prevention of undue drain by crops, and the regulation of 

 the necessary elements of plant food by the addition of fertilizers, 

 which is of vital importance in rational agriculture; secondly, 

 the immediate producing capacity, which is chiefly concerned 

 with immediate returns. The determination of these two fac- 

 tors is entirely different, even though the results and their 

 causes may usually be intimately correlated. 



The soil is the result of the degradation and disintegration 

 of rocks. Some of the inorganic plant food is still in the mineral 

 condition in which it was originally derived from the parent 

 rock. The degree of disintegration usually determines the 

 degree of availability. The inorganic plant food elements may 

 be classified with reference to whether they are (1) soluble in 

 water, (2) in acid, or (3) can be dissolved only by alkali- 

 carbonate fusion. 



(1) The water-soluble plant food elements constitute the read- 

 ily available portion, but an excess in this form would be dis- 

 advantageous, as the greater part would be lost by being leached 

 out and washed away by heavy rains. An excess of some soluble 

 salts is a great detriment, as shown by the poor growth of plants 

 on saline or alkali lands. 



'" This percentage is usually assumed as 0.1. It is evident that this is 

 influenced by the physical and chemical conditions. The circulation of an 

 abundant supply and subsequent evaporation of w^ater increases the con- 

 centration of the soil solution so that the requisite percentage is probably 

 less when the physical and chemical conditions are favorable. It un- 

 questionably varies with a great many interdependent factors which cannot 

 be determined without the employment of methods based on physical 

 chemistry. 



When the naturally accumulated concentration of nutrient solution is 

 too low, it can be efficiently increased artificially by the addition of an 

 easily soluble supply. 



