624 
Philippine Journal of Science 
1920 
of the processes that take place in this system, and proper 
fertilizer practice must depend upon a knowledge of these 
principles. Although investigators have been working for a 
century on the general problem of the salt nutrition of plants, 
nevertheless, scarcely more than pioneer work has thus far 
been done in this field. For the proper interpretation of the 
mineral requirements of even a single plant species, an enormous 
amount of research would be required. The present study sug- 
gests a number of problems that need to be subjected to careful 
investigation, some of which are mentioned below. 
In the study reported in this paper only one total concentra- 
tion of the fertilizer salts was tested. Similar experiments 
should be conducted with a range of total concentrations, from 
very low values to values that are well above the optimum.* In 
this way it will be possible to determine approximately the 
optimum proportions of the fertilizer salts at the optimum total 
concentration.® Such studies should enable us to determine the 
very best total concentration and the best proportions of these 
fertilizer salts for the growth of rice plants under the general 
conditions of these experiments. Studies should be made ex- 
tending the sets of proportions to include single salts and mix- 
tures of two of the salts used in the present study; since, under 
certain conditions, just as satisfactory growth may be obtained 
when only one or two of these salts are used as when all three 
are employed. Furthermore, such studies must be made to in- 
clude many types of soils that are different from the one used 
in the present work. This is requisite because the amount and 
character of growth made by a given plant species is strongly 
influenced by the physical and chemical characteristics of the 
soil, and these characteristics influence the effect produced by 
fertilizer additions. Soil temperature, soil water content, and 
soil air content are likewise important. The studies must also 
include an investigation of the effects of different methods of 
cultivation, and must be extended to field tests. This phase of 
the problem will necessitate a study of the physical, chemical, 
and biological changes that take place in the soil as the result 
* It must of course be borne in mind that optimum conditions for one 
plant process are likely to be different from those that represent optimum 
conditions for other plant processes, and also that the optimum total con- 
centration for one set of salt proportions may be different from that for 
another set. 
’See McCall, (10) p. 214; also Breazeale,(l) Livingston, (7) Livingston 
and Hawkins, (8) Pulling and Livingston, (11) and Trelease.(16) 
