626 Philippine Journal of Science 1920 
including the so-called natural fertilizers, such as manures, soy- 
bean cake, etc. 
It is a well-established principle of plant physiology that the 
amount and character of growth made by a given plant species 
is greatly influenced by climatic conditions. (12) With other 
aerial conditions, the comparative results recorded in the 
present paper might have been very different. Consequently, 
adequate studies of the salt requirements of the rice plant must 
involve a consideration of climatic variations. Such studies 
will necessitate tests during the different seasons of the year, 
and will require a correlation of changes induced by alterations 
in the salt supply with fluctuations in the conditions of air 
temperature, sunshine intensity, and evaporating power of the 
air. 
The preceding discussion has dealt exclusively with the con- 
ditions of the environment; first, the conditions that are ef- 
fective around the roots and, second, those that are effective 
around the aerial parts of the plant. Besides these external 
conditions, it is well known that the internal conditions influence 
the amount and quality of plant growth; and, before general 
interpretations may be obtained regarding the salt nutrition of 
the rice plant, it will be necessary to employ many varieties 
of rice and to investigate the salt requirements during the 
various developmental stages of each of these varieties. The 
best salt proportions for vegetative growth of rice may be ex- 
pected to be different from the best proportions for the period 
of flowering and fruiting. 
It will be seen that the conditions in this system, composed 
of the plant and its environment, are exceedingly complex, and 
that the investigations here briefly outlined will require an 
enormous amount of work and time. The conditions surround- 
ing the roots may be considerably simplified and more easily 
controlled by employing solution cultures or sand cultures in- 
stead of soil cultures ; and it is probable that many of the lines 
of investigation here mentioned can be best approached by 
means of solution cultures and sand cultures, under carefully 
controlled laboratory conditions. 
As yet we must depend largely upon the method of trial for 
determining what fertilizer salts and proportions must be used 
upon a given type of soil for a certain kind of plant growth. 
Thus far, general principles that may be applied to this problem 
have not been obtained ; but a gradual advance in our knowledge 
of salt requirements may be expected to result from such trials. 
