Hoagland — 4 — Plant Nutrition 



influence of biological catalysts, the enzymes, which 

 are themselves large and exceedingly complex protein 

 molecules. 



The study of plants growing under a natural or 

 agricultural environment of necessity must encounter 

 all the factors of the system, soil-plant-atmosphere, of 

 which I have spoken. Viewing the field as a whole, 

 the attack on problems of plant nutrition assumes wide 

 dissimilarity of method. At one time, the appropriate 

 tool may be a spade or a soil auger; at another time 

 a highly specialized and refined tool of physics or 

 chemistry may be required. For some purposes crude 

 measurements in terms of pounds or ounces may 

 suffice; for other purposes, a thousandth of a milli- 

 gram is of consequence, or even much less than that, 

 in the determination of the effects of certain plant 

 hormones in relation to nutritional or physiological 

 processes of the plant. 



Problems of the Soil Solution in Relation to Plant 

 Nutrition : — I shall not carry further these generali- 

 zations but at once embark on my discussion of specific 

 problems of plant nutrition. I wish to begin with 

 the early researches, in a period now about twenty- 

 five years past, of the laboratory with which I am 

 connected. Some justification for this procedure may 

 be offered, in that it will afford an opportunity to in- 

 dicate the underlying reasons and the stimulus for 

 undertaking much of the experimental work to be 

 described in the subsequent lectures. At the same 

 time some of the general trends of modem develop- 

 ments in the investigation of plant nutrition in certain 

 of its aspects can be brought to your attention, and 

 perhaps placed in a useful perspective. 



In a period shortly preceding that of which I have 

 spoken, several investigators had issued a series of 

 publications in which were set forth several far-reach- 

 ing conclusions concerning the nutrient interrelations 

 of soil and plant which appeared opposed to prevail- 



