The Plant as a Metabolic Unit in the 

 Soil-Plant System 



By D. R. HOAGLAND* 



THE RELATION o£ the plant to the soil as a medium for 

 plant growth concerns the general botanist, the ecologist, 

 the agriculturist, the forester, and indeed all who seek 

 an understanding of plant growth, plant distribution, or the 

 artificial modification of the plant's nutrient environment. But 

 it is seldom that the soil-plant system is contemplated as a whole 

 in its physiological aspects. Too frequently the absorbing root 

 cells of the plant are regarded, apart from their water-absorbing 

 functions, as merely passive receptacles for the dissolved mineral 

 constituents of the soil. Chief emphasis is frequently placed on 

 the solubility of these constituents in various solvents, and the 

 quest for simple laboratory methods of evaluating soil conditions 

 is unceasing. Single factors, for example, hydrogen ion concen- 

 tration, are often assigned great weight in explanations of plant 

 distribution or adaptation to soil conditions. Many other pro- 

 cedures are employed, in which some special phase of physico- 

 chemical equilibrium in the soil system is broadly interpreted in 

 physiological terms. 



Such methods of investigation do frequently yield results of 

 local practical application, or occasionally useful generalizations, 

 yet they are essentially nonphysiological, and therefore inade- 

 quate, in that they fail to deal with the soil-plant system as one 

 profoundly influenced by the growth and metabolism of the 



• It is desired to acknowledge the participation of T. C. Broyer and other mem- 

 bers of the staff of the Division of Plant Nutrition, University of California, in 

 the experimental v^^ork cited herein. 



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