Hoagland —2— Plant Nutrition 



tical study a comparable degree of complexity. In- 

 deed, scientifically considered, this complexity occa- 

 sionally takes on an almost appalling aspect. 



The multiphase system which the investigator of 

 plant nutrition explores is that of the soil-plant-atmos- 

 phere with its innumerable interrelations and inter- 

 reactions. Inherent in the green plant itself are all 

 the complexities common to living organisms and to 

 these must be added the complexities of the soil me- 

 dium in which the plant is anchored and finds some 

 of the substances essential for its nourishment. The 

 soil is a natural body, the product of geological and 

 . weathering agencies. It is extremely variable from 

 place to place, also changing with depth, and is com- 

 posed of inorganic and organic matter of utmost 

 chemical and physical diversity, existing in many 

 states of subdivision, including those states of ex- 

 tremely fine division that confer the properties of 

 colloidal behavior. The soil is not a dead or static 

 system. It is the nourishing medium of remarkably 

 varied forms of plant and animal macro- and micro- 

 organisms, whose life activities continuously modify 

 the non-living part of the soil and reciprocally are 

 modified by it. Finally, the whole system of plant 

 and soil is subject to the influence of another con- 

 stellation of factors in the atmospheric environment: 

 light, temperature, humidity, rainfall, air movement, 

 as well as carbon dioxide and other gaseous com- 

 ponents of the atmosphere. Under natural conditions 

 these are usually uncontrolled or uncontrollable fac- 

 tors. 



To envisage further the general nature of the rela- 

 tion of the environment to the development of the 

 green plant, it is first necessary to recall the special 

 functions of organic synthesis characteristic of this 

 kind of plant. From the simplest chemical substances, 

 of which carbon dioxide from the low grade source in 

 the atmosphere, and water from the soil, are quanti- 

 tatively most important, organic compounds are ere- 



