19 



SOIL-PLANT RELATIONSHIPS 

 AND PLANT NUTRITION 



A. G. Norman 



The soil environment. Although it has been estabHshed beyond all 

 doubt that higher plants can be successfully grown to maturity in the entire 

 absence of soil if the root system is immersed in a nutrient solution composed 

 of certain inorganic salts, the fact remains that soil is the normal root environ- 

 ment. Furthermore, the direction of evolutionary adaptation of plant roots 

 must have been toward accommodation to the nutritional and physical en- 

 vironment provided by soils. It would be unwise to assume that in water- 

 culture or nutrient-culture solutions higher plants necessarily find optimum 

 conditions for growth. However, the use of such solution cultures has been 

 of great value in determining which inorganic ions are essential for the welfare 

 of the plant, and in particular in ascertaining those needed only in minute 

 amounts. Solution-culture systems have been particularly helpful in establish- 

 ing and recognizing symptoms of deficiency or unbalance of particular nutri- 

 ents, because such symptoms can then be employed diagnostically on soils in 

 which these nutrients may be in poor supply or unavailable. Public interest 

 was aroused a decade or so ago in the practical use of soilless-culture systems 

 for the growth of flowers and vegetables. These went under a variety of names, 

 such as "nutriculture," "hydroponics," etc. Although there are circumstances 

 in which these may offer advantages over culture in soil, this usually arises 

 from some factor not directly related to the growth or fruitfulness of the plant. 



In addition to providing proof of the essentiality of particular nutrient ions, 

 solution-culture methods have made it certain that higher plants have no 

 requirement for organic substances for optimum growth, that they are not 

 directly dependent on the mineral particles that constitute much of the solid 

 phase of soils, and that micro-organisms, though ordinarily present in great 

 numbers on root surfaces, are not essential for growth. There is a danger, 

 however, in generalizing too broadly from such conclusions. One cannot safely 

 assume from such experiments that under natural conditions in soil soluble 



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