PREAMBLE TO CHAPTER 2
Chapter 1, P a r t 2, outlined some general considerations w h i c h flow from t h e growth of seed plants in soil-free culture media. Chapter 2 is focused u p o n t h e r e q u i r e m e n t s for specific elements, t h e symptoms incurred b y their deficient supply, a n d the evidence t h a t a given ele- m e n t is to be considered to be a n essential one. Although at first t h e criteria u p o n w h i c h these j u d g m e n t s w e r e based were, a n d often still are, visual, t h e t r e n d is n o w increasingly t o w a r d t h e additional use of metabolic criteria to characterize t h e plants w h i c h a r e g r o w n at dif- ferent levels of n u t r i e n t supply.
It will be evident from Chapter 2 t h a t still further discoveries on the role of m i n e r a l n u t r i e n t s a n d of the essential elements a r e fraught w i t h increasing difficulty; t h e y r e q u i r e ever m o r e fastidious control over the chemical e n v i r o n m e n t in w h i c h t h e plants a r e grown, a n d t h e y involve a n ever wider recognition of the interactions of n u t r i e n t elements w i t h each other a n d w i t h features of the e n v i r o n m e n t d u r i n g the growth of t h e plants. T h u s to comprehend all the possible effects of n u t r i e n t sup- ply, or deficiency, a n d to assess the specific effects of a n y or all of t h e prospective essential n u t r i e n t s a n d their interactions m a y r e q u i r e in- formation so detailed t h a t it m a y become t a n t a m o u n t to u n d e r s t a n d i n g growth a n d metabolism in all its aspects.
T h i s clearly raises t h e question, w h a t does n o r m a l n u t r i t i o n really m e a n ? H o w far is the m a x i m u m growth best determined b y the exo- genous n u t r i e n t supply, a n d h o w far is it preconditioned b y endogenous genetic factors? T h e s e considerations are necessary in t h e interpreta- tion of w h a t determines the maximum yield of agricultural plants.
T h e essential first step, however, is to k n o w w h i c h elements are neces- sary for t h e growth of plants a n d which, b y lack, cause abnormalities.
Chapter 2 furnishes this information in detail.
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