A Universal Method for Preparing Nutrient Solutions of a Certain Desired Composition

8/6/2019 A Universal Method for Preparing Nutrient Solutions of a Certain Desired Composition

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Pla nt and Soil XV, no. 2 Oct obe r 1961

A UNIVERSAL METHOD FOR PREPARINGNUTRIENT SOLUTIONS OF A CERTAIN

DESIRED COMPOSITION

b y A B R A M A . S T E I N E RHorticultural Experiment Station, Naaldwijk, Netherlands

INTRODUCTIONW ate r cu l tu res , o r o the r way s o f so il -les s p lan t cu l tu re , e.g. sand

cu l tu res , a re f requ en t ly used fo r phys io log ica l re sea rch . Refe renceto the l i t e ra tu re however , r evea ls a cons ide rab le d ive rs i ty in thecompos i t ion o f the nu t r ien t so lu tions used . He w i t t 2 g ives over ah u n d r e d f o rm u l ae , a n d S c h r o p p 4 a b o u t s i xt y, e x c h d i n g t h o s epresc r ibed fo r induc ing de f ic iency sym ptom s . T he d eve lop men tof prac t ica l so i l - less cul ture techniques before the las t war gaver i se to a ve r i tab le ava lanche o f fo rmulae . Up to the p resen t t ime ,a t o t a l o f a b o u t t h r e e h u n d r e d f o r m u la e h a v e b e e n p u b l i sh e d , m a n yo f w h ic h h a v e b e e n p a r t i c u l a r ly r e c o m m e n d e d f o r t h e g r o w th o fpa r t icu la r c rops , e.g. tomatoes , ca rna t ions , roses etc.

Phys io log ica l exper imen ts on nu t r i t ion have been made to in -v e s t i g a t e , a m o n g o th e r t h in g s , t h e i n f lu e n c e o f a w id e r a n g e o f

n u t r i e n t - s o lu t i o n c o m p o s it i on o n t h e d e v e lo p m e n t a n d b e h a v io u ro f p lan ts . F or exper im en ts o f th i s k ind , knowledge o f the p rob lem sunder ly ing the p repara t ion o f the des i red nu t r ien t so lu t ions i s af i r s t p re requ is i te s ince the ac tua l compos i t ion m ay no t a lwaysc o r r e s p o n d to t h a t g iv e n b y t h e f o r m u la . F o r e x a m p le , t h e r e a r emany so lu t ions , the fo rmulae fo r wh ich ind ica te the p resence o fr a u c h m o r e p h o s p h a te , s u lp h a t e , c a l ciu m , a n d m a g n e s iu m th a n c a nbe de te rm ined by ana lys i s ; thus , f rom tw o so lu t ions which , ac -co rd ing to the i r fo rmulae , shou ld bo th con ta in say , 2 mg ions o fphospha te pe r l i t r e , 1 .8 mg ions may be recovered f rom one , andon ly 0 .2 mg ions f rom the o the r . Th is i s due to the Iac t tha t the

- - 13 4 - -


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PREPARATI ON OF NUTRIENT SOLUTIONS 135

g re a t m a j o r i t y o f r e c o m me n d e d fo rmu l a e a r e n o t r e al iz a b le i nso lu t ion because the so lub i l i t y p roduc t s o f ce r t a in ion combina t ionsa re exceeded . The in t e rp re t a t ion o f exper im en t s i nvo lv ing suchcases wou ld the re fo re , be se r ious ly a f fec t ed . The ma in ob jec t o fthe p resen t i nves t iga t ion i s t o f ind a so lu t ion to these p rob lems .

Al though p resen t ing no d i rec t p rob lems per se, i t mus t a l so ben o t e d t h a t v e ry d i v e rg en t fo rmu l a e m a y p ro d u c e n u t r i e n t s o l u ti o n sof ident ica l comp osi t ion , e.g. 2 mmo l Ca (NO3) 2 and 4 m mo l KH 2 P O4g ive exac t ly the same concen t ra t ions o f K+ (4me) , Ca ++ (4me) ,NO3 ' (4 me) and H2 P0 4 ' (4 me) a s 4 mm ol KN 03 and 2 mm olCa(H2PO4)2: th is i s c lear ly seen when the ions are considere dind iv idua l ly .

J u d g i n g f ro m t h e i r p u b l ic a t i o n s a n d s t a t e m e n t s , i t w o u l d s e e mtha t many inves t iga to rs have g rown c rops in a so lu t ion chosenla rge ly a t random, and tha t , where the re su l t s have been success fu l ,t hey have recommended tha t so lu t ion a s be ing spec i f i c fo r t hepa r t i cu l a r c rop conce rned . Compar i sons be tween d i f fe ren t so lu t ionsas to the i r e ffec t on crop growth ha-~e , in fac t , been made only to al imi ted extent , whi l s t whe re th ey oecur~ p rec ip i ta tes rende r thec o r re spond ing fo rm ulae inva l id a s a gu ide to ac tua l comp os i t i on :th i s l a t t e r app l i e s t o many eXper imen t s i n wh ich compar i sons haveb e e n ma d e b e t w e e n d if f e r e n t f o rmu l ae .3 Fu r t h e rm o re , t h e o b s e r-va t io n th a t a c rop g rows we l l i n a pa r t i cu l a r nu t r i en t so lu t ion doesn o t p ro v e t h a t t h e r e i s n o t a mo re f a v o u ra b l e c o mp o s i t i o n fo r t h a terop .

Since the p lant i t se l f excerc ises cer ta in se lec t ive propert ies , theques t ion may be ra i sed a s t o t he ex ten t t o wh ich the compos i t i ono f t h e n u t r i e n t s o l ut i o n is i m p o r t a n t f o r g ro w t h a n d d e v e l o p me n t .How ever , be fo re pu rsu ing th i s po in t fu r the r , i t is necessa ry to ex -a mi n e m o re c l os e ly w h a t i s me a n t b y " c h e mi c a l c o mp o s i t i o n " .Th i s t e rm compr i ses (1) t he conce n t ra t ions o f t he comp onen t i ons ,(2) the to ta l ion ic conce nt ra t ion , and (3) pH . E xpe rim ent s wi thf lowing cul tures , in which fresh nut r ien t so lu t ion i s suppl ied con-t inuous ly to p l an t s , have shown tha t i nd iv idua l i ons a re t akenup e ren a t ve r y low coneen t ra t ions , bu t i t i s p rob ab le tha t fo reve ry ion , t he re is a ce r t a in m in imu m concen t ra t ion be low whichup take i s no longer poss ib l e a t t he requ i red ra t e . A t t he o the r ex -t r e me o f c o n c e n tr a t io n , l u x u r y c o n s u mp t i o n m a y l e a d t o i n t e rn a ltox ic i ty . The po in t a t i s sue he re i s whe the r t he re i s an op t imum


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136 ABRAM A. STEINE R

concentration of certain ions in solutiort for a certain crop grownunder given environmental conditions, or whether their relativeproportions and not absolute concentrations are the determiningfactors ?

Let us assume that absolute concentrations are decisive and thatin a particular experiment, optimum concentrations have beenfound corresponding to a me K+, b me Ca++ and c me Mg++ perlitre. This gives the rat io of K : Ca : Mg as a : b : C. However,this ratio may also be expressed as a/n b/n c/n, provided the valueof a + b + c is given; thus, the o pt imum composition can be ex-pressed in terms of both a sum and a ratio. The point is that evenif the absolute concentrations are considered to be determiningfactors, the relative proportions of the different ions can also beconsidered in the same way, if the total ionic concentration isincluded.

From the author's own investigations along these lines on car-nations, tomatoes, and gerberas, it appeared that the relative pro-portions of the anions were imp ort ant only within rather wide limitsbut tha t the relative proportions of the cations were very important.The total ion concerttration is important within the limits 4- 0.2atmospheres osmotic pressure. In many cases, the pH of the nu-trient solution was important within the limits 4- 0.2 pH units.

In the author's opinion, it is essential tha t in physiological re-search involving water cultures, the influence of the compositionof the nutrient solution be investigated systematically. Irr orderto do this, every possiNe combination of the variables, anionratio, cation ratio, total ionic concentration, and pH must beconsidered.

Superficially, the preparation of nutrient solutions satisfyingcertain requirements would appear to be straightforward. Thishowever is by no means the case; the pH of the solution, a veryimportant growth factor, introduces serious complications. Forexample, let us assume that a solution of a certain desired compo-sition has a pH of 5, hut that we require a pH of 6.5. This can beachieved by adding an alkali such as KOH. However, a great demof KO H m ay be required, especially if the solution contains a largeamount of phosphate. At pH 5, practically all the phosphate ispresent as H2P04-, as shown by the phosophate dissociation curve 1in Figure 1: in order to obtain a pH of 6.5, 22 per cent of this


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PRE PA RA T I O N O F N U T RI E N T SO L U T I O N S 137H 2 P0 4 - t a u s t b e c o n v e r t e d i n to H P0 4 - - , a c c o rd i n g t o th e d is so c i-at i on sequ ence , H,PO4~~-H2PO4-~~-HPO4-~~-~P04- . The largeam oun t o f K O H req u i red fo r t h i s wou ld the re fo re, se r ious ly upse tthe re l a t ive p ropor t ions o f t he ca t ions. T he answ er wou ld be to

H 3 P O 4

~ 7O

~I'

" 2 0

I0

0 'I 2

H 2 P O 4 - H P 0 4 - - P 0 4 - - -

3 4 5 6 7 8 9 tO il 12 13 14pH

Fig. 1. Phosphate dissociation curve.ad jus t t he pH by t i t ra t ing wi th a lka l i con ta in ing a l l ca t ions in t hesam e re l a t ive p ropor t ion s a s in t he o r ig ina l so lu t io n ; i n th i s casehowever , t he appr opr i a t e mix tu re o f K O H + Mg(OH)2 + Ca(OH)2wi ll 0n ly y ie ld a suff ic ien t ly d isso cia ted so lu t ion of Ca++ an d Mg ++a t ex t remely low concen t ra t ion .

As an a l t e rna t ive , t he des i red so lu t ion cou ld be p repa red wi th ap H o f o v e r 7 b y s u p p ly i n' g t h e p h o s p h a t e a s PO 4 - - - a n d t h e n t i-t ra t ing to the requ i red p H wi th a mix tu re o f a ll an ions in t he samere l a t ive p ropo r t ions a s befo re . The d i f f i cu l ty w i t h th is p rocedure ,a s wi ll app ea r l a t e r , i s t ha t a t p H 7 , on ly a l imi t ed nu m ber o f i oncombinations a re poss ib l e w i thou t p roduc ing a p rec ip i t a t e (e.g.of CaH P04 ) a t an osm ot i c p ressu re o f 0 .7 a tm. S ince the o p t im umfor many c rops t aus t su re ly no t be be low 0 .7 a tm, t h i s me thod canon ly be used to a l imi t ed ex ten t . T i t ra t ion wi th a so lu tion a l read y


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138 ABRAM A. STE IN ER

c o n t a i n i n g p h o s p h a t e p r e c i p i t a te s w i l l o b v i o u s l y y i e l d v e r y u n -r e l i a b l e r e s u l t s b e c a u s e o f t h e l o w d i s so lu t i on r a t e s o f t h e s e p h o s -p h a t e s .

T h e a l m o f t h is p a p e r is t o d e t e r m i n e h o w a p a rt ic u l a r n u t r i e n ts o l u t i o n c a n b e p r e p a r e d w h i c h s a t i s f i e s g i v e n r e q u i r e m e n t s a s t o( 1) r e l a t i v e c a t i o n r a t i o s , (2 ) r e l a t i v e a n i o n r a t l o s , ( 3) t o t a l i o n i cc o n c e n t r a t i o n , a n d (4 ) p H . H o w e v e r , b e f o r e t h i s p r o b l e m i s c o n -s i d e r e d f u r t h e r , i t w a s t h o u g h t d e s i r a b l e t o i n v e s t i g a t e h o w t h e s er e q u i r e m e n t s a r e m e t i n m a n u r i a l t r ia l s o n so i ls a n d t o w h a t e x t e n t ,t h e p o s i t i o n c a n b e ' i m p r o v e d i n t h e s e G a se s.

COMPARATIVE MANURIAL TRIALS ON SOLLS

I f o n e w i s h e s t o i n v e st i ga t e, f o r e x a m p l e , t h e e f fe c t 0 5 a n a d d i t i o n o f p o -t a s s i u m t o t h e s o ll o n t h e d e v e l o p m e n t o f a p a r ti c ul a r c r op , i t is c l ea r t h a tt h i s c a n n o t b e r e al i se d w i t h o u t i n t r o d u c i n g o t h e r f a ct o rs , e . g. e h lo r id e o rs u lp h at e . P o t a s s i u m , a t le as t, m u s t b e a d d e d a s a sa lt . A l t h o u g h o n e is i n-elined to attribute a resulting resp onse to the cation in question, in this casep o t a s s i u m , it m a y b e t h a t t h e e h l o r id e o r t h e s u l p h a t e i s a l s o i n vo l ve d , b e -sides the increase in the total ionic concentration. Be ca us e of the hi gh buf-fering capa eit y of the soil, ch an ge s in pI-I do not usuall y cau se trouble.

It is ho we ve r, possible to avoi d these difficnlties. n e ca n investigate thei n fl u en e e o f p o t a s s i u m w i t h r e sp e ct t o a n o t h e r c a t i o n, i. e. b y w o r k i n g w i t hr e la t iv e q u an t it i es ; a s a l r e a d y s u g g e s t e d i n t h e i n t r o d u c t i o n t o t h i s p a p e r ,t h is is j u st i fi e d v e n t h o u g h o n l y " a b s o l u t e qu a nt i ti e s" a r e e v a l u a t e d . T h u s ,i n o n e c a se , K 2 S O 4 m a y b e u s e d a n d in. t h e ot he r, a n e q u iv a l en t a m o n n t o fN a ~ S O a ; v ar ia ti on c a n b e a c h i e v e d b y s u p p l y i n g t h e s a m e t ot al q u a n t i t yo f s u l p h a t e b u t d i s tr ib ut ed t o d if fe re nt e x t e n t s b e t w e e n K ~ S O 4 a n d N a 2 S O 4 .In tliis ay botll the effects f different levels of s ulph ate an d of differentt o ta l i o n i c c o n c e n t r a t i o n s c a n b e e l i m i n a t e d . T h e l a tt e r, o f c o ur s e, is o n l yp o ss i bl e c o m p l e t e l y , if s a lt s e o n t a i n i n g c a t io n s o f t h e s a m e v a l e n e y a r e u s e d ,if i n o n e c as e, 2 g r a m e q u i v a l e n t s o f S O 4 " a r e s u p p l i e d a s M g S 0 4 , a n d i n t h eo t h e r a s K 2 S O 4 , t h e f o r m e r y ie ld s 2 g r a m i o n s a n d t h e l at te r 3 g r a m i o n s ,co nse que nt ly resulting in a difference in total ionie concentration, th ou ght o a sm a l l er e x t e n t t h a n w i t h e o n v e n t i 0 n a l m e t h o d s .

C u r io u s l y e n o u g h , t h e i m p o r t a n c e o f t h e a b o v e d o e s n o t a p p e a r t o h a v eb e e n g e ne r al l y a p p r e c i a t e d to a n y r e a l e x te n t. T a k e f o r e x a m p l e , e x pe r i-m e n t s i n v o l v i ng K : N r at lo s. I f t h e s e a r e m a d e u s i n g d if fe re nt a m o u n t so f K 2 S O 4 a n d C a ( N O 3 ) 2 , t h ei r i n te r pr e ta t io n i s n o t a t all s t r a i g h t f o r w a r d ;al th ou gh onl y the K : N ratio is usual ly considered, there are actually fourf ac to rs to b e t a k e n i nt o a c c o u n t ~ K + : N O a - , K + : C a + + C a + + : S O 4 - -a n d I K O a - : S O 4 - - . T h i s e o m p l i c a t i o n m a y b e e l i m i n a t e d b y a p p l y i n g n it ro -"g e n a s t h e a m m o n i u m i o n. T h u s , f o r t h e p u r p o s e s of e o m p a r i s on , t h e s a m et ot al a m o u n t o f I K 2 S O 4 + ( N H 4 ) 2 S O 4 is a pp li ed , w i t h t h e r at io


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P R E P A R A T I O N O F N U T R I E N T S O L U TI O N S 1 3 9

( N H 4 )2 S O 4 : K 2 S O 4 d i f f e r e n t i n e v e r y t r e a t m e n t ; i n t h i s c a s e i t i s c l e a r t h a tp r o v i d e d t h a t d i f f er e n c es in n i t r i f i c a t i o n d o n o t i n t e r f e r e w i t h t h e r e s u l ts , av a l i d t e s t i s b e i n g m a d e o f t h e N H 4 + : K + r a t i o . I f N I- I4 N O 3 is u s e d f o r i n -c r e a s i n g t h e N : K r a t i o h o w e v e r , t h e t o t a l i o n i c c o n c e n t r a t i o n i s a f f e c t e d ;i t t a u s t a l so b e p o i n t e d o u t t h a t o n c e r t a i n so ll s, t h e u s e o f a m m o n i u m s a l t si n s u c h e x p e r i m e n t s m a y l e ad t o c o n s i d e r a b l e c h a n g e s in p H .

M a n y m o r e d i f fi c u lt i es ar e e n c o u n t e r e d i n m a n u r i a l e x p e r i m e n t s o n so il s,b u t t h e f e w e x a m p l e s g i v e n a b o v e s e r v e t h e p u r p o s e o f i l l u s t r a t i o n .

T H E P R O B L E M W I T H N U T R I E N T S O L U T I O N SL e t u s n o w t a k e t h e c a s e of a n u t r i e n t s o l u t i o n c o r r e s p o n d i n g t o a p a r -

t i c u l a r f 0 r m u l a ti o n , i n w h i c h t h e p o t a s s i u m c o n t e n t i s t o b e v a r ie d . T h i sm a y b e d o n e b y t h e a d d i t i o n o f e x t r a p o t a s s i u m b u t i t i s c l e ar l y i m p o s s ib l et o d o t h i s w i t h o u t a f fe c t in g t h e p H . I n o r d e r t o m a i n t a i n t h e p H a t ad e s i r ed v a l u e , t h e a d d i t i o n o f a n a n i o n i s es s en t i al , e.g. s u l p h a t e o r c h l o r i d e .A s a r e s u l t o f th i s , o f c o u r s e, t h e e f f e c t of t h e s u l p h a t e o r c h l o r i d e m u s t b ec o n s i d e r e d b e s id e s t h a t o f t h e p o t a s s i u m . I n t h e c a s e o f n i t r o g e n , a t t e m p t sc a n b e m a d e t o a v o i d t h i s e ff ec t b y a d d i n g N H 4 N O 3 , b u t m a n y p l a n t s r e a c td i f f e re n t l y t o n i t r o g e n w h e n s u p p l ie d a s n i t r a t e o r a m m o n i u m i o n ; a ls o,b e c a u s e of t h e m o r e r a p i d u p t a k e o f a m m o n i u m b y t h e p l a n t, t h e r e m a y f ot -l o w a n a p p r e c i a b l e d r o p i n p H , i n w h i c h c as e, i f t h e p H i s t o b e m a i n t a i n e d ,a c a t i o n m u s t b e a d d e d w h i c h m a y i n t e rf e r e w i t h t h e e x p e r i m e n t .

Sys temat i c r esea rch in to the in f luence o f the chemica l compo-si t ion of the nut r ient so lut ion i s only just i f ied i f o l le deals wi thre la t ive r a t los o f the nu t r i en t s . There i s no ob jec t ion wha t s oev erto th i s approach . As a l r ead y d i scussed in the in t roduc t ion , wh a tev erthe values ass igned to the "absolute" levels , i t i s s t i l l possib le touse r e l a t ive r a t ios , p rov ided tha t the to ta l ion ic concen t r a t ion anda l so the pH , a r t t ake n in to aecoun t . Th i s does no t imply , a s wassuggested in the previous d iscussion on sol l , that K2SO4 can berep laced by Na~S04; in bo th cases , t he to ta l ion ic coneen t r a t ion i sinereased.

Co nsid erat io n of the rat io, N (as N Q ) : P : S : K : Ca : Mg ismeaning less s ince the ra t io IN(as N0~) + P + SJ : [K + Ca + Mg~is a l r eady de te rmined by the pH. Hence one t aus t dem wi th d i f f e r -en t r e l a t ive r a t los o f an ions in comb ina t ion wi th d i f f e r en t r e l a t ivera t ios o f ca t ions . M any peop le a t t a ch g rea t imp or tance to the K/ Nrat io ; a l tho ugh the s igni f icance of th is is open to arg um ent 5, i t canalso be considered on the basis of re la t ive ra t los , s ince h igh K +re la t ive to the o ther ca t ions and low NO3 - r e la t ive to the o theran ions means th e same as a h igh K /N ra t io . In p r inc iple , us ing a


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140 ABRAM A. STEIN ER

sys temat ic approach , i t i s des i rab le to take in to accoun t a s manycat ions and anions as poss ib le . In th is ar t ic le a t tent ion has beenr e c t r i c t e d t o t h e a nion s, N O a - , H 2 P 0 4 - ( H P 0 4 - - a n d P O 4 . . . ) a n dS 04 -- an d to the ca t ions , K +, Ca ++ an d Mg++.

3 0

K + / I \ C a ++

6 O ~ 7 0

Mg ++Fig. 2 . K+ : Ca++ : Mg++ r&Lios repres ented by the p o in ts a en b.

The rep re sen ta t io n of re la t ive ra t ios as single po in ts wi th in at r iangu la r sys tem of fe rs cons ide rab le adv an ta ge s. Th is c an be doneb y m a k in g t h e r a t i o s c o r r e s p o nd to t h e l e n g th o f t h e p e r p e n d ic u l a r sfrom each point to the 3 s ides of an equi la tera l t r iangle , each s iderepresen t ing the p ropor t ions o f a pa r t icu la r ion (c / Fig. 2); thera t io o f the 3 ions is then eas ily de te rm ined by re fe rence to there la t ive pos it ions o f the po in ts wi th in the t r i ang le . For conven ience ,each s ide i s d iv ided in to 10 equa l pa r t s and a t r i angu la r co -o rd ina tesys tem imposed . Refe r r ing to F igure 3 , eve ry po in t on the s ideAB for e xam ple corresponds to 0 per ce nt Mg ++, those on th e l ine


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PREPARATI ON OF NUTRI ENT SOLUTIONS 141

CD to 20 per cent Mg++, and so on; point N corresponds to a ratioof 20 per cent K +, 30 per cent Ca ++, and 50 per cent Mg++. Naturall yone can consider each ratio (or point) as a ratio of weights, as anion ratio, or as an equivalent ratio. Because of the connection with

M

A L Mq++ HFig. 3. Triang u]ar gra ph used for comp ari ng the K + : Ca ++ : Mg++ ratios.

pH adjustments to be discussed later, we prefer equivalent ratios.By locating all points within a cation and an anion triangle andcombining one from each, it is possible to classify sys tematica lly,all possible nutrient combinations.

For nutritonal-physiological investigations each combinationtaust correspond to a particular "formula" according to which acertain ionic concentra tion and a certain pH are obtainable with outaffecting the parti cular ratio used. Satisfying the pH requirementrepresents a serious stumbling block. For example, let us assumethat nutrient solutions are to be prepared according to two formu-


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142 ABRAM A. STEINER

lae g iv ing a par t icular to ta l ionic concentra t ion . I f a l l the phosphatei s supp l i ed as H2 P04 - , the so lu t ions wil l have low pH va lues (a -b o u t 5 ). H i g h e r p H v a l u e s m a y b e o b t a i n e d b y t i tr a t i o n w i t h K O H ,bu t then th e K : Ca : Mg ra t io t aus t be r ead jus te d i .e. t h e a m o u n to f KO H requ i r ed t aus t be conve r t ed in to the same K : Ca : Mgrat io as in the or ig inal so lu t ion; a t the same t ime, the to ta l ionicconcen t r a t ion , inc reased by the t i t r a t ion , has a l so to be r es to red .The r esu l t is tha t the pH i s no longer the same as imm edia te ly a f t e rthe t i t r a t ion .

The p rob lem of th is p H change ca ll s fo r a theore ti ca l t r e a tm en to f i n c o m p l e te d i sso c ia t io n a n d p H w i t h r e g a r d t o Ca H P 0 4 , Mg H P 0 4 ,Ca(OH)2 and o ther eompounds . Tb i s i s beyond the scope o f thep r e se n t a rt ic le a n d w il l n o t b e d is c u sse d h e re . T h e r e su l t o f su c ha t r e a t m e n t h o w e v e r , i n d i c a t e s t h a t t h e r e m u s t b e as y s t e m a t i c r e l a t i o n b e t w e e n t h e a m o u n t o f O H i o n sr e q u i r e d t o g i v e t h e d e s i r e d p H , t h e a b s o l u t e c o n c e n -t r a t i o n o f p h o s p h a t e a n d c e r t M n c a t i o n r a t i o s in t h es o l u t i o n .

I t wi l l be seen la ter that by means of cer ta in curves , i t i s possib leto f i t i n to a fo rmula , any ehosen combinaf ion and tha t in th i s way ,the fo llowing r equ i r emen t s can be sa t i s f ied wi th cons iderab teaccuracy : -

(1) a par t icu lar re la t ive cat ion ra t io(2) a par t icular re la t ive anion ra t io(3) a par t icular to ta l ionic concentra t ion

and (4) a par t ic ular pH .S ince the ac tua l cour se o f these eu rves in such a sys te m has to

be de te rmined exper imen ta l ly , a l a rge number o f so lu t ions mus t beprepared .REQUIRED COMBINATIONS OF ION RATIOS, TOTAL IONIC

CONCENTRATION AND PHEve n su pposing the app roac h descr ibed above was not en t i re ly successful ,

the fo rmulae obtain ed may yet f ind appl icat ion to phys iological s tudies onnutr i t ion. For example , i t would be poss ible to ma ke a se lection from thoseformulae giving a sui table pH, ere n i f this did not e xact ly correspo nd tothe or te required. I t was therefore cons idered des i rable to i l lc iude in thisinvest igatior) , a large numb er of 'combin at ion s ' ; these would a lso providean e ren be t t e r basi s for the cons t ruc t ion of the above -ment ioned curves .


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P R E P A R A T I O N O F N U T R I E N T S O L U T I O N S 143

F o l l o w i n g t h i s p r i n c i p l e , w e h a v e t r i e d t o i n c l u d e a s m a n y r a t i o s a s p o s s i -b l e ; a s s u m i n g 5 0 f o r t h e a n i o n s a n d 5 0 f o r t h e c a t i o n s , t h i s g i v e s u s 5 0 X 5 0= 2 , 5 0 0 p o s s i b l e e o m b i n a t i o n s . I f , i n a d d i t i o n t o t h e s e , w e w i s h t o i n c l u d es a y , 5 d i f f e r e n t t o t a l i o n i c c o n c e n t r a t i o n s a n d 5 d i f f e r e n t p H v a l u e s , t h e t o t a ln u m b e r o f s o l u t i o n s r e q u i r e d w o u l d b e 6 2 , 5 0 0 . A l t h o u g h t h e s e w i ll i n c l u d em a n y c o m b i n a t i o n s t h a t c a n b e r e j e c t e d o n t h e o r e t i c a l g r o u n d s b e c a u s e o fe x p e c t e d p r e e i p i t a t e s , t h e r e s t i l l r e m a i n s t o o 1 a r ge a n u m b e r t o b e d e a l t w i t hi n p r a c t i c e . T h e n u m b e r o f r a t i o c o m b i n a t i o n s t a u s t t h e r e f o r e b e r e d u c e d o rc e r t a i n r a n g e s o f r a t i o s w i t h i n t h e t r i a n g l e o m i t t e d . T h e p h o s p h a t e c o n t e n to I n u t r i e n t s o l u t i o n s u s e d i n p r a c t i c e , e x p r e s s e d a s m i l l i e q u i v a l e n t s *H 2 P O 4 - , g e n e r a l l y d o e s n o t e x c e e d 2 5 p e r c e n t o f t h e t o t a l c o n t e n t o f a n i o n s .

: 2 0

6 0 4 0 2 0 I 0 5 , ]v% H 2 P O 4 -F i g . 4 . T h e a n i o n r a t i o s u s e d i n t h e i n v e s t i g a t i o n . C o m b { n a t i o n s 1 t o 2 0.

* I n t h i s a r ti c le , p h o s p h a t e i s a l w a y s e x p r e s s e d a s m i l l i e q u i v a l e n t s - p e r c e n t , o r a sm i l l ie q u i v a l e n ts ( m e ) o f H 2 P O 4 e r e n t h o u g h , d e p e n d i n g o n t h e p H , i t m a y b e p a r t l yp r e s e n t a s H P O 4 . T h u s , a l t h o u g h t h e a d d i t i o n o f 2 m e H 2 P O 4 t o a s o l u t i o n m a y f o r e x -a m p l e , y i e ld 1 m e H 2 P 0 4 ' a n d 2 m .e . H P 0 4 " , t h i s s t i ll a m o u n t s t o 2 m g i o n s of p h o s p h a t e .C o n f u s i o n w il l b e a v o i d e d i f t h e p h o s p h a t e i s a l w a y s e x p r e s s e d a s m e o f H 2 P O 4 .


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! 4 4 A B R A M A . S T E IN E R

A l t h o u g h i t is n o t d e s i ra b l e t o e x c l u d e e n t i r e l y t h e h i g h e r p r o p o r t i o n s o fp h o s p h a t e b e c a u s e t h e s e m a y b e o f f u n d a m e n t a l p h y s i o l og i c a l i n t e re s t ,a t t e n t i o n c a n b e l a r g e ly d i r e c te d t o t h e l o w e r p r o p o r ti o n s . I n a n y c a se ,h i g h p r o p o r t i o n s o f p h o s p h a t e w i l l a l l o w f o r o n l y a f e w c o m b i n a t i o n s b e -c a u s e o f p r e c i p i t a t i o n e.g. o f C a H P O 4 .

T o b e g i n w i t h w e h a v e c h o s e n p h o s p h a t e c o n t e n t s c o r r e s p o n d i n g t o t h e5 , 10 , 20 , 40 , and 60 pe r cen t l i ne s i n F i gu re 4 . C om bi ned w i t h fou r d i f f e r en tN O s - : S O 4 - - r a ti o s, viz 80 : 15, 60 : 35, 40 : 55 and 20 : 75, thes e giv e 20d i f f e r e n t a n i o n c o m b i n a t i o n s ( N os . 1 t o 20, F i g . 4 ). T he 16 ca t i o n r a t i o sc h o s e n f o r i n v e s t i g a t i o n a r e s h o w n i n F i g u r e 5 ( a - p ) ; s i n ce i t i s a l r e a d y k n o w nt h a t b e s t r e s u lt s a r e o b t a i n e d w i t h l o w p r o p o r t i o n s o f M g , a t t e n t i o n h a sbee I1 m a i n l y focus sed on t he se .

8 0 : 1 0

6 0 4 O 2 0 I 0\ /'v

~,/o M g ++F i g . 5 . T h e c a t i o n r a t io s u s e d i n t h e i n v e s t i g a t i o n . C o m b i n a t i o n s a t o p .

T h e 2 0 a n i o n r a t i o s a n d 16 c a t i o n r a t l o s t o g e t h e r y i e l d 3 20 c o m b i n a t i o n s .W i t h 5 d i f f e r en t pH -v a l u es i nc l uded , na m e l y 5 . 0, 5. 8, 6. 5, 7 .1 , and 7 .6 , t het o t a l a m o u n t s t o 1 6 0 0 d i f f e r e n t c o m b i n a t i o n s .

O b v i o u s l y , i f t h e e f f e c ts o f d i f f e r e n t i o n i c r a t i o s a r e t o b e d i s t i n g u i s h e d ,


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PREPARATION OF NUTRIENT SOLUTIONS 145

the total 'strengths' of the solutions must be comparable. This raises thequestion as what "str engt h" actuall y means to the plant. Clearly it is notthe weight of the salts but their ionic equivalents; to some people howeverthis is a disput able marter. However, the calcul ations nece ssa ry to ensurecomparable total eq uivalent contents become very complicated, especiallywhen allowance has to be made for adjustments to the p H va he by titra tion ;amon g other factors, this is bou nd up wit h the ph ospha te dissociation series,HaPO4 ~- H2PO4- ~~- HPO 4-- ~- PO 4-- -. Part ly because of this compli-cation, but largely due to the fact that earlier experiments with differentcrops have demonstrated the great irnportance of total ionic concentration,as reflected in the osmotic values of the nutrient solutions, the total ionicconcentra tion has been kept constant. Because of the already very largenu mb er of inves tigat ions entailed, we have thereIore restricted ourselvesto a single concentration, orte in which very m an y plant s are kn own to thrive,correspond ing to 0.71 atm osmotic pressure. Assumin g a theore tically idealcomplete dissociation at 15C, this is equiva lent to a tota l co ncen trati on of30 mg ions per litte, provided no hydrolysis or precipitation occurs.

T h e 1600 c o m b i n a t i o n s h a v e b e e n i n c o r p o r a t e d i n t o f o r m u l a ea n d su b j e c t e d t o l a b o r a t o r y i n v e s t i g a t i o n . A d e t a i l e d d e sc r i pt i o nof the i r p rep ara t io n i s g iven be low.

P R E P A R A T I O N O F T H E T E S T S O L U T I O N C O M B I N A T I O N S

In o rder to c l a r i fy the p roc edure employed , fu ll de ta i l s a re g ivenin Tab le I fo r the So lu t ion _ra , co r r es pond ing to the Ani on Rat i oNo. z (c / Fi g . 4) an d Ca t ion Ra t io a (cf F ig . 5) , wi th a n osmot icpres sur e of 0.71 atm .

In the Col umn (z) the e qu iv a le n t con t en t o f each ton is ex -pressed as a percen tage o f the cor res pond in g to ta l an ion o r ca t ion

TABLE 1Combination . Ratio Absolute values

mg ions per litreza equiv, in % ions me per litre tcheck )( ) (2) (3 ) (4)

NOa-HzP04-S04--

80515Total 100

Ca++Mg++801010

8 057 .5

8055

13.1510.8222.46616.439I3.1511.6441,644

13.1510.8221. 23313.1510 . 8 2 20 . 8 2 2

Total 100 182.5 16,439 30.001


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146 A B R A M A . S T E I N E Re q u i v a l e n t c o n t e n t , j u s t a s i n F ig u r e s 4 a n d 5 . I n t h e C o l u m n( 2 ) t h e s e v a l u e s h a v e b e e n c o n v e r t e d t o m g i o n s ; t h i s c o l u m nt o t a l s 1 8 2 . 5 . F o r a n o s m o t i c p r e s s u r e o f 0 . 7 1 a t m , t h e t o t a l i o n i cc o n c e n t r a t i o n m u s t b e 3 0 m g i o n p er l i t re . T h e a b s o l u t e a m o u n t so f e a c h i o n s a s m i l l ie q u i v a l e n t s p e r li tr e a r e o b t a i n e d b y m u l t i -p l y i n g t h e v a l u e s i n t h e f ir st c o l u m n b y 3 0 / 1 8 2 . 5 ( g iv i n g a to t a l o f3 0 m g i o n s p e r l it re ) ; t h e v a l u e s a r e g i v e n i n C o l u m n ( 3) . A s a c h e c k ,t h e s e v a l u e s h a v e b e e n r e - c o n v e r t e d i n C o l u m n ( 4) in t o m g i o n s p erl i t r e , a n d t h e r e q u i r e d t o t a l o f 3 0 i s c o n f i r m e d .

S u m m i n g u p , i t w i l l b e s e e n t h a t C o l u m n (3 ) g i v es t h e n u m b e r o fm i l l i e q u i v . a l e n t s p e r l i t r e o f e a e h i o n c o r r e s p o n d i n g t o a p a r t i c u l a rr e l a t iv e r a t io o f a n i o n s a n d o f c a t i o n s a n d a to t a l i o n i c c o n c e n t r a t i o no f 3 0 m g i o n s p e r l i t r e . T h u s , e x c e p t f o r p H , t h e s o l u t i o n p r e p a r e da c c o r d i n g t o t h i s f o r m u l a s a t i s fi e s a l l r e q u i r e m e n t s .

I n o r d e r t o a p p l y t h e f o r m u l a t o a c t u a l s a lt s, a s c h e m e h a s b e e nd e v i s e d w h i c h e n a b l e s t h e p r o c e d u r e t o b e f o l l o w e d s y s t e m a t i c a l l ya n d w h i c h u t il iz e s t h e s m a l l e s t n u m b e r o f d i f fe r e n t s a l ts , g i v i n gp r e f e r en e e t o t h o s e w h i c h a r e e a s i ly s o l u b l e . F o r m o s t f o r m u l a et h i s h a s m e a n t 5 d i f f e r e n t s a l t s , b u t t h e r e i s a s m a l l e r g r o u p r e -q u i r i n g 4 s a l t s a n d a v e r y s m a l l g r o u p , 6 s a l ts . D e t a i l s o f th e s y s t e ma p p l i e d t o s o l u t i o n C o m b i n a t i o n I a a r e o u t l i n e d i n T a b l e 2 .

T A B L E 2/C o m b i n a t i o n ~ a ( 3 0 m g i o n s / 1 ) /

S a lt N o r m a j m l p e r ]l i t y ( N ) I 1 0 1 }

K I - I g P 0 4C a ( H ~ P O 4 ) 2 . l a q .M g ( H 2 P O 4 ) 2 . 3 a q .C a ( N O a ) ~ . 4 a q .C a S 0 4M g S 0 4 . 7 a q .M g ( N O 3 ) ~ . 6 a q .K N O 3K ~ S 0 4

10 .1

100 . 0 22111

M i l l ie q u i v a l e n t s p e r l i t r eK + C a + + M g ++ N O a - H 2 P O 4 - S O 4 - -

13.151 1 .644 1 .644 13.151 0 .8 22 2 .4668 . 2 2 0 . 8 2 2. 8 2 2

11.50~O . 8 2 ~

E 6 4 ,8 . 2 2

115 .078.22

1 .6441 .644

1 .644

11 .8071 .644

0 . 8 2 2A s f a r a s p o s s i b l e , t h e r e q u i r e d n u m b e r o f m i l l i - e q u i v a l e n t s o f

t h e v a r i o u s i o n s a r e o b t a i n e d p r e f e r e n t i a l l y f r o m s a l t s i n t h e o r d e r ,t o p t o b o t t o m ; t h i s o r d e r i s b a s e d f i r s t o n t h e e a s e w i t h w h i c h t h ep h o s p h a t e c a n b e s u p p l i e d i n m a n a g e a b l e f o r m , v i z K > Ca > Mg.T h e b a l a n c e o f c a lc i u m i s t h e n m a d e u p i n t h e o r d er NOa- > S 0 4 - - ,


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PREPARATI ON OF NUTRIENT SOLUTIONS 147and that of magnesium in the order SO4-- > NOt-; any nitrateoutstanding is then made up from KNO3. Final amendments, ifnecessary, are auto mat ical ly made up by adding equivalent a-mou nts of K + and SO4--.

To facilitate the pr eparation of these solutions, calculations havebeen made to determine how many ml of stock salt solution ofgiven normal ity are needed per 10 litres of nutri ent solution.

Att ent ion is drawn to the difficul ty of obtaining a clear solutionof 0.02N CaS04 if CaS04 is used: this can be avoided by dissolving1 g CaC03 in 200 ml 0.1N H~S04 and mak ing up to 1 1 with wate ra / t e r it has dissolved.In each case, 10 1 of the r equi red nutrie nt solution were preparedusing demineralized wate r (spec. conduc tivi ty usua lly 10-6 andnever < 10-5 ohm-lcm-1). The solution was then thoroughlyaerated for 24 hours so as to obtain equilibrium with air. The pHof the resulting solution was measured with a Radiometer pHmeter, Type M22P: all preparations were carried out in the labora-tory at a temperature of usually, about 21C.

Solution ~a had a pH value of 4.76 which, aft er 24 hours aerat ion,rose to 5.33. One litt e of this solution was then t it ra te d to pH 5 withHN03, the titrations were made with continuous measurementsof pH, and agi tat ion by magnetic stirrer. 0.14 rel. of HN03 (.0838N)were found necessary to obtain a pH of 5; this is equivalent to0.012 me NO3- per litre. Fur ther aliquots of 1 1 nut rient solutionwere ti tr at ed to obtain the othe r pH values - 5,8, 6.5, 7.1, and 7.6;the amounts of KOH required corresponded respectively to 0.060,0.296, 0.606 and 0.772 me K +. After each ti trat ion, the so lutionswere again aspirated for 1 hour and the pH remeasured, these gavenew pH values of 5.03, 5.86, 6.45, 6.85 and 7.30 respectively. Sinceit was decided to accept on ly a deviation of ~z 0.1 p H unit s fromthe required pH value the solution giving a pH of 6.85 instead of7.1 H 0.1 was again titrated with KOH to pH 7.1; this requireda fur th er 0.65 ml of KOH (.0837 N) ---- 0.054 me K +. After aspira-fing again for 1 hour, the pH fell to 7.0. The solution giving a pHof 7.30 ins tead of 7.6 H 0.1 was similarly ret it rat ed with KOH andan equivalent of 0.087 me K + was required in this case; af ter 1-hour's aspiration, the pH rose to 7.34, indicating incomplete dis-sociation. Further titrations were made and finally, after a totalof 0.949 me K + had been added, a solution was obta ined which


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148 AB R AM A. S TEI NER

a f t e r a e r a t i o n , g a v e a p H o f 7 . 54 i . e . w i t h i n t h e r a n g e 7 . 6 :j : 0 . I .A l t h o u g h t h e s o l u t io n s a t t h e tw o h i g h e s t p H v a l u e s w e r e c o m -p l e t e l y c l e a r v i s u a l ly , t h e r e w a s s o m e i n d i c a t i o n , a s m e n t i o n e d a b o v e ,

t h a t d i s s o ci a t io n w a s in c o m p l e t e . N o t e h a s b e e n t a k e n o f t h i s to -g e t h e r w i t h t h e o b s e r v a t i o n t h a t " s o m e o f t h e s o l u t io n s w e r e o p a l -e s c e n t o r a c t u a l l y t u r b i d . H o w e v e r , a ll 3 2 0 c o m b i n a t i o n s h a v e b e e nt r e a t e d i n th e s a m e w a y e v e n w h e r e i n c o m p l e t e d i ss o e i a t i o n o rp r e c i p i t a t i o n h a d b e e n e x p e c t e d o n t h e o r e t i c a l g r o u n d s . A l t h o u g ht h e a c t u a l t h e o r e t i c a l e a l cu l a t io n s a r e b e y o n d t h e s c o p e o f t h isp a p e r , i t c a n be s a i d t h a t t h e y h a v e g i v e n g o o d a g r e e m e n t w i t h t h er e s u l t s .T h e p r o c e d u r e d e s c r i b e d s o r a r g i v e s s o l u t i o n s s a t i s f y i n g t h e p Hr e q u i r e m e n t s b u t , o f c o u r se , a l t e r s t h e r e l a t i v e p r o p o r t i o n s o f t h ea n i o n s ( w h e r e H N O ~ w a s a d d e d ) o r o f t h e c a t i o n s ( w h e r e K O Hw a s a d d e d ) . F u r t h e r m o r e , t h e t o t a l io n ic c o n c e n t r a t i o n n o w e x -c e e d s t h e s t a t e d 3 0 m g i o n s p e r l i tr e . I n o r d e r t o r e c t i f y th e r a t i o st h e a m o u n t s o f N O a - o r K + a d d e d t o a d j u s t t h e p H h a v e b e e n c o n -v e r t e d t o eq u i v a le n t s o f N O 3 - , H 2 P Q - a n d S O 4 - - i n t h e o n e c a seo r K + , C a ++ a n d M g + + i n t h e o t h e r , i n t h e s a m e p r o p o r t i o n sa s in t h e o r i g i n a l s o l u ti o r t. T h e m e t h o d o f c o n v e r s i o n is i l l u s t r a t e db e l o w f o r S o l u t i o n r a ( T a b l e 1) w i t h a p H o f 6 .5 ( a c t u a l l y 6 . 45 ) ;t o o b t a i n t h i s p H , 0 .2 9 6 m e K + (s i m i la r ly O H ' ) h a d t o b e a d d e d p e rl i tr e o f s o lu t io n . T h e c o n v e r s i o n f a c t o r i s o b t a i n e d f r o m t h e r a t ioo f t h e t o t a l m e c a t i o n s ( i n c lu d i n g t h e e x t r a K + ) t o t h e o r i g i n a lt o t a l m e c a t i o n s : -

13 .151 -4 - 1 .644 @ 1 .644 q - 0 .296C o n v e r s i o n f a c t o r = = 1 .0 1 80 0 513 .151 + 1 .644 + 1 .6441 . 0 1 8 0 0 5 1 3 .1 5 1 m e K + = 1 3 . 3 8 8 m e K +1 . 0 1 8 0 0 5 1 . 6 4 4 m e C a + + = 1 . 6 74 m e C a + +1 . 0 1 8 0 0 5 1 . 6 4 4 m e M g + + = 1 . 6 7 4 m e M g + +

T h e s e c o n v e r t e d v a l u e s n o w g iv e th e s a m e r e l a t i v e p r 0 p o r t i o n so f K , C a , a n d M g a s in t h e o r i g in a l s o l u t i o n a n d t h e n e w I o r m u l ab e e o m e s "

In K + Ca ++ ] Mg ++ NOa - H2P O4- SO4- - Tot alme 13.388 1.674 , 1.674 13.151 0.822 2.466 -mg ions 13.388 0.837 0.837 13.15l 0.822 1.233 30.268H o w e v e r , t h e t o t a l i o n i c c o n c e n t r a t i o n h a s b e e n i n c r e a s e d t o


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P R E P A R A T I O N O F N U T R I E N T S O L U T I O N S 149

30.268 mg. ions per litre and the formula must therefore be read-justed to give 30 mg. ions. The final result is: -

I n K + C a ++ M g ++ N O 3 - H 2 P O 4 - ] S O 4 - - T o t a lme 13 .269 1 . 6 5 9 1.659 13.035 0.815 ] 2 . 4 4 4 -mg ions 13 .269 0 .830 0 .830 13 .035 0 .815 i 1 .222 30 .001

The amounts of the various ions can now be derived from theappropriate salts according to the procedure outlined earlier,making sure that the required surplus of cation (K) is in the firstplace, made up entirely Irom KOH; although the surplus cationcould also have been supplied as Ca(0H)2 or Mg(OH)2, KO H ispreferred because of its high solubil ity. The ac tua l surplus of cationsin the final solution amou nts to (13.269-k 1.659 -}-1. 659)- -(13.035 q- 0.815 -}- 2.444) = 0.293 me; this is added as KOH afterwhich the remaining equivalent amounts of cations and anionsare added as described earlier.

Ten litres of Solution za with pH 6.5 were now prepared accordingto this new formula. After aeration the pH changed to 6.58; it isclear that this change in pH has taken place because of the changefrom K+ to K + -k Ca ++ -k Mg++. This example, in which 0.815 meH2PO4- were present and a surplus of 0.293 me OH - had to be added,will be fu rthe r discussed lat er on.

Except for those solutions already turbid or giving a precipitateon titration with KOH, all 320 ion combinations have been con-verted in this way and tested according to the above scheine at5 different pH values.

C O M P U T A T I O N O F F O R M U L A EAs mentioned earlier there is apparently a systematic relation

between the amount of H2PO4- present in the solution and thesurplus OH ions which must be added in order to obtain a certainpH. This relationship however, is inIluenced by the relative cationratios, especially those between K + and Ca++. The pH dependslargely on the ratio of OH- to H 2P04- ions and this m ay be depictedby a curve which is displaced according to the K : Ca ratio.

In the 360 ion combinations, only four K : Ca ratios were in-volved, namely , 80 : 10, 60 : 30, 40 : 50 and 20 : 70; however,tests were made on another 80 combinations giving intermediate


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1 5 0 A B R A M A . S T E I N E R

K C a r a t i o s so as to d e t e r m i n e w i t h s o m e a c c u r a c y , t h e i n f l u e n c eo f t h is r a t i o o n th e d i s p l a c e m e n t o f t h e O H - / H 2 P 0 4 - c u rv e s .I n F i g u r e 6 , t h e p H ( ab c is sa ) is p l o t t e d a g a i n s t t h e a m o u n t o f

I 0 0 I,- A

B9 0 C ,

I 8 0

0[3_ 7OO4I0 6 O

II s o !~9CDCJ 2 4 0CC9Uu .C9 3 OCL

2 0

I O

DE ,

K : C a . r o t i oA = 8 0 : 2 0B = 7 0 : 3 0C = 6 0 : 4 0D = 5 0 : 5 0E = 4 0 : 6 0F = 3 0 : 7 0G = 2 0 : 8 0

5 . 0 5 . 5 6 . 0 6 . 5 7, O 7 . 5pH

F i g . 6 . T h e d e p e n d e n c e o f t h e O H - : H ~ P O 4 - ' c u r v e s u p o n t h e K : C a r a t i o .

O H - e x p r e s se d as a p e r c e n t a g e o f t h e a m o u n t o f H 2 P 0 4 - i n th es o l u t i o n ( o r d i n a t e ) ; c u r v e s a r e g i v e n f o r 7 d i f f e r e n t K : C a r a t i o s( A - G ) , a n d i n t e r m e d i a t e v a l u es c a n b e o b t a i n e d b y i n t e rp o l a t io n .T h e c u r v e s i l l u s t r a t e d h o w e v e r a r e o n t o o s m a l i a sc a le I o r p r a c t i c a l


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PREPARATION OF NUTRI ENT SOLUTIONS 151

p u r p o s e s a n d l a r g e r s ca le s a r e a v a i l a b le f o r t h o s e w h o w i s h t o u s et h e m .W i t h t h e a i d o f th e s e c u r v e s , i t i s n o w p o s s ib l e t o w o r k o u t a n yd e s i r e d I o r m u l a . S u p p o s e w e r e q u i r e a r a t i o ( in e q u i v a l e n t s ) o fN O t - : H 2 P 0 4 - : S 0 4 - - = 5 : 3 : 1 ( i . e . 5 5 . 5 5 6 : 3 3 . 3 3 3 : 1 1 . 11 1t o g i v e a t o t a l o f 10 0) a n d a r a t i o o f K + : C a + + : M g + + = 5 0 : 3 0 : 2 0w i t h a t o t a l i o n i c c o n c e n t r a t i o n o f 3 0 m g ! o rt s a n d a p H o f 6 .3 .

T h e r a t i o K : C a = 5 : 3 o r 6 2 . 5 : 3 7 . 5 ( t o g i v e a t o t a l o f 1 0 0)a n d t h e l i n e C i n F i g u r e 6 , r e p r e s e n t i n g a r a t i o o f 6 0 : 4 0 , i s t h en e a r e s t t o th i s . F o r p H 6 .3 th i s w o u l d in d i c a t e a n O H ' r e q u i r e m e n to f 2 6 .9 p e r c e n t o f t h e p h o s p h a t e c o n t e n t . I n t e r p o l a t i o n b e t w e e nc u r v e s C a n d B t o c o r r e s p o n d w i t h t h e a e t u a l 6 2 . 5 : 3 7 .5 r a t i o ,i n d i c a t e s a s o m e w h a t l o w e r v a l u e , n a m e l y , 2 6 . 0 p e r c e n t ; t h i s ist h e v a l u e a d o p t e d .

I n t h e d e s i re d so l u t io n , H s P O 4 - a m o u n t s t o 33 .3 3 3 p e r c e n t o ft h e t o t a l eq u i v a l e n t a n i o n c o n t e n t so t h a t t h e O H - c o r r e s p o n d i n gt o 2 6 .0 p e r c e n t o f t h is , a m o u n t s t o 8 . 66 7 O H i o n s . T h e s e O H i o n sa r e n o w d i v i d e d a m o n g t h e c a t i o n s i n t h e s a m e p r o p o r t i o n a s t h ec a t i o n r a t i o , i . e . 5 0 : 3 0 : 2 0 . T h i s y i e l d s 4 . 3 3 3 K + , 2 . 6 0 0 C a + + a n d1 .7 33 M g ++ a n d w h e n t h e s e a m o u n t s a r e a d d e d t o t h e c o r r e s p o n d i n gc o m p o n e n t s o f t h e o r i g i n a l c a t i o n r a t i o ( t o t a l 1 00 ), t h e r a t i o s t i llh o l d s g o o d . T h u s , b o t h c a t io n a n d a n i o n c o n t e n t s h a v e n o w b e e ne x p r e s s e d i n t e r m s o f m i l l i e q u i v a l e n t s : -

K + I C a + + I ~ < g + + I N O - / H 2 P O ~ - . I S O - -5 4 . 3 3 3 ] 3 Z e 0 0 I 2 ~ . 7 3 3 I S S . S S 6 I 3 3 . 3 3 3 I 1 1 .1 1 1

I n o r d e r t o o b t a i n a t o t a l io n i c c o n c e n t r a t i o n o f 3 0 m g i on s , th ea b o v e v a l u e s a r e fi r s t c o n v e r t e d i n t o m g i o n s : -

K + I C a + + I M g + + I ~ O 3 - I m P o 4 - 1 S O 4 - - I T o t al5 4 . 3 3 3 I 1 ~ . 3 0 0 I 1 0 . 8 6 7 F 6 6 . 5 5 6 I 3 3 . 3 3 3 I 5 . 5 ~ 6 I 1 7 5 9 4 5

S i n c e t h e t o t a l o f 1 7 5 .9 4 5 h a s t o c o r r e s p o n d t o 3 0 m g i o n s, t h ef in a l f o r m u l a is o b t a i n e d b y m u l t i p l y i n g t h r o u g h o u t b y 3 0 /1 7 5 .9 4 5.T h e r e s u l t is a f o r m u l a f o r a n u t r i e n t s o l u t io n s a t i s f y i n g t h e f o ll o w -i n g c o n d i t i o n s : -

(1) a d e s i r e d r e l a t i v e a n i o n r a t i o(2) a d e s i r e d r e l a t i v e c a t i o n r a t i o(3) a d e s i r e d t o t a l i o n i c c o n c e n t r a t i o n

a n d (4) a d e s i r e d p H w i t h a m a x i m u m t o l e r a n c e o f 4 - 0 .1 p H .


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152 ABRAM A. STEINER

A s s u m i n g t h a t t h e n u t r i e n t s o l u t i o n is p r e p a r e d a c c o r d i n g t o t h i sI o r m u l a , t h e p H a f t e r a e r a t i o n w i l l b e a b o u t 6 . 37 i . e . o n l y 0 . 0 7u n i t s h i g h e r t h a n r e q u i r e d .

I n t h e c a s e o f s o l u t i o n l a , w h i c h h a s b e e n g i v e n e a r l i e r a s a ne x a m p l e , t h e a m o u n t o f O H - a d d e d (0 .2 93 m e ) c o r re s p o n d s t o3 5 .9 p e r c e n t o f t h e a m o u n t o f H 2 P O 4 - ( 0 .8 1 5 m e ) p r e s e n t i n t h es o l u t i o n ; a c c o r d i n g t o c u rV e A i n F i g u r e s 6 , t h i s w o u l d i n d i c a t ea p H o f a b o u t 6 .5 8, w h i c h a g re e s e x a c t l y w i t h t h a t a c t u a l l y f o u n d .

E v e r y p o s s i b l y i o n c o m b i n a t i o n h a v i n g a t o t a l c o n c e n t r a t io no f 3 0 m g i o ns , w h i c h d o e s n o t p r o d u c e a p r e c i p i t a t e , c a n b e p r e -p a r e d i n t h is w a y w i t h i n a p H r a n g e c o v e r e d b y t h e K : C a r a t io sP r e c i p i t a t e s , iI a n y , a n d t h e s i g n i fi c an c e o f t h e a b o v e - m e n t i o n e dp H r a n g e w i l l b e d i s c u s s e d l a t e r .

R e g a r d i n g t h e e x t e n t t o w h i c h t h e p H o f t h e s o l u t i o n s a ti s Ii e st h e r e q u i r e d v a lu e , it m u s t b e s t a t e d t h a t n o b e t t e r a c c u r a c y t h a n-1- 0 : 2 p H u n i t s c a n b e g u a r a n t e e d w i t h i n t h e r a n g e 5 . 0 t o 5 .5 . T h i si s d u e t o t h e f a c t t h a t b e t w e e n t h e s e v a l u e s , t h e p h o s p h a t e d i s s o c i -a t i o n c u r v e ru n s p r a e t i c a l l y h o r i z o n t a l, i n o t h e r w o r d s , t h e s m a l l e s tc h a n g e i n O H - c o n c e n t r a t i o n p r o d u c e s a l a r g e c h a n g e i n p H .B e t w e e r t p H 5 .5 a n d 6 .0 , t h e p r e c i s io n is i 0 .1 5 a n d b e t w e e n p H6 . 0 a n d 7 . 0, 0 .1 . B e c a u s e o f t h e i n c o m p l e t e d i s s o c i a t i o n o fC a H P O 4 , a n d o t h e r s a l t s a b o v e p H 7 ,0 , t h e c c u r a c y f a ll s t o =L 0 .2 .

T h e m e t h o d d o e s n o t h o l d f o r e x t r e m e v a l u e s o r fo r so l u t io n sn o t c o n t a i n i n g c a l e iu m o r p h o s p h a t e .

I t w i l l b e o b s e r v e d i n F i g u r e 6 t h a t t h e c u r v e s c o v e r o n l y ac e r t a i n p H f a n g e w i t h a m i n i m u m a t p H 5 . 0 ; r e l a t i o n s h i p s b e l o wt h i s v a l u e w e r e , i n f a c t , n o t i n v e s t i g a t e d . H o w e v e r , b e t w e e n p H4 . 0 a n d 5 .0 , t h e r e i s l i t t l e p o i n t t o t h e i n v e s t i g a t i o n s i n c e t h e d i s s o c i-a t i o n c u rv e f or H 2 P O 4 - # H P 0 4 - - t h e n r un s h o r iz o n ta l l y a n d int h i s r e g io n t h e s o l u t i o n is n o l o n g e r b u f f e r e d b y p h o s p h a t e . B e l o wp H 4 .0 , t h e a c c u r a c y i n c r e as e s a g a i n (c f F i g u r e 1). A b o v e t h e p Hr a n g e c o v e r e d b y t h e c u r v e s , f e w s o l u t io n s a r e p o s s i b l e w i t h a t o -t a l i o n i c c o n c e n t r a t i o r t o f 3 0 m g i o n s p e r l it r e ; o n l y in t h e c a s e o fe x t r e m e r a t l o s a r e h i g h e r p H v a l u e s p o s s i b l e e . g . i n s o l u t i o n sc o n t a i n i n g v e r y l i t tl e C a . T h i s i s b e c a u s e o f th e o c c u r r e n c e o f t u r -b i d i t y o r a c t u a l p r e c i p i t a t io n . N e v e r t h e l e s s , a f e w s o lu t i o n s a r es ti ll p o s si b le w i t h p H v a l u e s g r e a te r t h a n t h o s e i n d i c a t ed b y t h ec u r v e s, b u t t h e s e r e q u ir e m o r e O H i o n s t h a n w o u l d n o r m a l l y b en e c e s s a r y t o a c h i e v e t h e d e s i r e d p H . I n s u c h c a s e s , t h e d i s s o c i a t i o n


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PREPARATI ON OF NUTR IENT SOLUTIONS 153

of certain ion combinations is also very incomplete and a stage isfinally reached when the addition of OH ions does not produceany rise in pH but even a fall, because of precipitation of CaHP04and other compounds. Eren within the pH range covered by thecurves, certain combinations are also not possible because of pre-cipitation. On the whole, about 58 per cent of all possible combi-nations ean be used around the point where the appropriate curveends. Also near the 'lower' regions of the curves, a number of combi-nations are not possible; for Curve A, this involves about 15 percent of all possible combinations at pH 7.1 and for Curve C, about15 per cent at pH 6.5.Whether a particular combination at a certain pH is possibleor not, can be determined with some precision. No combinationwith the product of Ca ++ and S 04- - greater than 60 mg ions perlitre is possible, independent of the pH of the solution. However,this concerns on]y a few combinat ions ; in Figures 4 and 5, it appliesonly to the Anion Ratios z6 and 17 in eombination with CationRatlos m and n. Solutions with the product of Ca ++ and HP O4 --amounting to about 2 .2 mg ions per li tre are also doubt ful ; if Ca ++

. X H P 0 4 - - is greater tha n 2.2, the solution becomes turbid. Thereare however, border-line cases in which precipitation may or maynot occur depending on the other ratlos. The absolute contentsof Ca ++ and tota l phosph ate are readily determined from the Ior-mula calculated; the absolute content of HP 04 - - may then be foundfrom its percentage of the total phosphate read from the phosphatedissociation curves in Figure 1, at the appropriate pH.

The method described above holds good for a total ionic concen-tration of 30 mg ions per litre. Although the immediate objectiveof this investiga tion has therefore been achieved, tests have alsobeen made on solutions with a total ionic concentration of 20 mg.ions per litre (0.47 atm osmotic pressure). IM these cases also, thecurves can be used with a certain accuracy. No tests have beenmade on solutions with lower concentrations; in any case, thistaust be looked upon as a separate investigation since the curvesneed then to be extended to regions of higher pH. At these lowerconcentrations, many more combinations are possible, espeeiallyat the higher pH values.When the total ionic concentration is raised above 30 mg ionsper litre, only a limited number of combinations are possible. Thus,


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1 5 4 P R E P A R A T I O N O F N U T R I E N T S O L U T I O N S

a t 1 a t m o s m o t i c p r es s u r e ( a b o u t 4 2 m g i o n s p e r l it r e) , o n l y a f e wn u t r i e n t s o l u t i o n s c a n b e m a d e , v i z . o n l y t h e c a t i o n c o m b i n a t i o n sa , b , c a n d d ( F i g u r e 5 ) i n c o m b i n a t i o n w i t h c e r t a i n a n i o n r a t l o s .A b o v e p H 6 . 5, o n l y v e r y f e w e x t r e m e i o n r a t lo s a r e s t i ll p o s s i b le .

I n t h i s p u b l i c a t i o n , o n l y 3 a n i o n s a n d 3 c a t i o n s h a v e b e e ni n v e s t i g a t e d . T e s t s h a v e b e e n m a d e a s t o t h e p o s s i b i l it y o f i n c l u d i n ga f ou r t h c a t i on , N a ; i n t h i s case , t h e sam e K : C a r a t i o e an b e u se d ,b u t t a k i n g ( K + N a ) : C a i n t o a c c o u n t i n s t e a d , i . e . t h e s u m o fs o d i u m a n d p o t a s s i u m i s t r e a t e d a s p o t a s s i u m . A s f a r a s w e h a v eb e e n a b l e t o d e t e r m i n e , t h i s d o e s n o t u p s e t t h e r e l i a b i l i t y o f t h em e t h o d . T h e c a l c u l a t io n s i n v o l v i n g f o u r c a t i o n s a r e id e n t i c a l w i t ht h o s e f o r t h r e e . N o i n v e s t i g a t i o n h a s y e t b e e n m a d e a s t o w h e t h e rN H 4 io n s , f o r e x a m p l e , c a n b e d e a l t w i t h b y t h e s a m e c u r v e s ; i nt h i s c a se m u c h m o r e a t t e n t io n w o u l d b e n ec e s s a ry t o t h o s e c o m b i -n a t i o n s p r o d u e i n g p r e c i p i t a t e s .

S U M M A R Y

So as to provi de a basis for nutr it io nal and general physiological experi-ments on plants , a systema tic invest igat ion has been made in to the prepa-rat ion of nutr ient solutions sat is fying certain require ments as to the relat iverat los of the nutr ient ions , the to tal ionie concentrat ion and the pH, witho utcomplicat ions due to precipi tat ion. A met hod has be en developed wherebyit is possible to c ompu te in advance , a fo rmula for the co mposit ion of thenutr ient solut ion which sat is f ies these requirements .

A C K N O W L E D G E M E N T

The author is much ind ebted to Dr. L . Leyto n Dep ar tme nt of Forestry ,Imperial For estry I ns t i tu te , Univer s i ty of Oxford, for reading and improvin gthe text of th is paper.Received October 28, 1960

LITERATURE CITED1 Goeting, W. A. J., Orinterende berekening van de maximale concentraties in x,oe-dingsoplossingenvoor plantentee lt zonder aarde. Algemene Technische Afdeling T.N.O.Rapport T 1343 (1949).2 He wi tt , E. J., Sand and watet eulture methods used in the study of plant nutrition.Commonwealth Bureau of Hort. Plantation Crops, Tech. Commun. No. 22 (1952).3 Kiplinger, D. C. and Laurie, A., Growing ornamental greenhouse crops in gravelculture. Ohio Agr. Exp. Sta. Bull. 634 (1942).4 Schr opp , W., Der Vegetationsversueh I. Die Methodik der Wasserkultur hhererPflanzen. Neumann Verlag, Radebuel und Berlin (1951.)5 S teine r , A. A., Problemen rond de K : N verhouding, Kali 35, 186-188 (1958}.

A Universal Method for Preparing Nutrient Solutions of a Certain Desired Composition

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