SCHEME I Soil organic'matter
V. Nutrient Uptake from Soil
T h e r e a r e t h r e e possible sources from w h i c h roots can extract their nutrients: t h e soil solution, t h e exchangeable ions, a n d t h e readily de-composable minerals.
A. S O I L S O L U T I O N
T h e soil moisture w i t h the salts a n d gases dissolved in it is c o m m o n l y considered to be t h e soil solution. Sometimes reference is m a d e to the
" i n n e r " a n d " o u t e r " soil solution. T h e t e r m i n n e r solution refers to t h e moisture in i n t i m a t e contact w i t h t h e colloidal soil particles, in w h i c h t h e concentration a n d composition of t h e solutes a r e in equilibrium w i t h t h e solid phase. By t h e outer solution is m e a n t t h e liquid in the
4.0pH 4;5V e ry 5.0 5.5 6.0 6.5 V er y 7.0 7.5 8.0 8.5 9.0 9.5^ pH 1.0
FIG. 10. Chart showing general trend of relation of reaction to availability of plant nutrients. From Truog (252).
larger capillary spaces, w h i c h is considered to be m u c h m o r e dilute t h a n t h a t in close contact w i t h t h e colloidal particles.
Schloesing, in 1866, was the first to use a displacement m e t h o d for collecting soil solutions. H e used w a t e r colored w i t h c a r m i n e as the displacing solution. Ischerekov, in 1907, used ethyl alcohol as t h e dis-placing liquid a n d obtained results w h i c h indicated t h a t t h e displaced liquid was t h e t r u e solution in a n u n a l t e r e d condition. Moist soil w a s packed in a glass t u b e w h i c h h a d a piece of linen tied over t h e bottom.
After alcohol h a d been placed on top of t h e soil c o l u m n the soil solu-tion soon began to drop from t h e bottom of t h e tube. Ischerekov found t h a t successive portions of the displaced solution w e r e of t h e same
com-position, a n d t h a t the concentration of t h e soil solution w a s inversely proportional to t h e moisture content of the soil. V a n Suchtelen, 1912, modified Ischerekov's method b y using paraffin oil as a displacing liquid a n d b y a p p l y i n g suction to hasten displacement. M o r g a n , 1916, used a combination of pressure a n d displacement methods, in w h i c h a h e a v y oil w a s used as t h e displacing liquid a n d a n applied pressure of about 500 pounds per square inch w a s used to force the oil into the packed soil. Several methods h a v e been suggested for d e t e r m i n i n g t h e concen-tration of t h e soil solution directly in t h e soil: these include electrical conductance a n d freezing point depression methods ( 1 8 2 ) .
P a r k e r (182) compared t h e displacement, w a t e r extraction, a n d freezing point methods for determining t h e concentration a n d compo-sition of t h e soil solution from a n u m b e r of soils. Some of t h e m o r e i m p o r t a n t conclusions derived from his results are given below:
1. E t h y l alcohol was found to be m o r e satisfactory as a displacing liquid t h a n water, m e t h y l alcohol, acetone, or liquids immiscible w i t h water.
2. T h e composition of the soil solution obtained b y displacement w a s not influenced b y t h e displacing liquid used.
3. Successive portions of the displaced solution gave the same freez-ing point depression a n d contained t h e s a m e a m o u n t of total salts.
4. T h e concentration of t h e displaced solution w a s found to be in-versely proportional to t h e moisture content of the soil.
5. T h e displacement method gave t h e same a m o u n t of n i t r a t e nitro-gen a n d approximately t h e same a m o u n t of total salts as a 1:5 w a t e r extraction of t h e soil.
6. T h e freezing point method does not give a m e a s u r e of t h e concen-tration of t h e soil solution directly in t h e soil at o r d i n a r y moisture con-tents.
B u r d a n d M a r t i n (50) h a v e pointed out t h a t a soil's effective solu-tion m a y attain a relatively constant total concentrasolu-tion, as m e a s u r e d b y freezing point depression or conductivity, while undergoing large changes i n t h e concentration of its solutes, notably in its bicarbonate a n d n i t r a t e content.
It m a y be asked w h e t h e r , in general, t h e concentration a n d composi-tion of soil solucomposi-tions from productive soils are of such a n a t u r e as to be adequate w h e n a p a r t from t h e solid phase of t h e soil. H o a g l a n d (109) m a d e such a comparison b y growing barley plants in artificial solutions side b y side w i t h plants g r o w n in soils; h e c a m e to t h e conclusion t h a t w h e n used i n sufficient a m o u n t s t h e y w e r e adequate for supporting n o r m a l growth.
B u r d a n d M a r t i n (51) used a w a t e r displacement m e t h o d to obtain
the soil solutions from cropped, fallowed, a n d stored soils a n d concluded t h a t continuous cropping i n v a r i a b l y decreases t h e concentration of t h e solutions. O n t h e other h a n d , fallowing increases t h e concentration.
T h i s is clearly illustrated i n T a b l e I I I . T h e quantitative composition of the soil solution i n soils u n d e r crop is continuously changing (Table I V ) . Burd a n d M a r t i n ( 5 1 ) point out t h a t n u t r i e n t solutions m a d e u p in imitation of t h e soil solution as it exists at t h e beginning of t h e
T A B L E III
AVERAGE COMPOSITION OF DISPLACED SOIL SOLUTIONS FROM CROPPED ( A ) , FALLOWED ( B ) , AND STORED ( C ) SOILS AFTER 8 YEARS*
Soils %
Anions (meq) Cations (meq)
Total ions (meq) Soils % N 03 H C 03 CI S 04 P 04 SiO.3 Ca Mg Na Κ
Total ions (meq) A 12.6 3.72 1.84 0.00 12.53 0.08 1.61 10.14 7.10 1.84 0 .68 39.54 Β 16.3 29.56 1.02 1.44 9.66 0.07 1.48 27.88 10.99 2.77 1 .61 86.49 C 16.2 18.36 1.31 8.10 5.43 0.19 1.38 19.02 8.80 5.04 1 .92 69.56
"From Burd and Martin (51).
T A B L E IV
SEASONAL CHANGES I N THE COMPOSITION OF DISPLACED SOIL SOLUTIONS FROM CROPPED SOILS*
Parts per million of displaced solution Soil
no. Date Moisture pH N 03 H C O 3 S 04 P 04 Ca Mg Na Κ 7 April 30, 1923 10.7 7.4 174 83 655 1.1 283 106 49 24 Sept. 4, 1923 12.5 7.6 58 155 432 0.6 193 47 40 9 8 April 30, 1923 9.6 7.4 274 93 633 2.5 267 93 31 20 Sept. 4, 1923 8.4 7.6 88 143 275 1.4 153 56 28 11
aF r o m Burd and Martin (51).
season cannot represent soil solutions d u r i n g t h e later stages of t h e growth of crops. W i t h t h e exception of bicarbonate ion, all other de-termined ions decrease between April a n d September.
Because n i t r a t e is entirely contained i n t h e soil solution, a n d is r a p i d l y absorbed b y plants, t h e r a t e at w h i c h it is replenished is m o r e i m p o r t a n t t h a n t h e total a m o u n t of nitrate nitrogen present in t h e soil.
Concentrations of phosphate i n t h e soil solution a r e low—of t h e order of 1 p p m of solution. I n order to m a i n t a i n the phosphate supply to growing plants it is necessary to postulate complete r e n e w a l of t h e soil solution phosphate at least ten times daily.
Studies with solution potassium a n d p l a n t uptake indicate t h a t water-soluble potassium alone is insufficient for m a i n t e n a n c e of adequate plant growth.
B . E X C H A N G E A B L E C A T I O N S
T h e relative quantities, as m i l l i g r a m equivalents, of t h e major ex-changeable bases in soils follow the order, calcium > m a g n e s i u m >
potassium. T h e content of exchangeable sodium m a y be either larger or smaller t h a n t h a t of potassium. T h i s is shown in T a b l e V, w h i c h gives a s u m m a r y of results of different investigators. Soils of h u m i d regions m a y contain substantial quantities of exchangeable hydrogen a n d a l u m i n u m , so t h a t the degree of base saturation is less t h a n 1 0 0 % .
T A B L E V
EXCHANGEABLE BASES I N MILLIGRAM EQUIVALENTS IN TYPICAL SOILS"
Soils Ca Mg κ Na Authority
25 Dutch soils 30 5.0 0 .8 2.5 D. J. Hissink 17 Scottish soils 9.95 0.78 0 .24 0.27 A. M. Smith
7 Neutral soils, U.S.A. 13.92 4.83 0 .75 1.48 W. P. Kelley and S. M. Brown 5 Alkali soils, U.S.A. 0.0 0.80 1 .65 6.88 W. P. Kelley and
S. M. Brown 6 Acid soils, U.S.A. 1.06 0.68 0 .13 0.51 W. P. Kelley and
S. M. Brown
aF r o m Robinson (204).
As exchangeable bases are released to the soil solution, a n d removed by p l a n t uptake, t h e y are continually being replenished from nonex-changeable sources a n d minerals. Illite a n d montmorillonite clays con-tain nonexchangeable m a g n e s i u m , a n d illite concon-tains nonexchangeable potassium. Kaolinite is of no value as a source of these nutrients. T h e ease w i t h w h i c h a particular cation is released to t h e soil solution de-pends u p o n the n a t u r e of the c o m p l e m e n t a r y ions present on the colloid.
T h e behavior of t h e individual cations in this respect is described b y t h e complementary-ion principle, w h i c h m a y be stated as follows: " T h e proportionate release of a given cation from the exchangeable form to the solution in a n incomplete exchange reaction increases w i t h the in-creasing strength of bonding of the c o m p l e m e n t a r y exchangeable cations" ( 2 6 ) .
T h e effect of c o m p l e m e n t a r y ions on t h e release of a given ion is illustrated in Table V I from data of Jarusov ( 1 1 8 ) . I n this experiment 5 m e q of a m m o n i u m chloride was added to soils, each sample of w h i c h
contained 0.5 m e q of exchangeable calcium a n d 0.5 m e q of a comple-m e n t a r y ion. T h e displacecomple-ment of t h e exchangeable calciucomple-m was greatest w i t h hydrogen, intermediate w i t h m a g n e s i u m , a n d least with sodium as t h e c o m p l e m e n t a r y ion. By inference, h y d r o g e n w a s attached most strongly a n d sodium least strongly.
T h e over-all c o m p l e m e n t a r y ion effects in n a t u r a l soils t h a t contain a variety of cations m a y be s u m m a r i z e d by the series N a > Κ > M g >
Ca, w h e r e sodium is released most readily a n d calcium least readily in
T A B L E V I
INFLUENCE OF THE COMPLEMENTARY ION ON THE RELEASE OF EXCHANGEABLE CALCIUM FROM A CHERNOZEM SOIL0
Exchangeable cations in soil sample
(meq)
Amount of calcium displaced from soil Exchangeable cations in
soil sample
(meq) Milligram equivalents Per cent of total
0.5 Ca + 0 . 5 Η 0.30 60
0.5 Ca + 0 . 5 Mg 0.18 36
0.5 Ca + 0.5 Na 0.09 19
"From Jarusov (118).
T A B L E V I I
CALCIUM CONTENT OF WHEAT SEEDLINGS GROWN ON SOIL SATURATED WITH CALCIUM OR WITH CALCIUM AND DIFFERENT COMPLEMENTARY IONS
Exchangeable cations Calcium in seedlings
in soil sample (mg)
100% Ca 9.7 60% Ca + 4 0 % Η 8.6
60% Ca + 4 0 % Mg 8.1 60% Ca + 40% Na 5.2 Control (sand without soil) 5.2
β After Black (26) from data of Ratner.
a n incomplete exchange. T h u s sodium is released m o r e readily if it is accompanied b y a high proportion of calcium t h a n b y a high proportion of potassium; m a g n e s i u m is released m o r e readily if it is accompanied b y a high proportion of calcium t h a n of potassium. T h i s explains in p a r t the effect of high exchangeable K : M g soil ratios on the induce-m e n t of induce-m a g n e s i u induce-m deficiency in crops.
T a b l e V I I shows the u p t a k e of calcium b y plants from soil in w h i c h a fixed q u a n t i t y of exchangeable calcium was accompanied b y different c o m p l e m e n t a r y ions. T h e u p t a k e of calcium was greater w i t h hydrogen
t h a n w i t h m a g n e s i u m as t h e c o m p l e m e n t a r y cation, a n d greater w i t h m a g n e s i u m t h a n w i t h sodium.
M a n y attempts h a v e been m a d e to relate t h e exchangeable cations in soils to their u p t a k e b y crops, a n d to t h e response b y crops of further additions of cations as fertilizers. B r a y (42) studied the quantitative relation of exchangeable potassium to crop yields a n d of crop response to potash additions. T h e potassium extraction technique consisted of shaking 5 g m of soil with 10 m l of either 2 2 % N a C 1 04 or N a N 03 in w a t e r for 1 m i n u t e , followed b y filtration. H e found only a fair cor-relation between t h e increase in corn (Zea mays) yield obtained w i t h potash fertilizers a n d t h e total exchangeable potassium in the surface soil, expressed in pounds per 2,000,000 pounds of soil. If, however, the yields from t h e nonfertilized plots, expressed as a percentage of the yields from the fertilized plots, w e r e plotted against t h e exchangeable potassium, a better correlation was obtained. T h e curve could be ex-pressed b y a modified Mitscherlich equation
Log (A — y) = Log A — C i & i
w h e r e cx = t h e proportionality constant, bx = a m o u n t of n u t r i e n t in the surface soil as m e a s u r e d b y t h e soil test, A = yield w h e n potash is not deficient, y = yield w h e n no potash is added.
T h e above relationship holds w i t h approximately the same value for C i w h e r e m a n y physical a n d chemical soil properties v a r y w i t h i n a r a t h e r w i d e r a n g e a n d w h e r e t h e u l t i m a t e yields u n d e r full t r e a t m e n t also v a r y considerably.
H a r d i n g (94) studied the relationship between exchangeable K, N a , M g , a n d Ca in t h e soils of California orange (Citrus sinensis) orchards, a n d in leaf composition. A better correlation was found between per-centage saturation of K, N a , a n d M g in the soil colloids t h a n between t h e actual concentrations in t h e soil, expressed as m i l l i g r a m equivalents, a n d the respective concentrations in the leaf tissue (see T a b l e V I I I ) . T h e most extensive work on predicting t h e proportionate content of bases in plants from m e a s u r e m e n t s on soils has been done b y Mehlich a n d his co-workers in N o r t h Carolina ( 1 6 2 ) . T h e y showed that the cation content of plants is related to: (a) the concentration of m e t a l cations in t h e exchange complex, (b) t h e distribution of cations present, (c) t h e relative e n e r g y w i t h w h i c h the cations are retained, a n d (d) the total a n d proportionate cation r e q u i r e m e n t of different p l a n t species.
T h e cation content of plants should be predictable if the characteristics of (d) a r e k n o w n a n d t h e soil properties u n d e r (a) to (c) are deter-mined. T h e y tested out their concepts w i t h Crotalaria striata g r o w n on five soils, of v a r y i n g cation exchange capacity, t y p e of colloid, a n d
C a : M g a n d C a : Κ ratios. T h e proportionate content of t h e various bases i n Crotalaria w e r e estimated from t h e soil m e a s u r e m e n t s b y m e a n s of a n equation, w h i c h m a y be simplified to t h e following form for cal-cium ( 2 6 ) :
Capiant C a n c i
Capiant + Mgplant + Kplant C aHC l + « M g H C l + j8KHCl
w h e r e C apia nt , M gpia n t, a n d Kpia nt r e p r e s e n t t h e m i l l i g r a m equivalents of t h e respective bases p e r 100 g m of p l a n t m a t e r i a l ; w h e r e CaHci5 Mgnci, a n d KHc i represent t h e m i l l i g r a m equivalents of t h e respective
T A B L E V I I I
CORRELATION BETWEEN CATION CONTENT OF LEAVES AND OF SOIL FROM 72 CALIFORNIAN ORANGE ORCHARDS"
Soil
Cations in leaves Cation Determination
Depth (inches)
Correlation coefficient (r)
Κ Κ % Saturation 0-6 + 0 . 6 6 9b
Κ Κ % Saturation 6-18 + 0 . 7 ΙΟ6
Κ Κ meq/100 gm 6-18 + 0 . 1 8 3
Na Na % Saturation 0-6 + 0 . 5 6 66
Na Na % Saturation 6-18 + 0 . 7 3 4b
Na Na meq/100 gm 6-18 + 0 . 5 1 36
Mg Mg % Saturation 6-18 + 0 . 4 9 36
Mg Mg meq/100 gm 6-18 + 0 . 3 3 76
Ca Ca % Saturation 6-18 - 0 . 0 3 56 aF r o m Harding (94).
6 Significant at 0.1% level.
bases i n t h e extract b y shaking 100 g m soil for 15 m i n u t e s w i t h 500 m l of w a t e r containing 1 m e q of hydrochloric acid; a n d w h e r e a a n d β a r e constants. A n analogous equation w a s used to estimate t h e propor-tionate content of t h e other bases i n t h e plants. A high correlation for Ca ( r = 0.98) a n d M g ( r = 0.98) between calculated a n d determined values w a s obtained. T h e calculated a n d determined values for Ca a n d M g failed to agree w h e n e v e r t h e percentage Ca saturation w a s too low for o p t i m u m p l a n t growth.
I t is generally recognized t h a t t h e exchangeable cations act as a source of readily available p l a n t n u t r i e n t s , b u t t h e r e is some difference of opinion as to their mode of transfer from t h e soil colloids to t h e root surfaces. T h e e a r l y workers regarded t h e exchangeable bases as being in equilibrium w i t h t h e soil solution: as n u t r i e n t s w e r e w i t h d r a w n from the soil solution b y p l a n t roots, t h e equilibrium w a s restored b y
ex-changeable ions going into solution. J e n n y a n d Overstreet (122) ques-tioned this concept of nutrition in soils. T h e y pointed out t h a t the ca-tions on t h e surface of clays a r e not held rigidly. As a result of t h e r m a l agitation t h e y oscillate a n d at times m a y be at a considerable distance from t h e surface, b u t t h e y r e m a i n in t h e field of force e m a n a t i n g from the colloid. Although the ions are surrounded b y w a t e r molecules, t h e y are not in solution in t h e sense t h a t t h e y can diffuse freely. J e n n y and Overstreet postulated t h a t w h e n a root surface makes intimate contact w i t h soil colloids, interchange of ions takes place b y contact exchange, i.e., without t h e ions necessarily going into solution. H y d r o g e n ions on the root surface a r e exchanged for cations on t h e soil colloid. J e n n y
(121) quotes evidence showing t h a t the uptake of radioactive sodium,
Meq Να added Meq NH4added Meq Κ added
FIG. 11. Comparison of cation uptake by roots from clay suspensions and salt solu-tions having equal cation content. From Jenny (121).
at higher concentrations, is decidedly greater in clay suspensions t h a n in chloride or bicarbonate solutions of equal concentrations. T h e u p -take of a m m o n i u m b y t h e roots is n e a r l y t h e same for the two systems, but potassium chloride provides a better source of potassium t h a n potassium clay (see Fig. 11).
I n a recent paper, Olsen a n d Peech (180) described experiments which cast doubt on the validity of t h e contact theory. T h e y tested the significance of the suspension effect (greater cation concentrations or activity in t h e soil suspension t h a n in t h e equilibrium dialyzate) in determining the uptake of cations b y p l a n t roots b y comparing the rate of u p t a k e of R b+ a n d C a+ f b y excised roots of barley (Hordeum vulgare) a n d m u n g beans (Phaseolus aureus) from a suspension of clay, or cation exchange resin, w i t h t h a t from t h e corresponding equilibrium dialyzate. T h e y found t h a t although t h e cation concentration of the clay, or resin suspension, greatly exceeded t h a t of t h e corresponding equilibrium dialyzate, the r a t e of u p t a k e of R b+ a n d C a+ + b y t h e roots
was exactly t h e same from both t h e suspension a n d dialyzate. T h e results for C a+ + a r e given i n T a b l e I X . These results a r e a t variance w i t h t h e prediction of t h e contact-exchange theory. Olsen a n d Peech concluded t h a t t h e composition of t h e soil solution, or t h e equilibrium dialyzate, should completely characterize t h e ionic e n v i r o n m e n t of plant roots i n soil-water systems. T h i s conclusion does n o t necessarily m i n i m i z e t h e value of assessing t h e exchangeable cations i n soil, for t h e y constitute t h e immediate reserve supply a n d determine to w h a t
T A B L E I X
T H E ABSORPTION OF C A++ BY EXCISED M U N G BEAN {Phaseolus aureus) ROOTS FROM CLAY AND RESIN SUSPENSIONS AND THE EQUILIBRIUM DIALYZATE0
Concentration of C a+ + in Relative uptake of Dialyzate Suspension Concentra- C a+ + by Suspension (mg/liter) (mg/liter) tion ratio6 rootsc
2% Kaolinite 0.60 32 53 0.94
2% Montmorillonite 0.60 400 670 1.06
2% Amberlite IR-120 0.60 660 1100 1.03
2% Amberlite IRC-50 0.60 3960 6500 1.26
aF r o m Olsen and Peech (180).
6 Ratio of concentration of Ca4+ in the suspension to that in the dialyzate.
0 The amount of Ca++ absorbed by roots from the suspension divided by the Ca++
absorbed from the dialyzate.
extent t h e low concentration i n t h e soil solution will b e replenished a n d m a i n t a i n e d u p o n removal b y the plant roots.
C . C A T I O N E X C H A N G E P R O P E R T I E S O F R O O T S
T h e F r e n c h chemist Devaux, 1 9 1 6 ( 1 6 3 ) , w a s t h e first to report t h e existence of root cation exchange properties; h e attributed it to t h e pres-ence of pectose i n t h e walls of t h e root hairs. M o r e recently, cation ex-change capacities have been m e a s u r e d b y a n u m b e r of different tech-niques ( 2 7 7 ) . I n general, t h e cation exchange values a r e m u c h higher for dicotyledons t h a n for monocotyledons (see Table X ) . Values r a n g i n g from 9 m e q for w h e a t to 9 4 m e q p e r 1 0 0 g m d r y m a t t e r for larkspur h a v e been recorded b y M e h l i c h a n d D r a k e ( 1 6 3 ) . Roots w i t h high ex-change values have been shown to h a v e bonding energies for calcium t h a t a r e m o r e t h a n double t h e bonding e n e r g y for potassium. T h e higher t h e exchange capacity of roots, t h e greater is t h e relative adsorp-tion of calcium over potassium. T h e root colloid a n d t h e soil colloid compete for cations, a n d t h e cation u p t a k e b y t h e p l a n t depends p a r t l y upon t h e relative exchange capacities of t h e root a n d soil colloid ( 7 1 ) .
T h e valency effect can be nullified by greatly increasing the cation concentration of t h e solution a n d enhanced b y dilution. Schuffelen
(211) demonstrated the valency a n d dilution effect in relation to the u p t a k e of potassium a n d m a g n e s i u m b y fruit trees. I n a wet spring m a g n e s i u m deficiency is greater t h a n in a d r y spring. According to the
T A B L E X
T H E CATION EXCHANGE CAPACITY OF PLANT ROOTS I N MILLIEQUIVALENTS PER 100 GM DRY MATTER"
Cation Cation exchange exchange Dicotyledons capacity Monocotyledons capacity
Larkspur, Delphinium ajacis 94 .0 Orchard grass, Dactylis glomerata 24 .9 Lettuce, Lactuca sativa 65 .1 Timothy, Phleum pratense 22 .6 Soybean, Glycine max 65 .1 Oats, Avena sativa 22 .8 Blue lupin, Lupinus angustifolius 53 .3 Red top, Agrostis alba 17. 3 Carrot, Daucus carota var. sativa 51. .7 Rosen rye, Secale cereale 15. ,1 Red clover, Trifolium pratense 47. 5 Barley, Hordeum vulgare 12. .3 Buckwheat, Fagopyrum
esculentum 39 .6 Millet, Panicum miliaceum 12 .2 Tomato, Lycopersicon esculentum 34.6 Winter wheat, Triticum vulgare 9. 0
α From Mehlich and Drake (163).
T A B L E X I
T H E EFFECTS OF MOISTURE CONTENT ON THE RATIO OF K : M G IN THE SOIL SOLUTION"
Sandy soil Clay soil
Moisture content K: Mg Moisture content K: Mg
15% 0.50 11 0.21
30 1.45 17 0.20
60 1.69 22 0.42
28 0.45
"From Schuffelen (211).
D o n n a n r u l e the ratio of K : M g in the soil solution should be higher in a wet soil t h a n in a d r y soil of t h e same composition. This was con-firmed experimentally ( T a b l e X I ) . A similar effect is noted with
"sand d r o w n , " a m a g n e s i u m deficiency of tobacco on light soils.
Cation u p t a k e b y plants from n u t r i e n t solutions is not subject to this i m p o r t a n t competition of soil colloid with p l a n t root colloid for adsorbed cations.
D . BONDING ENERGY OF CLAYS
It has been shown t h a t a specific cation on a given clay m i n e r a l can be held w i t h a wide r a n g e of bonding energies ( 1 5 7 ) (see Fig. 1 2 ) . Although t h e activity of C a+ + in a H+- C a+ + montmorillonite clay changes v e r y little from 3 0 to 7 0 % saturation, it almost triples as the calcium
1300 1200
Δ Ρ 1100 (calories |0 00
P e r, A9 0 0 equivalent)
8 0 0 7 0 0 6 0 0 5 0 0
t
τ-•χ.
a* 10"20 40 60 80 100 120 140 MilMequivalents C a ( 0 H L per 100q.clay
FIG. 12. Clay titration curves for 1.07% Wyoming bentonite with calcium hydroxide. A, pH titration curve. B, Calcium ion activity plotted against base added.
C, Mean free energy of calcium ions (per equivalent) plotted against base added.
From Marshall (157).
saturation increases from 7 0 to 9 0 % . A given clay, such as montmoril-lonite, has different bonding energies for different cations, and different clays h a v e widely different bonding energies for t h e same cation.
F u r t h e r m o r e the c o m p l e m e n t a r y ion has a n i m p o r t a n t effect on bond-ing energies ( 1 6 3 ) .
E . PHOSPHORUS N U T R I T I O N
T h e phosphorus nutrition of plants in soil has been studied inten-sively ( 6 4 ) . Studies using culture solutions have shown that p l a n t
growth is retarded with solution concentrations of 0.1 p p m phosphorus or less. On the other h a n d , crops appear to grow n o r m a l l y on soils w h e r e t h e concentration of phosphorus in t h e soil solution is less t h a n 0.1 p p m . This m a y be due to one or m o r e reasons: (a) T h e displaced solution is not t h e t r u e solution, (b) T h e p l a n t roots exert a solvent action on the soil particles a n d thus b r i n g m o r e phosphorus into solu-tion, (c) T h e r e is a higher concentration of phosphorus at t h e solid-solution interphase t h a n in the displaced soil solid-solution.
Fried a n d Shapiro (79) consider t h a t t h e u p t a k e of phosphorus b y plants from a soil system m a y be divided into four stages: first, t h e release of the phosphate ion from t h e solid phase into t h e soil solution;
second, t h e m o v e m e n t of the phosphate ion from a n y point in t h e soil solution to t h e vicinity of the root; third, t h e m o v e m e n t of t h e ion from the vicinity of the root into t h e root; a n d fourth, the m o v e m e n t of t h e phosphate ion to t h e top of the plant. E a c h of these steps involves r a t e constants, a n d the over-all process m a y be rate-limiting at a n y of the transfer points. T h e soil solution phosphorus in agricultural soils m u s t be renewed m a n y times per d a y b y the solid phase in order to give the total u p t a k e of phosphorus noted in plants. T h i s ability to r e n e w the phosphorus in solution is a capacity factor. T h e relationship between the intensity factor (concentration in the soil solution) a n d t h e capacity factor, a n d the level of these factors, distinguishes the phosphorus sys-t e m in one soil from sys-the phosphorus syssys-tem in anosys-ther. Shapiro a n d Fried (212) developed a relationship between t h e phosphorus capacity factor (soil-P) a n d the phosphorus intensity factor ( P ) as shown in the equation
w h e r e Κ = a n a p p a r e n t dissociation constant for t h e system at the particular p H specified; 2 soil = a m o u n t of phosphorus adsorbed per g r a m of soil w h e n all the adsorption sites a r e saturated w i t h P ; a n d soil-P = a m o u n t of phosphorus adsorbed p e r g r a m of soil. 5 soil a n d Κ characterize t h e soil a n d p e r m i t t h e calculation of changes in phosphorus concentration w h e n t h e soil phosphorus system is placed u n d e r stress.
It has been shown t h a t organic acids such as citric, oxalic, tartaric, malic, malonic, a n d galacturonic acids are h i g h l y effective in solubiliz-ing phosphate a n d t h a t one or m o r e of these acids m a y be produced b y microorganisms in the rhizosphere. Chelation of calcium, from insoluble calcium phosphates, is also a n i m p o r t a n t factor. It has been shown recently t h a t 2-ketogluconic acid is formed b y certain organisms in the
rhizosphere ( 6 7 ) . This acid is v e r y effective i n chelating calcium, t h e r e b y also liberating phosphorus.
P l a n t roots t h a t possess high bonding energies for calcium, high cation exchange capacities, a n d h i g h acid dissociation would be ex-pected to obtain m o r e phosphorus from insoluble forms t h a n would those roots low in these properties.
F . I O N U P T A K E
U p t a k e of ions b y p l a n t roots h a s been shown to consist of two phases, adsorption a n d accumulation. Adsorption is a physiochemical p h e n o m -enon: it is nonmetabolic a n d is p r e d o m i n a n t l y concerned w i t h cations.
It is considered to be a n exchange process, h y d r o g e n ions generated b y respiration being released into t h e culture m e d i u m i n exchange for
cations. Accumulation, however, is dependent on respiration a n d is concerned w i t h t h e active u p t a k e of both cations a n d anions. I n t h e in-terpretation of active u p t a k e of ions, specific carrier compounds or sites h a v e been invoked, b u t their n a t u r e is still u n k n o w n , although various suggestions h a v e been m a d e , such as phosphorylated energy-rich nitro-gen compounds a n d ribonucleoproteins. T o w h a t extent accumulation is dependent on adsorption is not certain, b u t according to Laties ( 1 4 1 ) , adsorption exchange as w e k n o w it h a s little or n o t h i n g to do w i t h ac-cumulation. This general subject is dealt w i t h i n detail b y Steward a n d Sutcliffe (Vol. I I , C h a p t e r 4 ) ( 2 3 1 ) .
VI. Methods for Determining the Nutrient Requirement of Crops in the Field