Plan t and Soil 50, 363-370 (1978) Ms. 3607

THE Q U A N T I T Y - I N T E N S I T Y RELATIONS OF POTASSIUM IN SOILS FROM PLOTS HAVING

NINE F I X E D CROP ROTATIONS FOR SIX Y E A R S

by BIJAY SINGH, K. N. SHARMA and D. S. RANA

Department of Soils, Punjab Agricultural University,

Ludhiana 141004, India

SUMMARY

Soil samples collected 6 years af ter rais ing of var ious cereals, pulses, oil seed and tube r crops in nine f ixed ro ta t ions were used to s tudy the quan t i ty - in tens i ty re la t ions of potass ium. Potass i tun ac t i v i t y ratio, where the soil ne i ther gains nor looses K, was correla ted in a h ighly signif icant manner (P ~ 0.01) wi th the to ta l a m o u n t of K fert i l izer applied dur ing 6 years. The K buffer ing capaci ty , the slope of the Q/I curve, when the soil nei ther gains nor looses K, was pos i t ive ly correla ted (P = 0.05) wi th K sa tura t ion of the to t a l and inorganic cat ion exchange capaci t ies to a s imilar extent . Super- imposing Q/I curves showed no appreciable difference be tween samples f rom different t rea tments . Desorp t ion of po tass ium wi th increasing soil : solution (0.01M CaC12) rat io followed a Langmui r type of equa t ion and suppor ted the conclusions drawn from quan t i t y - in t ens i ty curves.

INTRODUCTION

The potassium quantity/intensity (Q/I) relationship of a soil relates change (+AK) in the exchangeable potassium (Q) to acti- vity ratio 12 (AR = aK/a i ca + M,). The addition or removal of quan- tities of labile potassium in fertilizer or by cropping change AR e (activity ratio at AK = 0) in the directions expected. But the amount of K that must be added to cause a given change in AR e in a soil is indicated by Q/I relation, of which the gradient represents the buffering capacity of the soil.

This paper describes measurements on soils taken from plots of a 6 years old experiment consisting of nine fixed crop rotations. An attempt has been made in explaining ARe and buffering capacity

364 BIJAY SINGH, K. N. SHARMA AND D. S. RANA

in t e r m s of f e r t i l i ze r p o t a s s i u m be ing a d d e d a n d soil p a r a m e t e r s .

S o m e d a t a on d e s o r p t i o n of p o t a s s i u m f r o m the soil e x c h a n g e c o m -

p l e x h a v e also been inc luded .

MATERIALS AND METHODS

Soil samples were taken from 9 treatment plots of a long term experiment progressing at Punjab Agricultural University farm, Ludhiana. The treat- ments consisted of nine fixed crop rotations as described in Table 1. The ex- periment was started in June-July 1970 with 9 different crops in a Tolewal loamy sand (Typic ustochrept). Rates of fertilization to various crops are given in Table 2.

Soil (0-15 cm), at the start of the experiment consisted of 79~/o sand, 16% silt and 5% clay; with textural class as loamy sand. It tested pH (1:2 soil water ratio) 8.4, electrical conductivity ( 1 : 2 soil water ratio) 0.20 mmhos/cm, organic carbon la 0.28%, cation exchange capacity 5 (at pH 8.1) 6.23 meq/100g of soil and exchangeable potassium 0.17 meq/100g of soil. The soil samples were taken from 0-15 and 15-30 cm depths after 6 cycles of each of the nine rotations. These were air dried and subjected to the following measurements.

The Q/I relations were investigated following the methods suggested by B e c k e t t L Soil samples were equilibrated in 2.5 g portions with 25 ml ot 0.01 ]ll CaCI~ containing a range of potassium concentrations from nil to 120 mg K/1. The suspensions were shaken for an hour and equilibrium solu- tions were separated by filtration. The filtrates were analysed for potassium by flame photometry and for calcium and magnesium by t i t rat ing with 0.01 N EDTA. From the initial and final concentrations of K, Ca and Mg, the equi- librium activity ratio and the amount of potassium gained or lost by the soil were calculated for each suspension. From these data graphs have been plott- ed to show the Q/I relations for the top soils (0-15 cm) and subsoils (15-30 cm) for each of the nine sites.

Exchangeable I( was estimated by shaking for an hour 5 g soil with 40 ml neutral N ammonium acetate in an erlenmeyer flask. Suspension was filtered into a 250 ml volumetric flask and soil was washed with 25 ml aliquotes of ammonium acetate till volume. Cation exchange capacity including the or- ganic contribution (CECo+~t) was measured at pH 8.1 by B a s c o m b ' s 5 method which uses Ba-Mg exchange. Cation exchange capacity excluding the organic contribution (CEC~t) was measured also at pH 8.1 by a method des- cribed by A d d i s c o t t 2 which uses Ca-K exchange.

The topsoil samples from the nine treatments were also shaken for an hour with 0.01M CaC12 solution in soil solution ratios of 5, 10, 20, 50 and 100 ml/g of the soil. The suspensions were filtered and potassium estimated flame pho- tometrically in the filtrate.

Q/I R E L A T I O N S OF P O T A S S I U M

T A B L E 1

Various crop ro ta t ions and the a m o u n t of po t a s s ium appl ied in 6 years

365

T r e a t m e n t Crop ro t a t i on A m o u n t of

No. K applied (kg K/ha)

T1 Tz

Ta

T4

T5 Ti;

T7

T8

T9

Moog - Maize (g) - 'vVIleat 398

Cowpeas - Maize(g) - W h e a t 398

Maize(f) - Maize(g) - W h e a t 498

Maize(f) - G r o u n d n u t - W h e a t 407

Maize(f) - Soybean - W h e a t 365

Moong - Maize(g) - Po ta to - Po ta to 1538

Fal low - Maize(g) - W h e a t 349

Fal low - G r o u n d n u t - W h e a t 257

i',loong - Maize(g) - Po ta to - W h e a t 927

T A B L E 2

Ra te of fer t i l izat ion (kg]ha) to d i f ferent crops*

Crop N P K

Wheat (Triticum aestivum L. va r . Ka lya ns ona ) 120 26.2

Maize-gra in (Zea mays L. va r . G a n g a 5) 120 26.2

Maize-fodder (Zea mays L. v a t . G a n g a 57 60(100) a 17.4( 13.1)8

P o t a t o (Solanum tuberosum L. va r .

Kufr i C h a n d r a m u k h i ) 180 39.3

Moong (Phaseolus attreus L. va t . Pusa baisakhi) 20(12.5)a 26.2(10.9)a

Cowpeas (Vigna sinensis va t . FOS 68) 20 26.2

Soyb ean (Glycine max va t . Bragg) 20(30)b 26.2

Groundnut(Arachishypogaeavar. M 145) 20( 15)b 26.2(17.4)b

24.9

24.9(49.8)b

24.9

74.4(148.8)a

16.6(0)a

16.6(0)a 16.6(0)b

16.6(20.8)b

* Figures in pa ren thes i s ind ica te changed ra tes a) since Oc tober 1973, b) since J u n e -

J u l y 1974.

R E S U L T S A N D D I S C U S S I O N

Figs. 1 and 2 show the results of visually superimposing the Q/I curves for all plots in the topsoil and subsoil respectively. Treatment numbers on AK axis indicate the positions of AK = 0 for each plot relative to the superimposed curves. There was no discernible dif- ference between the shape of the Q/I curves for the various treat- ments. The difference, however, was observed in the shape of the curves in the topsoil (Fig. 1) and in the subsoil (Fig. 2).

The values of activity ratio at AK = 0 (ARe) for different treat- ments are listed in Table 3. Although this parameter gives an idea of only immediately available potassium, no account can be taken

366 BIJAY SINGIt, K. N. SHARMA AND D. S. RANA

of the possible effects of the fixation or release of non-exchangeable potassium. The high AR e indicates a high intensity of the labile pool and suggests that any potassium residue would be loosely bound and readily available for crop growth or leaching. High ARe in the topsoil suggests more availability of potassium to the crop plants as compared to that in the subsoil. However it may be observed that trend in the values of AR e under different treatments is quite similar in both the layers.

A coefficient of correlation as high as 0.933** (n = 9) has been found to exist between AR e of the top soil and the amount of po- tassium added as fertilizer during the 6 years of the experiment (Table l). It indicates that K fertilization has affected strongly the AR e but not the shape of the Q/I curves. Effect of crop rotation on Q/I relations is not visible, possibly 6 years are too short a time. In- crease in AR e with K fertilization has been observed by H ag in and Baze l e tS and M a c l e a n 11.

Cation exchange capacity measurements with (Ba-Mg exchange) and without (K-Ca exchange) organic contributions did not show any differences among treatments (Table 3). This may be attributed

Fig. 1.

o~ o . - t -o

o t D ~

�9 -- E)'-

~o E

I I I p

0 . 8

0 .6

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~ > I I L c~

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L - ~ i l l

(3) - - 3(i EL- - <3 r 0 0 4 0 8 0 1 2 0 1 6 0

ARK_Ca ( x I 0 -4 ( M/I .) 0.5

The super imposed Q/I curves for 0-15 cm soil samples f rom the nine t r e a t m e n t s .

Q/I R E L A T I O N S OF POTASSIUM 3 6 7

F ig . 2.

o) O

- t - o o 6 -

. - - ( y "

-t- (D ~o E 9 -

0) t.. tn

(1) o >

II ~ - m' �9

x / ~ 8 - ",%

o E

L ~ (1) - - , ' . - -

0 ,4O

Oo e o o I

...:':.:"

I

�9 II

rO.8

0 -6

0.4

0-2

BO 120 0,5

(xi0-4 (M/I.) )

160

ARK_Ca

T h e s u p e r i m p o s e d Q/I c u r v e s fo r 15 -30 c m so i l s a m p l e s f r o m t h e n i n e

t r e a t m e n t s .

to very small organic matter contents of the soils. Also changes in the organic matter content in the subtropical climate which exists at the site of the experiment, are extremely slow.

B e c k e t t ~ defined potential buffering capacity as the slope of the Q/I curve at large activity ratios, where the curve is linear for some soils. However, Q/I curves with clearly defined linear parts do not exist in the present soils, and the slope of the tangent to the Q/I curve when the soil neither gains nor loses K, (dQ/dI) z~K=0, seems to be more useful. It may be called tangential buffering capacity or TBC4. The Q/I curves for the subsoils from each site are slightly steeper than those for the topsoils, indicating a rather high tangen- tial buffering capacity. Data in Table 3 also exhibit this fact. High TBC means that a greater amount of potassium would be needed to increase the readily labile pool of potassium. It may be noted that the highly fertilized soil in t reatment T6 possesses lowest tangential buffering capacity in both layers. Since the shape of the Q/I curves is similar in all the treatments, TBC is bound to decrease with in- creasing AR e . It would mean that those parameters which are re-

~ 6 8 B I J A Y SINGH, K. N. SHARMA AND D. S. RANA

TABLE 3

Act iv i ty ratio at AK = 0, tangent ia l buffering capaci ty (TBC), exchangeable potassium and cation exchange capaci ty (with and without organic contribution) in the topsoil and

subsoil sainples from nine t rea tments

Treat- ARe(XI0 -'t TBC(meq/100g Exch.potass ium CECo+~t CEC~t(meq/ ment (M/I)t) (M/I)-J) (meq/100g soil) (meq/100g) 100g)

Top- Sub- Top- Sub- Top- Sub- Top- Sub- soil soil soil soil soil soil soil soil

Top- Sub- soil soil

T1 22.1 14.2 ..52.0 91.2 0.186 0.166 6.42 6.87 5.23 5.87 T~. 16.2 11.0 57.3 112.5 0.183 0.160 6.80 6.91 5.48 5.79 T3 14.3 10.0 61.3 98.3 0.176 0.163 6.24 6.66 5.03 5.52 T4 28.6 17.2 42.1 82.5 0.183 0.160 6.32 6.73 5.41 5.79 T6 20.8 13.0 48.1 99.8 0.192 0.151 6.43 6.83 5 . 1 , 5 5.68 T6 137.2 28.2 30.6 79.5 0.407 0.192 6.02 6.30 4.75 5.41 T7 25.1 14.1 48.3 88.5 0.183 0.163 6.12 6.33 5.25 5.75 Ts 32.3 22.3 39.3 86.5 0.202 0.170 6.70 6.75 5.47 5.71 T9 87.3 29.0 35.3 74.6 0.314 0.183 5.98 6.16 4.90 5.46

lated to ARe also correlate significantly with TBC, but with an opposite sign.

Tangential buffering capacity is related to the K saturation of the CEC and this relationship was measured previously for soils from various Rothamsted and Woburn experiments 1. A d d i s c o t t 3 also observed that TBC was better related to the K saturation of the CEC when the organic contribution was omitted from the CEC than when it was included, suggesting that K-Ca exchange measured by Q/I curves occurs mainly on the non-organic part of the CEC. In the present study, however, coefficients of correlation with Kex/CEC• and Kex/CEC0 + M were 0.77* and 0.76* respectively; these were almost equal. As stated earlier, it is because of very low organic matter content of the soils under study.

Data pertaining to desorption of potassium from soils of differ- ent treatment plots as a function of soil:solution (0.01 M CaC12) ratio have been analysed according to the L a n g m u i r equationL

v / s -- b/St + V/St (1)

where S is the quanti ty of K desorbed (~ eq/g of soil) into a solution volume V (ml/g of soil), St is the total quanti ty of cation which can be desorbed following the Langmuir equation and b is a constant. The statistical significance of the correlation coefficient for V/S v s V was

Q/I R E L A T I O N S OF P O T A S S I U M 3 6 9

the criterion by which the fit of the desorption data to Eq. 1 was tested. The fit was found to be excellent for all the soils (r = 0.95** to 0.99"*, n = 10).

The Langmuir coefficients b and St calculated from the equations for the best fitting straight lines are recorded in Table 4. These para- meters, like Q/I relations, also characterize the adsorbed potassium in terms of its availability to crop plants or leaching. As K h a s a w - neh and A d a m s 9 have found, St increases when the K content in soil is increased by fertilizations. Hence St can be expected to be a function of K input into a soil compartment. The constant b is a measure of the binding energy of K with the soil. A value b = 0 indicates pure dilution of salt. The greater the value of b, the greater is the strength with which K is adsorbed by the exchange complex. Magnitudes of b and St as shown in Table 4 thus support the con- clusions already drawn from Q/I measurements. A very high value of b in the treatment T~ cannot be interpreted on the basis of pre- sent investigation.

Langmuir coefficients St and b define the soil potassium in terms of the ease with which it is desorbed into a K deficient soil solution. On the other hand, ARe and TBC characterize the soil potassium in terms of its reaching an equilibrium with a soil solution having po- tash potential equal to that of soil K. Therefore, the possibility of a simple geometric relationship between Langmuir and Q/I parameters is small. However, magnitude of the parameter dS/dV is proportio- nal to the amount of readily available and loosely bound K, so that it should correlate with AR e in a significant manner. I t has been

T A B L E 4

Cha rac t e r i za t i on of absorbed p o t a s s i u m in topsoi ls of d i f fe ren t t r e a t m e n t s

T r e a t m e n t b (ml/g) St(lxe/g )

T1 20.75 2.58 To. 68.03 4.94 T3 24.88 2.91 T,i 38.95 3.42 Ts 26.96 3.21 To 10.91 5.14 T7 26.65 3.16 T s 24.95 3.22 T 9 17.18 3.82

370 Q/I RELATIONS OF POTASSIUM

found that dS/dV computed at soil: solution ratio of 1 : I0 (the ratio at which Q/I relations have been worked out) has a significant coe- ficient of correlation (0.74*) with AR e. This hypothes i s gets further support from a highly significant coefficient of correlation (0.88**) between AR e and supply parameters. The later quant i ty has been defined by K h a s a w n e h and C o p e 1 a n d 10 to characterize the ability of a soil to supply an adsorbed nutrient. As it includes S, V and dS/dV and has been formulated as (SV) 0.5 (St b) -~ These relations thus suggest that Q/I curves and Langmuir equat ion have character- ized the same fraction of immediate ly available soil potassium through different approaches.

Received 13 October 1977

REFERENCES

1 A d d i s c o t t, T. M., The potassium Q/I relationships of soils given different K manu r ing. J. Agric. Sci. Camb. 74, 131-137 (1970).

2 A d d i s c o t t , T. M., A note on resolving soil cation exchange capacity into mineral and organic fractions. J. Agric. Sci. Camb. 75, 365-367 (1970).

3 A d d i s c o t t , T. M., Potassium: calcium exchange in soils of tbe Broadbalk experi- ment at Rothamsted. J. Agric. Sci. Camb. 75, 451-457 (1970).

4 A d d i s c o t t , T. M. and T a l i b u d e e n , O., The buffering capacity of potassium reser- ves ill soils. Potash Rev. Subject 4, 45th suite (1969).

5 B a s c o m b , C. L., Rapid method for the determination of cation exchange capacity of calcareous and non-calcareous soils. J. Sci. Food Agric. 15, 821-823 (1964).

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7 B e c k e t t , P. H. T., Potassium potentials. In Soil Potassium and Magnesium, Min- istry of Agriculture, Technical Bulletin No. 14, 32-38, H.M.S.O., London (1967).

9 H a g i n , J. and Baze l e t , M., Potassium status in some fertilized soils. Potash Rev. Subject 4 (1964).

9 K h a s a w n e h , F. E. and A d a m s , F., Effect of dilution on calciunl and potassium contents of soil solutions. Soil Sci. Soc. Am. Proc. 31, 172-176 (1967).

10 K h a s a w n e h , F. E. and C o p e l a n d , J. P., Cotton root growth and uptake of nu- trients. Relation of phosphorus uptake to quantity, intensity and buffering capacity. Soil Sei. Soc. Am. Proc. 37, 250-254 (1973).

11 Mac lean , A. J., Water soluble K, per cent K saturation and pK-�89 (Ca + Mg) as indices of management effects on K status of soils. Trans. 7th Int. Congr. Soil Sci. 3, 86-91 (1960).

12 S c h o f i e l d , R. K., A ratio law governing the equilibrium of cations in the soil solu- tion. Proc. l lth Int. Congr. pure appl. Chem. London 3, 257-261 (1947).

13 Wa lk l ey , A. and Black , I. A., An examination of the Degtjareff method for deter- mining soil organic matter and a proposed modification of the chromic acid titra- tion method. Soil Sci. 37, 29-38 (1934).