P l a n t a n d Soll XV, no. 2

O c t o b e r 1961

NITRATE

DISTRIBUTION

IN TROPICAL

SOILS

II. E X T E N T

OF CAPILLARY ACCUMULATION OF NITRATE DU1RING A L O N G DIRY PE1RIOD

b y R. W E T S E L A A R Division of Land Researeh and Regional Survey, Commollwealth Scientific and Industrial Research Organization, Canberra, A.C.T. Austra]ia

INTRODUCTION

Accumulation of nitrate near the surface of a bare fallow soff at Katherine, N.T., was described in the first of this series of papers ( W e t s e l a a r S ) . It was shown that neither by photo-chemical, physico-chemical, nor b y biological formation could this phenomenon be explained. Accumulation of nitrate b y physical means was suggested. This paper reports investigations on three soff types in the Katherine region - Tippera, Blain and Florina solls. Tippera is a lateritic red earth which has been described in Part I of this series ( W e t s e l a a r a). Florina is a sandy lateritic podzolic soff (St e w a r t3), Blain a sandy lateritic red earth ( S t e w a r t , personal communication), both with a clay content of about 5 per cent in the top soll. EXPERIMENT 1

The changes in nitrate content during the dry season were compared with those of an anion of similar solubility that is not affected biologically. The chloride ion was chosen for this purpose. A t t h e b e g i n n i n g of t h e wer season, D e c e m b e r 1958, s o d i u m chloride a t t h e r a t e of 2000 p o u n d s p e r acre was h a n d b r o a d c a s t e d on t r i p l i c a t e plots, 4 × 12 feet on each soll t y p e . I m m e d i a t e l y a d j a c e n t t o t h e s e p]ots (the "'chloride p l o t s " ) were t r i p l i c a t e plots, 8 × 12 feet, receiving no salt (the ù n i t r a t e p l o t s " ) . I n o r d e r to avoid chloride c o n t a m i n a t i o n f r o m t h e NaC1

- -

121

- -

122

R. WETSELAAR

application during broßdcasting and subsequent possible horizontal diffusion in the soll, chloride and nitrate plots were not randomized among each other. The cultivation history for all plots and for all soil types was exactly the same. Each had been bare Iallow for orte year, after clearing from virgin land the previous season. On 1st December, 1958, all plots were ploughed to 8 inches, and then harrowed. Two days later NaC1 was applied to the chloride plots. No further cultivation took place during the wet season. Weeds »vere controlled manually. Three sarnples per plot were colleeted for chloride and nitrate on the respective plots during the dry season from the 0-1, 1-2, 2-3, 3-6, 6-9, and 9-12 inch soil levels, using tubes diminishing in diameter with increasing depth. For each plot the samples were bulked, mixed, and sub-sampled in the field. On each sampling date the Florina samples were collected between 8 and 9, the Blain samples between 9 and 10, and the Tippera samples between 10 and 11 a.m. Extraction for soil nitrate took place within 4 hours after sampling on 10-g subsamples. Nitrate was estimated by the method described in Part I of the series (~Wetselaar 5). The rest of the subsample was used to determine soil watet content gravimetrically. The chloride subsamples were first oven dried at I05°C to determine soil watet content. The dry soil was then ground and mixed, 20 ml distilled watet was added to a 10-g subsample, ~ollowed by shaking for one hour. Chloride concentration was estimated in the supernatant, using a modification of the method of S t e r n and S c h w a c h m a n 4 The first sampling took place on 21st April, when it was expected that the rains for the season w0uld have finished. However, on 18th ~ a y Tippera received 0.08 inch rain, and on 24th May Tippera received 0185 inch, Blain 0.51 inch, and Florina 0.54 inch. The final sampling was on 21st July. Disturbance of t h e soil crust during sampling was avoided by working Irom planks which rested on the soil on the edges of the plots. The sampling holes were filled up again with the residue of the bulked samples after sub-sampling. The holes were then marked to avoid re-sampling in the same position. C h a n g e s i n n i t r a t e , c h l o r i d e a n d soil w a t e r c o n t e n t b e t w e e n 21st A p r i l a n d 21st J u l y o n T i p p e r a , F l o r i n a , a n d B l a i n solls a r e s h o w n i n F i g u r e s 1, 2, a n d 3 r e s p e c t i v e ! y . F o r B l a i n , c h l o r i d e c o n t e n t a n d soil w a t e r c o n t e n t o n c h l o r i d e p l o t s are o m i t t e d , since i t w a s f o u n d t h a t p r a c t i c a l l y all c h l o r i d e a p p l i e d h a d b e e n l e a c h e d o u t of t h e t o p t w o Ieet of t h i s soil d u r i n g t h e w e t season. T h e r e s i d u a l c h l o r i d e f l u c t u a t e d b e t w e e n 6 a n d 10 p p m a t all s a m p l i n g d e p t h s . T h i s diff e r e n c e w a s too s m a l l to e v a l u a t e t r e n d s of a c c u m u l a t i o n a t a n y depth. The s a m p l i n g period covered three phases, a d r y i n g phase from 21st A p r i l u n t i l 13th M a y , a r a i n y p e r i o d f r o m 13th M a y t o 2 8 t h

123

NITRATE DISTRIBUTION IN TROPICAL SOILS. II NO~-(ppm ove»dry soil) IOO

200

S.E. 300

-SO

wot~r(%

ovendry 5oil)

"~-,~\/ 5

+50

I0

15

S.E. 20_~

+1

x\~, \\ \\ \\'~,

,ii

\

\ ',,'t:

i,, ,dL .....

w~ter(.% ovendry soir) S.E. 5 I0 15 20-I +t

\ä

"ii')~\\\

/

?

..........

9

....... . . . .

21 April J3 MOg 2B May 23 J u n e

6 9

21 July

2

C

:g~,iZù2,otù'l~ ! I i I[....... p~o,,, D

Fig. 1. Nitrate (A) and chloride (C) stratification in the top foot of bare fallow Tipera clay loam, together with changes in soil water on nitrate (B) and chloride (D) plots, during the 1959 dry season.

May, and a second drying phase from 28th May to 21st July. The Tippera soll dried to approximately the water content at 15 atm to 9 inches by 13th May. The May rain wetted the soil to 6 inches. This water was lost through evaporation by 23rd June, when the water content profile approached that of 13th May. By 21st July the whole 12-inch profile was below the 15-atm level. Florina and Blain showed similar eycles, but data on 15-atm level and Iield capacity are not yet available for these solls. On Tippera soll, changes in nitrate and chloride content with time were significant in the 0-1, 2-3, and 3-6-inch zones, and changes in ehloride were also significant in the 9-12-inch zone (Figures lA

124

R. W E T S E L A A R

0

NO~'~ppm oV•ndr7 5 I0

soll) 15

w~tert'O/o 2.5

S. E, 2 0 _B

+5

own¢lry soH~j $,E. 5 7.5 _t +1 J

°-

i

i

;r ,»" / ] I I / /

'

•

'

i,/,,i

i,ii\\ -

L7~

._23

Juni

"5

2. I Ju[y

9

•

12 L "

O

°

°

~

«

C I - (ppm ov¢ndry soil) lOO 200 t"

.~...~~"

wat«r(a/o ov«n8ry soil) $. E, +t 2,5 5 ~ 5 _4

S. E, 3 0 0 3(IO

~

+ IIO

,

"

O;~. R,,ä

] ~6

"õ

I! "-, I {i~"-,.. \

]

J

"

"

\,~\ / ',, ~\\/

•

•

"

f

~:'t'A/ \i

i

"

\

• Xi:,,

õ]i\

,

"

g

2 2C

2D

Fig. 2. Nitrate (A) and chloride (C) stratification in the top foot of bare fallow F]orina soil, together with changes in soll watet on nitrate (B) and chloride (D) plots, during the 1959 d r y season,

.......

N O Z ( p p m ovlnd~y soi O 20 40 60

ù,. \. ........ - ~ ".._.~_~~" V/ ,

i

!!.'\. /Y Ih/~

Ic,

•

t - -

S.E.

eC _,2 eo

,

+'F

water(°B cvRndry soH) S.E. 2 4 6 -0.5 +O.S

O

~~~.%~-,, \

;i\,7,+

................. 28 May

~3J,,ù« 21July

A

B

Fig. 3. Nitrate (A) and soil watet (B) stratification in the top foot of bare fallow Blain soil, during the 1959 dry season.

NITRATE

DISTRIBUTION

125

I N T R O P I C A L SOLLS. I I

and 1C). Significant changes and trends for the drying phases and the rainy period are shown in Tables 1 and 2 respectively. TABLE

1

E f f e e t of d r y i n g cycles (21.IV- 13.V 1958 a n d 2 8 . V - 2 1 . V I I I 1958) on c h a n g e s in n i t r a t e a n d ehloride c o n t e n t in e a c h s a m p l i n g l a y e r on each soll t y p e . T a b l e shows depth(s) of soll l a y e r s 0 in inches w h e r e a e h a n g e has been o b s e r v e d as i n d i e a t e d in the first c o l u m n Chloride

Cha•ges in . . . . SoiI t y p e . . . . .

Tippera

Dryingphase . . . S i g n i f i e a n t * increase . . . . . . T r e n d t o w a r d s increase . . . . . .

2nd

0-1

0-1

Ist

.

.

.

.

.

0-3 .

.

.

6-9

.

.

.

.

.

.

.

.

.

.

.

2rld

Florina

Tippera 1st

2nd

1st

0-1

1-2

.

Trend towards decrease . . . . Significant * deerease . . . . .

1st

Nitrate

_

Florina

.

1-2 3-6

.

.

.

.

.

.

.

.

0-6

0-2

9-1;

2-1~

Blain Ist

2nd 1-2

1-2

0-2

0-1

0-1 2-3

2-12

2-3

3-1"-

2nd 1-2 0-1

.

5-12

1-2 5-12

36

3-6

2-6

5-I7~

* P = 0.05 TABLE 2 E I f e e t of r a i n b e t w e e n 13.V a n d 28.V 1958 oi1 c h a n g e s in n i t r a t e a n d ehloride e o n t e n t in e a c h s a m p l i n g l a y e r ( d e p t h in inehes on e a c h soll t y p e C h a n g e s in . . . . . . . . . . .

Chloride

Soll t y p e . . . . . . . . . . . .

Tippera

Significant * increase . . . . . . T r e n d t o w a r d s inerease . . . . . .

2-3 1-2

Florina 3-12

Nitrate Tippera 2-6 0-2 6-12

Florina 2-6 1-2 6-12

Blain I

2-6 6-12

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

T r e n d t o w a r d s decrease

0-1 2-3

. . . . .

Signifieant * deerease . . . . . .

0-1

•

0-1 I-2

* P = 0.05

During the first drying phase, nitrate and chloride content on Tippera increased in the top 2 inches and decreased in the lower layers. Following the May rain, chloride decreased in the top 1 inch and increased in the 1-3-inch layer, but nitrate content tended to inerease throughout the profile.In the second drying phase, both nitrate and chloride again increased in the top 1 inch and decreased in the lower layers. The pattern of change on Florina soll was very similar to that on Tippera (Figures 2A and 2C).

126

R. WETSELAAR

On Blain soil (Figure 3A) gross changes in nitrate stratification during the drying phases were similar to those on Tippera and Florina, but in this instance the zone of maximum nitrate accumulation was at 1-2 inches depth. Discussion

From the results it is dear that the general eIfect of a drying phase is a marked accumulation of both chloride and nitrate anions near the surface, accompanied by a decrease in the subsurface. The reverse effect, surface depletion and subsurface accumulation caused by rain, is clearly shown with chloride, but does not fully hold for nitrate, since on Tippera and Florina there was no decrease in nitrate in the top inch of the soll after wetting. However the fact that the increase in subsurface anion concentration was rauch greater for nitrate than for chloride indicates t h a t some nitrate was formed locally during the wet period on these soils when water content was adequate. Though the changes in total amount of nitrate and chloride with time in the top 12 inches, given in Table 3, were not significant, total nitrate content was nearly doubled on all three soil types during the wet period between 13th May and 28th May. TABLE 3 Changes in t o t a l n i t r a t e a n d chloride e o n t e n t s (ppm) in top 12 inches ! Soll

i"

Nitrate

Chloride

Tipper~

Bl~in

194 166 312 268 229 33.7 n.s.

38 41 70 6 3 63 9.1

18 11 23 26 24 3.4

Il.S.

il.S.

Florina

Tippera

Florina

Date 21.IV/

[ l a.:v ~ «ry

ù'/28v ] 23.VI ~ dry

21.Vlil S.E

1838 1959

1725 1559 2118 232.6 n.s.

1465 912 1022 622 1252 125.7 n.s.

The effect of rain in redistributing anions can be ascribed to leaching eren if nitrification during the rainy period is taken into account, since changes in stratification of the chloride ion, which is not subjeet to biological action, were parallel to those of the nitrate ion (Figures lA and IC). Both the ehloride and nitrate

N I T R A T E D I S T R I B U T I O N IN TROPICAL SOILS. I I

127

stratifications at 28th May are in agreement with the results of other experiments (Wetselaar in preparation), in which parallel distribution of these anions with depth was recorded. During the drying phases, total nitrate colltent over the 0-12 in. profile did not increase (Table 5), indicating that biological nitrate formation ceases altogether under these soil temperature and water conditions. On Tippera soll there was a tendency f o r the total nitrate coritent to decrease. A significant reduction in total nitrate content was showI1 under laboratory conditions in dry Tippera soil exposed to high temperatures ( W e t s e l a a r 5). Detailed triplicate soll temperature measurements at the surface and at ~,1 1, 1!2, 2, 3, and 6 inches depth, using thermistors, revealed that during the sampling period maximum temperatnres at 1½ inch or below never exceeded 40°C. Assuming that biological nitrate formation can contillue up to this temperature, it is clear that the cessation of biological activ'ity was due to low soll water content. However, during these drying phases, nitrate content increased in the surface and decreased in the subsnrface layers. Thus it appears that redistribution of nitrate takes place during a period in which water content levels are too low for biological nitrate formation, and during which no change in total anion content took place. This and the faet that changes in chloride stratification can parallel to those of nitrate sllggest a common physical callse. The accumulation of chloride and nitrate near the surface durillg drying phases can therefore only be explained in terms of capillary movement upwards of liquid water plus solute along a marked moisture gradient towards the surface, with a consequent decrease in anion content in the subsurface. M a r s h a l l 1, reviewing the literature on upward movement of salts, states that water can move in the liquid phase under very rauch drier conditions than was originally assumed. M a r s h a l l and G u r t ~' obtained a shift of 50 per cent of the chlorides, origillally in the bottom 1 cm, towards the top cm in a 2-cm block of soil at a water content 0nly slightly above the 15-atm level. The shift took place withill 24 hours. The results presented in this paper collfirm this, and seem to indicate that eren below the 15-atm level some liquid movemellt upwards can occur. It can be collcluded therefore that the main cause of nitrate accumulation llear the su~rface during a long dry period is capillary movement upward of the nitrate formed in the subsurface.

128

R. WETSELAAR

Since there is no apparent increase in total chloride content in the top foot during a drying phase (Table 3), all increases in chloride content near the surface can be accounted for as changes within the top foot of soil. Nitrate measurements in the field at the end of the dry season on a fallow soil high in nitrate content invariably revealed a marked decline to near zero at about one foot depth and a subsequent increase in the deeper layers (W e t s e 1a a r, unpublished data). Furthermore, on t h e same fallow soil, water contënt below the 1½-2-ft soiI layer showed scarcely any decrease during the dry season ( A r n d t , personal communieation). Thus it can be concluded that, under Katherine conditions, the upwards movement of soil water in the liquid phase during the dry season is mainly restricted to the top 1½ feet. EXPERIMENT

2

From field observations it seemed that the zone of maximum nitrate accumulation was associated with the base of the soil crust, Iormed b y raindrop puddling. This crust consists of an upper cemented layer, ~-~1 1 inch thick, overlying a layer that is looser and more open in structure and apparently lower in clay content. This texture differentiation is caused b y differences in the speed of settling after rain puddling, between coarse and fine particles. The aim of the experiment was to investigate any relation that might exist between location of physical discontinuity of the soil and location of maximum anion accumulation in the top 2 inches. I n a d d i t i o n t o t h e n i t r a t e plots o n all soll t y p e s a n d t h e chloride p l o t s o n T i p p e r a soll used in E x p e r i m e n t 1, w h i c h h a d n o t b e e n c u l t i v a t e d since t h e p l o u g h i n g a n d h a r r o w i n g in t h e b e g i n n i n g of t h e p r e e e d i n g w e r season, m e a s u r e m e u t s for t h i s e x p e r i m e n t were also carried o u t o n a b l o c k of t h r e e o t h e r chloride plots o n T i p p e r a soil, t h a t h a d b e e n h a r r o w e d r e g u l a r l y u n t i l o n e m o n t h before t h e e n d of t h e w e t - s e a s o n rains. I n t h i s w a y i t b è c a m e possible t o m a k e e o m p a r i s o n b e t w e e n c r u s t f o r m a t i o n o v e r a full w e t season, a n d a f t e r t h e l a s t r a i n s only. To m e a s u r e t h e v a r i a t i o n in p h y s i c a l e o n t i n u i t y a specially d e s i g n e d p e n e t r o m e t e r was used, w h i c h is d e s c r i b e d a n d i l l u s t r a t e d (Figure 4) in t h e App e n d i x . T h e r e a d i n g s of t h i s i n s t r u m e n t r e p r e s e n t t h e m i n i m u m force req u i r e d t o p e n e t r a t e a soll face in h o r i z o n t a l direction, a n d are e x p r e s s e d i n d y n e s c m -2. P e n e t r o m e t e r r e a d i n g s were o b t a i n e d in t h e t o p } i n e h a n d in e a c h } i n c h t o a d e p t h v a r y i n g f r o m 1}--2½ inches. Soll s a m p l e s were collected in ¼-inch layers t o a d e p t h of 1} i n c h o n t h e n i t r a t e plots a n d o n t h e u n c u l t i -

NITRATE

DISTRIBUTION

IN TROPICAL

S O L L S . II

129

v a t e d and c n l t i v a t e d chloride plots of the Tippera soil. T h e m e t h o d o f c o l of these sämples has been described b y W e t s e l a a r a. T h e averages g i v e n in Table 4 and 5 are based on three replicas per plot. l a c h replica p e n e t r o m e t e r reading w a s again the average o f t w o m e a s u r e m e n t s . A l l readings and samples were t a k e n o n 1 7 t h A u g u s t , t h a t is 4 w e e k s after the last s a m p l i n g in E x p e r i m e n t 1. N o r a i n f e l l during this period. lection

The penetrometer readings on nitrate plots of the three soil types are given in Table 4, together with the readings on the different chloride plots on Tippera soil. NO statistical comparison can be made between the different depths for one soil type. However, the TABLE

Depth (inches)

Penetrolneter readings in top 2½ inches, in nitrate and chloride plots, in dynes c m -2 Nitrate plots Chloride plots, Tippera Florina Blain Tippera Unharrowed Harrowed

o-} }_14

-}

1

2

2½

4

1700 ± 444 460 190 760± 166 342 1300 i 1140± 420 7900 :}7 2824 4734 13400 1 5 3 6 0 B 3716 1 7 1 0 0 ± 2610 23000 ± 1000

* 3 0 8 0 ± 640 6140B3054 8800 ~ 3726 12460 ~ 4086 14860 i 2420 17340 ~ 4444 > 24000 > 24000

> 24000 9540B3850 12600 £: 7090 > 24000 > 24000 > 24000 >24000 > 24000

4 0 0 0 ~ 1266 > 24000 > 24000 > 24000 > 24000 >24000 > 24000 > 24000

> 24000 B24000 22000B2190 4 6 6 0 ~ 730 > 24000 > 24000 >24000 > 24000

* Loose sand, penetration resistanee zero

measurements on the Blain soil indicate a marked change in penetrability below 1 inch depth, while on the nitrate plots of the Tippera soll penetrability decreased significantly between ~ and } inch depth On Florina soll there was no sudden change at any depth. The readings on the unharrowed chloride plots of the Tippera soff revealed a thin crust less than ~ inch thick, immediately underlain by a very hard soll. The harrowed chloride plots, however, had a thin hard surface about ~ inch thick, underlaid by a loose zone between } and a inch depth on top of hard soil. Table 5 gives nitrate content on Tippera soll for the different top soll layers, as well as the chloride concentrations on the unharrowed and harrowed plots. There was a marked increase in nitrate content from the top } inch towards ~ inch depth, followed

130

R. WETSELAAR

b y a decline towards I I inch. The peak in nitrate content occurred in the ½-}-inch layer. TABLE

5

N i t r a t e a n d c h l o r i d e c o n t e n t s ( p p m ) , as o n 1 7 . V I I I , o n T i p p e r a soil Depth

(i~.) o-~ ~-~ ½-~ i-I I .ii 1¼--1½

1½-1~

Chloride Nitrate 50 251 353 258 82 36 10

± 35.4 4- 156.3 -Az 179.5 ± 8.2 ± 33.2 ± 15.8 -L 2.6

Unharrowed 4121 2961 2229 1642 1039 859 230

~z i ~ i :~ ~ ~

607.2 414.5 554.7 190.7 523.1 436.8 27.2

Harrowed 2009 1068 3770 3827 437 89 82

~ 497.0 ~_ 167.0 ± 2265.0 ± 760.0 z- 174.0 ~ 39.0 I 1.0 z

Maximum chloride content was located in the top } inch on the unharrowed plots and in the }-l-inch layer on the harrowed plots. The coincidence between the zone of maximum anion accumulation and location of the base of the crust, as revealed b y the penetrometer readings, is given in Table 6. Discussion

When these soils are dry the non-puddled part sets hard, the clay fraction acting as a cement between the sand grains. The penetrometer readings can be considered as a measure of the degree of this cementation. Thus the softer, easier penetrable layer at the base of the crust has less clay cementation, i.e. tess physical continuity, than the soil below it. This would result in a break in the continuity of a capillary water film containing solutes. Thus the base of the crust tends to become the interface between capillary movement below and vapour movement above it, with solutes concentrating at the bottom of, or just under, the interface. The results, as given in Table 4 and 5, and particularly in Table 6, fully confirm this. Both nitrate and chloride are affected b y the type and thickness of the crust. In case of nitrate this is clear from the difference in depth of the accumulation zone between Tippera and Blain soil (Table 6) and in the case of chloride in the diIference between harrowed and unharrowed plots (Table 6). This again, points to a common cause of accumulation near the surface, for which again capillary movement is the only feasible explanation. Apart from the capillary movement upward the anion would

NITRATE DISTRIBUTION IN TROPICAL SOILS. II

131

tend to move downwards against the concentration gradient by diffusion. As was discussed in Part I of this series ( W e t s e l a a r 5) downward diffusion results in a logarithmic decline in concentration TABLE 6 Coineidenee between zone of maximum anion accumulation and location of base of erust Tippera Blain Chloride plots Nitrate plots Nitrate plots Unharrowed Harrowed Base of crust revealed by penetrability, inches . . Zone of maximum aeeumulation of anion, inehes . . . . . . . 1--2 0-~ ~-1 ½-3

with increasing depth. In this way the logarithmic decline in nitrate content between ~ and 3 inches, as given in Part I, is fully explicable, being the result of accumulation upward to the bottom of the crust and downward diffusion against the concentration gradient. SUMMARY Changes in n i t r a t e a n d chloride c o n t e n t were followed in t h e 0-1, 1-2, 2-3, 3-6, 6-9- a n d 9-12 inch zones d u r i n g t h e 1958-59 d r y s e a s o n on t h r e e d i f f e r e n t soll t y p e s a t K a t h e r i n e , N.T. Chloride was a p p l i e d as NaC1 t o t h e soil surface a t t h e b e g i n n f n g of t h e p r e c e d i n g w e t season, w h i c h allowed m o s t of it t o leach t o w a r d s t h e subsoil. Generally, d u r i n g d r y i n g p h a s e s n i t r a t e a n d chloride i n c r e a s e d in t h e surface layers a n d d e c r e a s e d in t h e s u b - s u r f a c e layers. T h e o n l y r a i n y period r e s u l t e d in a reverse effect. B o t h anions were a f f e c t e d similarly b y w e a t h e r conditions. I n a d d i t i o n t o t a l n i t r a t e t e n d e d t o increase i m m e d i a t e l y a f t e r rain due to biological f o r m a t i o n in t h e r e w e t t e d zone. M a x i m u m a c c u m u l a t i o n of b o t h anions d u r i n g d r y i n g p h a s e s o c c u r r e d in t h e 0 - I i n c h layers on t w o soil t y p e s a n d in t h e 1-2-inch l a y e r on t h e third. P e n e t r o m e t e r m e a s u r e m e n t s r e v e a l e d t h a t t h e a c t u a l zone of a c c u m u l a t i o n is g o v e r n e d b y t h e location of t h e b o t t o m of t h e crust, w h i c h is easily p e n e trable. I t is c o n c l u d e d t h a t t h e high n i t r a t e c o n t e n t generally f o u n d in t h e surface of t r o p i c a l solls a f t e r a d r y p e r i o d is due t o eapillary m o v e m e n t u p w a r d s of n i t r a t e f o r m e d biologically in t h e s u b s u r f a c e d u r i n g p e r i o d s of a d e q u a t e w a t e r c o n t e n t . T h e n i t r a t e a c c u m u l a t e s u n d e r t h e zone w h e r e t h e p h y s i c a l c o n t i n u i t y of t h e soil is broken.

R. WETSELAAR

132

ACKNOWLEDGEMENTS T h a n k s are due t o Mr. R. J. W r e n for bis c o n t i n u o u s assistance in t h e field a n d l a b o r a t o r y . T h e assistance in d e v e l o p i n g a n d actual m a k i n g of t h e p e n e t r o m e t e r b y Mr. E. F. H. M u r r a y is acknowledged. Received September 28, 1960 REFERENCES 1 Marshall, T. J., Relations between water and soll. Commonwealth Bur. Soll Sci. (Gt. Brit.) Teeh. Commun. No. 50 (1959). 2 Marshall, T. J., and Gurr, C. G., Movement of watet and chlorides in relativity dry soil. Soll Sci. "/~, 147-152 (1954). 3 S t e w a r t , G. A., Solls of Katherine-Darwin region, Northern Territory. Australian Commonwealth Sei. Ind. P,eseareh Org. Soils Publ. No. B, (1956). 4 S t e r n , M., and S h w a c h m a n , H., Potentiometric measurement of pC1. AnM. Chem. 30,

15o6-15io

(195s).

5 W e t s e l a a r , R., Nitrate distribution in Tropical Soils I. Possible causes of nitrate aceumulation near the surface after a long dry period. Plant and Soil 15, 110-120 (1961). APPENDIX T h e a p p a r a t u s (Fig. 4) w i t h w h i e h changes in p e n e t r a b i l i t y in a horizontal direction of t h e Surface soll were m e a s u r e d c o n s i s t e d of an 1½-inch wide p l a t e A w i t h a ¼-ineh w i d e v e r t i c a l slot d o w n its centre, t h r o u g h w h i c h t h e 1/10-inch t h i c k rod H of t h e p e n e t r o m e t e r P could r e a c h t h e soil, w h e n t h e l a t t e r was m o v e d in its b e a r i n g D h o r i z o n t a l l y t o w a r d s t h e soil face. This calibr~ated r(}d (G) to

h01d

I (b¢oring(D) for p¢n~tromcter

/

with ~ ] "Im ~ut(F)to ~«t«o~~~ ~~_L I . . . . . . . . 6. . inch¢s ...... caIibrc~t¢d

sle¢v~(E)

.

/support (B)

A

for support p¢n¢tr~romctcr

pl o t e(.~,') w i r k

ver t iCQI slot

/ cylinfler(L) B

f~iston(K)

/

....

/ ¢olibrotcd

r

"sprlng(S)

rod ( H )

i ,o"

~c

Fig. 4. I n s t r u m e n t as placed in t h e soil (A) for p e n e t r o m e t e r readings. B is a l o n g i t u d i n a l cross-section t h r o u g h p e n e t r o m e t e r .

NITRATE DISTRIBUTION IN TROPICAL SOLLS. II

133

b e a r i n g was held b y a c a l i b r a t e d r o d G w h i c h could b e set a t t h e r e q u i r e d d e p t h b y a n u t F in a sleeve E . A f t e r carefully c u t t i n g a w a y t h e t o p t w o inches of t h e soil w i t h a s m a l l t r o w e l t h e a p p a r a t u s was h a m m e r e d i n t o t h e soll w i t h t h e p e n e t r o m e t e r a t its h i g h e s t position. T h e horizolltal s u p p o r t B t h e n r e s t e d parallel to a n d o n t h e soil surface, secured w i t h a peg C. T h e soil b e h i n d P l a t e A was t h e n r e m o v e d a n d t h e p e l l e t r o m e t e r lowered t o t h e r e q u i r e d d e p t h . T h e p e n e t r o m e t e r (Fig. 4B) consisted of a c y l i n d e r L in w h i c h t h e m o v e m e n t of a p i s t o n K a w a y f r o m t h e soit face was o p p o s e d b y a s p r i n g S. M e a s u r e m e n t s of p e n e t r a b i l i t y were t a k e n b y p r e s s i n g t h e cyli n d e r L t o w a r d s t h e soll n n t i l t h e c a l i b r a t e d r o d I would n o t e x t e n d a n y f u r t h e r . T h e c a l i b r a t i o n o n t o d f was t h e n read. T h e s e c a l i b r a t i o n s were o b t a i n e d b y a p p l y i n g k n o w n p r e s s u r e t o 1/10-inch t h i c k t o d H in t h e labo r a t o r y for t h e different springs used. I n m a n y cases t h e soil was so h a r d t h a t e v e n w i t h t h e h e a v i e s t springs p e n e t r a t i o n was n o t a c h i e v e d evei1 a t m a x i m u m pressure. I n t h e s e i n s t a n c e s t h e r e a d i n g s are referred to as " h i g h e r t h a n 24000."