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Fertilizer Research 22: 63-70, 1990.© 1990 Kluwer Academic Publishers. Printed in the Netherlands. 63

A n e v a l u a t i o n o f u r e a r u b b e r m a t r i c e s a s s lo w r e l e as e f e r t i l iz e r s

Z. A . Hassan, S.D . You ng, C. Hepburn* R. ArizalSchool of Agriculture, Nottingham University, Sutton Bonington, Loughborough, Leicestershire,England

Loughborough University of Technology, Loughborough, Leicestershire

Received 30 August; accepted in revised form 7 January 1990

Key words: Fertilizer, slow-release, rubber matrix, urea, residual

Abstract

The viability of a 'urea-rubber matrix' (URM) as a slow-release nitrogen fertilizer was assessed by fieldtrials and incubation studies. Encapsulation of urea in the rubber matrix apparently prevented theinhibition of seed germination experienced at high temperatures (>20°C) following high urea applica-tions. The release of urea from URM increased with temperature and was well described by a diffusionmodel which allowed for the te mperatur e-depend ence of both the diffusion coefficient in water and thesaturated concentration of urea. Initial results suggest that the effect of varying the size of URMcuboids on both their release characteristics in moist soil and N-supply to plants is also reasonably wellpredicted by the diffusion model. In a ryegrass field trial over 24 weeks, the URM gave higher drymatter yields than either prilled urea or NH4)2SO 4 (following a single application at sowing) throughefficient matching of nitrogen supply and crop demand.

Introduction

Losses of nitrogen following urea application arerelatively high and are associated mainly with theprocesses of ammonia volatilization, nitrateleaching and denitrification. Urea hydrolysis isquite rapid in soils and proceeds via conversionto ammonium carbona te [4]. As (NH4)2CO 3 ac-cumulates, soil pH values increase resulting involatilization of ammonia gas [3]. Volatilization

losses have been repo rted as being 50 within2-3 weeks of application [13]. The remainingNH4, which is retained in the soil, will be nit-rified to NO 3 which is then subject t o lossesthrough leaching and, under anaerobic condi-tion, denitrification.

To reduce such potential losses, it is desirableto maintain as low a concentration of inorganic-N (NH 4 and NO3 ) in soil-water systems aspossible, without imposing a nutrient stress oncrops. Slow release fertilizers can fulfil this func-

tion and avoid the need for repeated applicationsof conventional fertilizers. There are also otheradvantages associated with specifically with sus-taining a low available nitrogen concentration.These include a lower chance of over stimulationdue to luxury consumption, reduced disruptionof nutrient balances and minimal injury hazardto germinating crops from high application rates[12].

One approach to controlling nitrogen availa-

bility involves the coating of conventional Nfertilizer granules with slowly permeable materi-als such as plastic, wax, resin or elemental sul-phur. The best known and most widely testedproduct is sulphur-coated urea or SCU [2]. How-ever, the cost of producing SCU has limited itsdevelopment as a commercial fertilizer [14] andso limited its use in rice fields because of thecost/benefit ratio incurred [11]. The coating onSCU may crack during shipment [10] resulting infailure following application. Similarly, excessive

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impact or abrasion in use may cause breakdownof the coatings and may alter or destroy the slowrelease characteristics of the product [7].

An alternative to a single protective shell maybe the use of a foam matrix to encapsulatewater-soluble fertilizers. A new slow-releasemedium for urea fertilizer called 'Urea-RubberMatrix (URM )' has recently been developed [7].A natural rubber (grade SMR 20) was used toencapsulate prilled urea (46 N) in the form ofa foam matrix, so that the dispersed phase con-sists of urea crystals. A special mixing techniquein rubber technology known as 'split feeding' wasused to produce the material [6, 7, 8].

The objective of this paper is to appraise theagronomic significance of URM through the fol-lowing studies.a. Germinat ion trials to test URM against pril-

led urea in terms of the damaging effect togerminat ing seedlings caused by NH 3 genera-tion at high temperatures and urea applica-tions.

b. Incubation trials in soil and 0.01 M CaC12 so-lution to quantify the rate of urea releasefrom URM at different temperatures and sodevelop a predictive model of urea solubiliza-tion from the slow release matrix.'

c. A micro-plot ryegrass field trial of URM

against NH4)2SO4and urea to examine theusefulness of the slow release mechanismunder field conditions.

a t e r ia l s a n d m e t h o d s

Material

Urea-rubber matrix was prepared in strips 0.4 cmthick as described by Arizal [1] and cut intosmall cuboids according to sizes required. Astandard size was adopted to 0.5 x0 .5

x0.4) cm.

Seedling germination tr ial

Wick soil series (sandy loam, pH 5.5) was takenfrom an experimental plot of the University ofNottingham, air dried to a moisture content ofaround 8 9mg g -1 and sieved to

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w i t h d i s t i l l e d w a t e r f o r 1 . 5 0 h t o r e m o v e f r e eu r e a f r o m s u r f a c e p o r e s s o t h a t t h e s l o w r e l e a s eo f u r e a c o u l d b e s t u d i e d w i t h g r e a t e r a c c u r a c y.T h e u r e a r e l e a se d a f t e r 1 .5 0 h s h a k in g a m o u n t e dt o 9 . 0 o f t o t a l f o r t h e s t a n d a r d s i z e o f U R M .

T h e c u b e s w e r e t h e n p l a c e d i n t h e p r e - i n c u b a t e dC a C I 2 s o l u t i o n . T h e r e s u l t in g s u s p e n s i o n s w e r er e g u l ar l y s h a k e n a n d s a m p l e d t o m e a s u r e u r e aa n d a m m o n i u m c o n c e n t r a t io n s . T h e u r e a i n t h ea n a l y t i c a l s a m p l e s w e r e h y d r o l y s e d b y i n c u b a t i o nw i t h 0 . 2 u r e a s e f o r 2 h at 2 5 ° C a n d t h e r e s u l t-i ng N H 4 a n a l y s e d u s i n g s t a n d a r d a u t o m a t e d f lo wa n a ly s i s. A t t h e e n d o f t h e i n c u b a t i n g p e r i o d t h eU R M c u b e s w e r e r e m o v e d , o v e n d r i e d a t 8 0 °Ca n d t h e n d i g e s t e d t o d e t e r m i n e t h e r e s i d u a l N .

Diffusion model of urea release

To d e s c r ib e d i f f u si o n o f u r e a o u t o f t h e r u b b e rf o a m c u b o i d s a s i m p l e n u m e r i c a l s o l u t i o n t oF i c k ' s f i rs t l a w w a s u s e d . E a c h c u b o i d w a s e n v i s -a g e d a s 6 'r i g h t p y r a m i d s ' w i t h b a s e s f o r m i n g t h eo u t s i d e f a c e s a n d a p e x e s m e e t i n g i n t h e c e n t r e o ft h e c u b o i d . E a c h p y r a m i d w a s t h e n d i v i d e d i n t o1 0 s h e l l s o r ' f i n i t e t h i c k n e s s p l a n e s ' ; t h e v o l u m ea n d i n t e r r a c i a l a r e a o f e a c h s h e l l w a s c a l c u l a t e df r o m t h e o v e r a l l c u b o i d g e o m e t r y. T h e i n i t i a lu r e a c o n t e n t o f e a c h s h e l l w a s c a l c u l a t e d f r o m

t h e t o t a l u r e a c o n t e n t o f t h e c u b o i d a n d s o th ep o r e s p a c e o f e a c h s h e ll w a s d e t e r m i n e d f r o m a na s s u m e d s o l i d u r e a d e n s i t y o f 1 . 3 2 g c m -3 .

I n c r e m e n t a l m o v e m e n t o f u r e a b e t w e e n s h el lsf r o m t h e a p e x t o t h e b a s e o f e a c h p y r a m i d w a sd e s c r i b e d b y :

A U = - D A ( A U J A x ) A t (4 )

w h e r e A U = u r e a t r a n s f e r r e d ( g ) d u r i n g t im e i n -c r e m e n t A t

D = t h e d i ff u s io n c o e f f i ci e n t (c m 2 s - 1 )A = t h e ' s o u r c e s h e l l ' p l a n a r a r e a ( c m 2 )

A U c = t h e d i f f e r e n c e i n s o l u t i o n c o n c e n -t r a t i o n b e t w e e n t h e s o u r c e a n d s i n ks h e l l s ( g c m -3 )

A x = t h e d i s t a n c e b e t w e e n t h e c e n t r e o ft h e s o u r c e a n d t h e s i n k s h e l l s ( c m )

A t = t h e t i m e i n c r e m e n t ( s e c o n d s ).T h e s o l u t i o n c o n c e n t r a t i o n i n e a c h s h e l l w a sa s s u m e d t o b e t h a t o f a s a t u r a t e d s o l u t i o n u n t i lt h e u r e a c o n t e n t d i v i d e d b y t h e s h e l l p o r e v o l -

65

u m e f e l l b e l o w t h is f ig u r e . T h e c o n c e n t r a t i o n o fu r e a i n t h e e x t e r n a l s o l u t i o n w a s a l w a y s t a k e n a sz e r o . T h e s a t u r a t e d u r e a c o n c e n t r a t i o n w a s ca l -c u l a t e d f r o m t h e e q u a t i o n p r e s e n t e d b y J a r re la n d B o e r s m a [ 9 ] :

U c s a t) = 6 . 9 6 × 1 0 - 3 ) T +0 . 4 5 ( 5 )

w h e r e T = t h e t e m p e r a t u r e i n ° C .T h e p r e v i o u s a u t h o r s h a v e a l s o d e v e l o p e d a

s i m p l e a l g o r i t h m t o d e s c r i b e t h e t e m p e r a t u r ed e p e n d e n c e o f t h e u r e a d i f f u s i o n c o e f f i c i e n t i ns o l u t i o n ( D 1

-5D 1 = ( 5 . 5 5 x 1 0 ) Tab S e x p ( - 2 1 3 5 / T ~ u s ) ( 6 )

w h e r e Tab S i s t h e a b s o l u t e t e m p e r a t u r e .T h e d i f f u s i o n c o e f fi c i e n t ( D ) o f a n o n - a d s o r -

b e d s o l u t e m o v i n g t h r o u g h a p o r o u s m e d i u m i sd e s c r i b e d a s t h e p r o d u c t o f D 1 a n d a t o r t u o s i t yf a c t o r ( f ) :

D = 7 )

C o m b i n i n g e q u a t i o n s 4 a n d 7 t o m e a s u r e t h eu r e a m o v e m e n t b e t w e e n s h e l l s , a n d h e n c e t h er e l e a s e t o t h e o u t s i d e s o l u t i o n , t h e o n l y u n -k n o w n p a r a m t e r is f. T h e m o d e l w a s t h e r e f o r e

f it t ed t o t h e e x p e r i m e n t a l d a t a f o r e a c h t e m p e r a -t u r e b y a d j u s t i n g t h e t o r t u o s i t y f a c t o r t o m i n i -m i z e t h e e r r o r s u m o f s q u a r e s . A A t v a l u e o f8 6 4 0 s e c o n d s w a s u s e d s o t h a t a t o t a l o f 2 0 0 0t i m e i n c r e m e n t s w e r e e m p l o y e d o v e r t h e t o t a ld i f f u s io n p e r i o d o f 2 0 0 d a y s .

Micro-plot field trial

A n a r a b l e p l o t o f Wi c k s e r i e s so i l w a s u s e d t os t u d y t h e N - n u t r i t i o n o f r y e g r a s sLolium peren-

ne)g r o w n i n m i r o - p l o t s . P l a s t ic c o l u m n s ( 2 0 c m

l e n g t h a n d 1 c m d i a m e t e r ) w e r e i n s e r t e d t o ad e p t h o f 1 7 c m f r o m t h e s o i l s u rf a c e a t d i s t a n c e so f 7 5 c m . T h e f e r t i l iz e r s , p r i l l e d u r e a ,

N H 4 ) 2 S O 4 a n d U R M o f s t a n d a r d s iz e w e r ea p p l i e d a t r a t e s o f 0 , 5 0 , 2 0 0 , a n d 5 0 0 k g N h a - ~t o a d e p t h o f 2 c m f r o m t h e s o i l s u r f a c e . T h eo t h e s s i ze s o f U R M u s e d i n t h e so i l i n c u b a t i o nt r i a l w e r e a l s o u s e d a s a t r e a t m e n t a t a s i n g l er a t e o f 2 0 0 k g N h a- 1 . Ry e g r a s s s e e d s w e r e s o w no n 2 0 t h M a y , 1 9 88 a t a r a t e o f 0 .2 g p e r c o l u m n

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66

and cove red w i th 1 cm o f so il one w eek a f t e rf e r ti l iz e r app l i cat i on . W a te r ing o f 0 .5 cm wasdone a s neces sa ry up t o 1 mo n th a f t e r sowing .A l l t he t r ea tmen t s we re r ep l i ca t ed 5 t imes u s ingan RCBD des ign . The g ra s s was ha rves t ed 3

t imes w i th each cu t be ing t aken a t 8 we ek i n t e r-va l s . The g ra s s s ample s we re oven d r i ed a t 80°Cfo r 3 days , we ighed , t hen g round and d iges t edfo r n i t r ogen ana ly s is . The U R M cubes we reremoved f rom the f i e ld a f t e r 30 weeks i n t hemic rop lo t s , washed , and ana ly sed fo r r e s idua l Naf te r Kje ldahl d iges t ion .

r epo r t ed t ha t u r ea se ac t iv i ty inc r ea sed l i nea r lywi th t em pera tu re and r eached an op tima l l eve l a t37°C. H ow eve r, in t he ca se o f U R M , u rea r e -l ea se f rom the rubbe r ma t r i x i s no t immed ia t e ,so tha t the urea concent ra t ion in the v ic in i ty of

the s eeds is l ow. Dam age t o s eeds f rom am moniafo l l owing u rea hyd ro lys i s i s t he r e fo re p robab lyneg lig ib l e . Th i s sugges ts t ha t U R M ma y be s a f e -ly appl ied a t the t im e of sowing, and a t h igh Nappl ica t ion ra tes , wi thout s igni f icant r i sk ofseed l ing i n ju ry o r r e t a rded ge rmina t ion .

Urea release in solution

e s u l t s a n d d i s c u s s i o n

ffec t of URM on seedl ing germinat ion

Th e germ inat ion of ryegrass was signi ficant lyin f luenced by t he t ype and r a t e o f f e r t il i z e r-Napp l i ed . An inc rea se i n p r i l l ed u r ea concen -t ra t ion cau sed s ignif icant p = 0 .01) germ inat ionfa i lu r e o f ryeg ra s s wh i l e U R M d id no t r educegerm inat ion ra tes s ignif icant ly Tab le 1).

Th e fa i lure of germ inat ion a t h igh appl ica t ionra t e s o f p r i ll ed u r ea shows the po t en t i a l l ydamag ing e f f ec t o f i ts u se . Dur ing t he expe r i -men t , ave rage da i ly t emp era tu re s t he sum o f

t h e m i n i m u m a n d m a x i m u m d a il y t e m p e r a t u r ed iv ided by two) i n t he g l a s shouse we re h igh20.5 -+ 1 . I °C) ; the averag e da i ly range was 10 . 3-

30 .6°C . H igh t emp era tu re s f av ou r rap id u r eahydro lys i s and NH 3 evolu t ion , Gould e t a l [5]

Ta b l e 1 Effect of nitrogen fertilizers on the germination ofryegrass

Seedlingnumbers

Treatments kg N ha - 1 % of control DMRT b

Cont rol 0 100 AUR M 100 93.6 AUR M 300 98.8 AUR M 500 92.2 AUrea 100 93.5 AUrea 300 55.6 BUre a 500 0.0 C

a Avera ge of 5 replicatio n, 12 days after sowing.b Duncan's multiple range te st - any 2 means having a com-mon letter are not significantly different at the 5% level ofsignificance.

A good f i t t o t he da t a was ach i eved u s ing t hedi ffus ion m ode l F ig . 1 ) . Ja re l l and Bo ersm a [9]h a v e s h o w n t h a t t h e t e m p e r a t u r e d e p e n d e n c e o fu rea r e l ea se f rom SCU can be d i r ec t l y r e l a t ed t othe product of the l iqu id d i ffus ion coeff ic ien t andthe s a tu r a t ed concen t r a t i on D I Csa t). The U R Mdiffers f rom SC U in tha t ins tea d o f a s ingle

U R E A RE L E AS E FR O M R U B B E R C U B O I D EDIFFUSION MODEl__

o D7 o • • ~

o~

tt~

g

0 5 10 15 2o 25TINE, WEEKS

Fig 1 Urea release from rubber cuboids as a function oftemperature and time, Solid lines are generated by thediffusion model. Data points for different tepmeratures in-clude (N) 1°C; (A) 9.8°C; (~) 16.5°C; (0) 18.8°C; (©)26.4°C.Y axis: Ur ea Rele ased (g x 10 -2)X axis: Time (Weeks)

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Ta b l e 2 Variation in diffusion model parameters with tem-perature

Temperature D 1 C s a t Tortuosity factor (f)(°C) (x lO -6) (xl O -4)

1.0 2.88 1.23

9.8 4.30 1.2216.5 5.71 1.1718.8 6.28 1.6926.4 8.46 1.92

* From equations of Jarell and Boersma (9).

chamber containing urea there are many, allinterconnected via small pores. Hence the needfor a 'tortuosity factor' in the description ofdiffusion. However, because the temperature-dependence of D 1 Csa is al lowed for in themodel the tortuosity factor should be constant atall temperatures for the model to be consistent.In fact there appears to be a slight increase in thetortuosity factor at high temperatures (Table 2)so that changes in D 1 and s a t may not fullyexplain the greater release rate with increasingtemperature. It is possible that the morphologyof the rubber pore walls changes with increasingtemperature, perhaps causing widening of inter-connecting channels and hence a higher tortuosi-ty factor. Nevertheless, most of the change inrate of release can be ascribed to the tempera-

ture-dependence of D 1 C s a tA further test of the accuracy of the diffusion

model lies in the estimation of residual ureacontained in cuboids equilibrated in soil. Weassumed that at 10.7 soil moistu re neit her therubber-water contact area nor the diffusion ofurea away from the source limited the urearelease from the rubber matrix. The latter as-sumption should certainly be valid as tortuosityfactors in soil tend to be of the same order ofmagnitude as the volumetric water content; thusf(soil) would be around 1000 times f(URM)- Itshould therefore be possible to use the tortuosityfactors derived for the solution experiment toestimate residual urea in cuboids incubated insoil. A comparison of model and actual residualurea in URM is given in tables 3 and 4; in bothcases the figures given by the model are correc-ted for urea released instantaneously (9.0 oftotal).

The effect of soil temperature and cuboiddimensions are shown to be reasonably well

67

Ta b l e 3 The effect of cuboid size on residual urea in soil-incubated URM (Temperature 18.8°C)

Cuboid dimensions

(cm)

Urea release after 27 weeks

Model Measured

0.25 x 0.50 × 0.40 98 99

0.50 x 0.50 x 0.40 97 970.50 x 1.00 x 0.40 85 861.00 x 1.00 x 0.40 74 76

predicted by the diffusion model given that thetortuosity factors used were derived from a sin-gle experimen t using only 1 size of cuboid (0.5 x0.5 x 0.4 cm) incubated in a completely differentbackground medium (0.01 M CaC12).

Plant uptake of nitrogen from UR

The nitrogen uptake and ryegrass yield as afunction of nitrogen application are shown forthe 3 consecutive harvests in Figures 2 and 3.The slow-release properties of URM were appar-ent in comparison with urea and ammoniumsulphate in Figure 2. Less nitrogen was taken upby the first cut of ryegrass growing in soil fertil-ized with URM. There was also a slight reduc-tion from an otherwise linear relationship be-

tween uptake and application at the highest ureaapplication. However second and third cutsclearly demonstrated the continued supply ofnitrogen by the URM throughout the growingseason. Nitrogen uptak e from (NH4)2SO 4 andprilled urea fell well below that from URM afterthe first cut. The total nitrogen uptake over all 3cuts (Fig. 2d) showed no significant differencesbetween N sources except in the case of thehighest urea application rate. The lower uptakein the latter case is thought to be due to a

Ta b l e 4 The effect of soil temperature on residual urea insoil-incubated URM (0.50 x 0.50 × 0.40 cm)

Soil temperature

(oc)Urea release after 27 weeks

Model Measured

1.0 74 609.8 84 78

16.5 89 9918.8 97 9726.4 99 99

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550,

~.50.

350.

Z50.

I S 07

o

5 o

o

18,<

1 0 0

d )

60,

a 1 3 0 0 ] b

/ f o o t

°

5Q

, l , O

0 100 ZOO3 6 0 kbO ~0

c 8oo-

l . S . O ( 1 ) I / 60o-

t , . O 0

2 0 0

6 8

6 o z 6 o a 6 o ~ b o ~ 6 o

N , K g ha 1

Fig 2 Nitrogen uptake by ryegrass grown in microplots at

different levels of nitrogen fertilizer: a) First cut, b) Secondcut, c) Th ird cut, d) O ver all uptake. Fertilizers include 11)Am monium Sulphate ; [~) Pr il led Urea; 0) U RMX axis: Nitrogen Levels kg ha -~)Y axis: Nitrogen U ptake rag micro plot ~)

z 0

16

1Z:

7

2

10.

= 8

a 2 z b

1 8 1

l td .

I0

6

1~0 z~0 3d0 ~ 0 s00 z ,a z ; i ; ~ 6 s c o

c 50,

kO

30

?.0.

10

100 2.00 300 ~,00 SO0

~)

l . s ~

1~0 zb0 360 ~60s o

N, Kg h a-1

Fig 3 Yields of ryegrass grown in microplots at different

levels of nitrogen fertilizer: a) First cut, b) Second cut, c)Th ird cut, d ) O verall yield. Fertilizers include: ram) Am -monium Sulphate; D) Pr il led Urea; a ) URM .X a xis: Nitrogen Levels kg ha -~)Y ax is: Dry w eight Yields grams m icroplot -~)

com bina t i on o f amm onia vo l a t il i z at i on and t hee ff ec t o f s eed l ing damage a s shown by t he l oweruptake for the f i r s t harves t .

The y i e ld a t f i r s t ha rves t a ch i eved f rom theth ree N sou rces (F ig . 3a ) fu r the r demons t r a t e sthe po t en t i a l l y dam ag ing e f f ec t o f p ri l led u r ea a th igh concent ra t ion• There was a s igni f icant re -duc t i on i n p l an t g rowth a t t he h ighes t u r ea l eve l .I t i s a l so no t i ceab l e t ha t a l t hough N up t ake fo rURM fe l l we l l be low tha t f o rN H 4 ) 2 S O 4(Fig.2a) , the respec t ive y ie lds a re much c loser (F ig .3a) . This i s incons is ten t wi th the poorer e ff ic ien-cy o f n i t r ogen u se i n d ry ma t t e r p roduc t ionexpec t ed fo r l a rge app l i ca t i ons o f ' s o lub l e ' Nfer t il i zers. Th e re la t ive ly h igher y ie lds of rye-

g ra s s on URM p lo t s ach i eved fo r t he s econd andth i rd cu ts (F ig . 3b and 3c) i s expec ted g iven tha tt he p l an t g rowth i s p roba b ly l im i t ed by ava i l ab l en i t rogen a t tha t s tage . F ina l ly, the va lue of themo re even sp read o f n it r ogen ava i l ab i li t y ac ros sthe growing season i s re f lec ted in the h igherove ra l l y i e ld ach i eved w i th URM fo r t he twohigh N appl ica t ion ra tes (F ig . 3d) . F or 200 and5 0 0 k g N h a -1 , t h e U R M g a v e a 1 7.3 a n d 2 9 .1h ighe r y i e ld t han (NH 4)~SO 4 r e spec t ive ly ; t he sef igu re s a r e c l ea r ly h ighe r i f t he U R M i s com-pa red w i th u r ea a t app l i ca t i ons l a rge enough todamage t he c rop .

The d iscuss ion of the f ie ld t r ia l so fa r hasconce rn ed a ' s tanda rd s i ze ' o f UR M (0 .5 x 0 .5 x

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Ta b l e 5. A comparison of theoretical urea release and meas-ured N uptake by ryegrass, under fie ld conditions, over 24weeks

Cuboid dimension Ure a release N uptake(cm) as SV* as SV*0.25 x 0.50 x 0.40 (SV)* 100 1000.50 x 0.50 x 0.40 94 970.50 x 1.00 x 0.40 82 831.00 x 1.00 x 0.40 69 74

* Smallest volume

0 . 4 c m ) c u b o id . T h e s a m e 4 d i f f er e n t c u b o i dv o l u m e s d e s c r i b ed p r e v i o u s ly w e r e a l so t e s t e da n d s h o w e d a n a b i l it y to s u p p l y N o v e r t h e 2 4w e e k s g r o w t h p e r i o d w h i c h w a s f a i rl y c o n s i s t e n tw i t h t h e e x t e n t o f u r e a r e l e a s e p r e d i c t e d b y th ed i f f u s i o n m o d e l . I n t a b l e 5 w e p r e s e n t b o t h t h eu r e a r e l e a s e ( 2 4 w e e k s , t e m p e r a t u r e 1 3 .4 ° C) a n dt h e t o t a l n i t r o g e n u p t a k e b y r y e g r a s s as a p e r c e n -t a g e o f t h a t f o r th e s m a l l e s t c u b o i d v o l u m e( 0 .2 5 x 0 . 5 x 0 .4 c m ) . T h e a v e r a g e s o il t e m p e r a -t u r e ( 1 0 c m d e p t h ) o v e r th e g r o w i n g se a s o n( 1 3 .4 ° C ) w a s t a k e n f r o m t h e S u t t o n B o n i n g t o nm e t e o r o l o g i c a l s ta t i o n . A g a i n , t h e m o d e l v a l u e sa r e c o r r e c t e d f o r t h e s m a l l a m o u n t o f u r e a w h i c his r e l e a s e d i n s t a n t a n e o u s l y. C l e a r l y t h e t h e o r e t i -c a l u r e a r e l e a s e g i v e n i n Ta b l e 5 m u s t b e r e -g a r d e d a s a p p r o x i m a t e a s t h i s w i l l d e p e n d o n

s p e c if i c s o il t e m p e r a t u r e f l u c t u a ti o n s ; t a k i n g a na v e r a g e t e m p e r a t u r e i s n o t s t r ic t l y v a l id . T h e s o ilm o i s t u r e c o n t e n t w i ll a ls o a f f e c t t h e d i f f u s i o n o fu r e a f r o m t h e c u b o i d ; i n a n o t h e r s e r i e s o f in c u -b a t i o n t r ia l s it w a s f o u n d t h a t t h e r a t e o f u r e ar e l e a s e f e l l d r a m a t i c a l l y w h e n t h e v o l u m e t r i c s o ilw a t e r c o n t e n t f e ll b e l o w 0 . 0 7 4 f o r Wi c k s e r i e ssoi l .

I t is p o s si b l e t h a t t h e a v a i l a b i l i ty o f u r e a f r o mU R M i s a f f e c te d b y p ro c e s s e s o t h e r t h a n s i m p l e

Ta b l e 6 Changes in mass of standard size of rubber cuboidswith temperature for UR M incubated in soil over 27 weeks

Temperature Mass of rubber*(°C) (g)1.0 0.0349.8 0.036

16.5 0.04118.8 0.03526.4 0.033

* Corrected for residual urea

69

P l a t e 1 Clustering of ryegrass roots around URM cuboid.

d i f f u si o n w h e n i n a r h i z o s p h e r e e n v i r o n m e n t .

Tw o p o s s ib i l it i e s i n c lu d e b i o l o g i c a l d e c a y o f t h er u b b e r m a t r i x a n d r o o t p e n e t r a t i o n o f t h ec u b o id . T h e s oi l i n c u b a t io n e x p e r i m e n t s h o w e dt h a t t h e f in a l m a s s o f r u b b e r o f e a c h c u b o i d w a sn o t t e m p e r a t u r e - d e p e n d e n t ( T a b l e 6) . S o m e d e -g r e e o f b i o l o g ic a l d e c a y m a y o c c u r u n d e r f ie l dc o n d i t i o n s b u t th i s m a y b e p e r i p h e r a l t o u r e as o l u b i l i z a t i o n . H o w e v e r , r o o t - p e n e t r a t i o n o fU R M w a s c o n s id e r a b l e , ( P l a t e s 1 a n d 2 ). T h ec u b o i d s w e r e f i r m l y h e l d w i t h i n r o o t m a t s r e c o v -e r e d f r o m t h e f ie ld e x p e r i m e n t ; m o s t o f t hes t o r a g e p o r e s w i t h i n t h e r u b b e r m a t r i x a p p e a r e dt o b e f i ll e d w i t h f i n e r o o t m a t e r i a l . I n a d d i t i o n ,l a r g er ro o t s p e n e t r a t e d t h e U R M s t r u c tu r e( P l a t e 2 ) . I t is l i k e ly h o w e v e r , t h a t r o o t p e n e t r a -t io n o c c u r e d a f t e r sw e l li ng o f th e U R M c a u s e db y i nf l ux o f w a t e r a n d f o l l o w i n g r e l e a s e o f u r e ab y d if f u s io n . T h e s u p p l y o f u r e a w o u l d t h e r e f o r es ti ll b e l i m i t e d b y d i f f u s i o n a c r o s s t h e i n t e r -c o n n e c t i n g r u b b e r w a l l s s e p a r a t i n g u r e a s t o r a g ep o r e s w i th i n th e U R M .

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Plate 2 P e n e t r a t i o n o f U R M c u b o i d b y i n d i v i d u a l r y e g r a s sroo t .

o n c l u s i o n s

The r e l ea se o f u r ea f rom U R M in mo i s t so il i sa f f e ct e d b y t e m p e r a t u r e a n d m a t r i x v o l u m e i n amanne r wh ich i s qu i t e we l l p r ed i c t ed by a d i f fu -s ion mode l . I t shou ld t he re fo re be pos s ib l e t ou t i l ize mat r ix volume (s ize of cuboid) as thede t e rmin ing v a r i ab l e fo r r equ i r ed u rea - r e l ea sera t e s unde r d i f f e r en t t em pera tu re s . Fu tu re r e -s ea rch cou ld p ro f i t ab ly be d i r ec t ed t owards i n -ves t iga t i ng the r e l a ti onsh ip be tw een ma t r i x t o r-t uos i t y and t he chem is t ry o f d i ff e r en t ma t r i xfo rmu la t i ons . Ce r t a in ly t he advan tages o f si ng le ,la rge fe r t i l i zer appl ica t ions g iv ing a cont ro l ledsupp ly and min ima l l o s s o f n i t r ogen m ake f ind ing

a cheap a l t e rna t i ve t o na tu ra l r ubbe r a wor th -whi le goa l .

R e f e r e n c e s

1 . Ar iza l R 1988) A s low- re leased u rea f e r ti l ize r basedoa n a t u r a l r u b b e r m a t r ix . P h D t h e si s. L o u g h b o r o u g h U n i -v e r s i t y o f Te c h n o l o g y U K

2 . C r a s w e ll E T a n d V l e k P L G 1 9 7 9 ) G r e e n h o u s e e v a l u a-t ion of ni t rogen fer t i l izers for r ice . Soi l Sci Am J 43:1184-1188

3 . F e n n L B , M a t o c h a J E a n d Wu E 1 9 8 1 ) A c o m p a r i s o no f c a lc i u m c a r b o m a t e p r e c i p i t a ti o n a n d p H d e p r e s s i o n o nc a l c i u m r e d u c e d a m m o n i a l o s s f r o m s u r f a c e - a p p l i e du r e a . S o i l S c i S o c A m J 4 5 : 11 2 8 - 11 3 1

4 . F e n n L B , Ta y lo r R M a n d M a t o c h a J E 1 9 8 1) A m m o n i aloss f rom su r face -app l i ed n i t rogen fe r t il i ze r as con t ro l l edb y s o l u b le c a l c iu m a n d m a g n e s i u m : G e n e r a l T h e o r y. S o ilS c i S o c A m J 4 5 : 7 7 7 - 7 8 1

5 . G o u l d W D , C o o k F D a n d We b s t e r G R 1 9 7 3 ) F a c t o r sa ffec t ing u rea hydro lys i s in seve ra l A lbe r t a So i l s . P lan ta n d S o i l 3 8 : 3 9 3 - 4 0 1

6 . He pb urn C and Ar iza l R 1988) Enc apsu la t ion o f f e r ti l -i z e r b y r u b b e r. I n : I n t e r n a t i o n a l R u b b e r C o n f e r e n c e ,H a r r o g a t e U K

7. He pb urn C and Ar iza l R 1987) S low- re lease f e r t il i ze r sb a s e d o n n a t u r a l r u b b e r. B r i ti s h P o l y m e r J 2 0 : 4 8 7 - 4 9 1

8 . H e p b u r n C , Yo u n g S D a n d A r i z a l R 1 9 8 7 ) R u b b e rmat r ix fo r the s low re l ease o f u rea f e r t i l i ze r. Po lymerP r e p r i n t s 2 8 : 9 4 - 9 6

9 . J a r e ll W N a n d B o e r s m a L 1 9 8 0 ) R e l e a s e o f u r e a b ygranu les o f su lphur coa ted u rea . So i l Sc i Soc Am J 44 :4 1 8 - 4 2 2

1 0. O t e y F H , Tr i m n e l l D , We s t h o f f R P a n d S h a s h a B S1984) S ta rch ma t r ix fo r con t ro l l ed r e l ease o f u rea f e r t i l -

i z e r. A g r i c F o o d C h e m J 3 2 : 1 0 9 5 - 1 0 9 811. S a v a n t N K , J a m e s A F a n d M c C l e l la n G H 1 9 8 3 ) U r e a

re l ease f rom s i l i ca t e and po lymer-coa ted u rea in wa te ra n d a s i m u l a t e d w e t l a n d so il . F e r t R e s 4 : 1 9 1 - 1 9 9

1 2. Ti s d a le S L , N e l s o n W L a n d B e a t o n J D 1 9 8 5 ) S o i l a n dfer t i l izer ni t rogen. In: Soi l fer t i l i ty and fer t i l izer, pp112-188 . 4 th ed i t ion . New York : Macm i l l an

1 3. V l e k P L G a n d C r a s w e l l E T 1 9 7 9 ) E f f e c t o f n i t r o g e ns o u r c e a n d m a n a g e m e n t o n a m m o n i a v o l a t i li z a t io n lo s se sf rom f looded r i ce - so i l s sys t em. So i l Sc i Soc Am J 43 :

3 5 2 - 3 5 81 4. Yo u n g d h a l L J , L u p i n M S a n d C r a s w e l l E T 1 9 8 6 ) N e w

deve lopment s in n i t rogen fe r t i l i ze r s fo r r i ce . Fe r t Res 9 :1 4 9 - 1 6 0