PertanikaJ. Trop. Agric. Sci. 16(2): 87-94 (1993) ISSN: 0126-6128© Universiti Pertanian Malaysia Press

Urea as the Nitrogen Source in NFT Hydroponic System

AMINUDDIN H., KHALIP R.1, NORAYAH K. and ALIAS H.2 : ^ ° *Department of Soil Science, Universiti Pertanian Malaysia

1 Agronomy Department, Universiti Pertanian Malaysia2Petroleum Research Institute, PETRONAS

Keywords: Hydroponics, urea, hydrolysis, urease, ammonium

ABSTRAKUnsur nitrogen dalam larutan nutrien hidroponik cara NFT (nutrient film technique) menjalani proses hidrolisisdengan pengeluaran ammoniam. Hidrolisis urea berlaku dengan pesat dari hari ketujuh dan tamat pada harikeduapuluh. Dalam masa yang sama jumlah ammoniam dalam larutan meningkat dan mencapai tahapmaksima pada hari keduapuluh. Penitratan berlaku serentak dan kepekatan nitrat mencapai maksima pada harikeduapuluh juga. Berat kering tanaman pada peringkat matang tidak berbeza antara larutan rawatan urea danrawatan nitrat. Kepekatan ammoniam dalam daun dan akar rawatan urea adalah tinggi dari rawatan nitrat.Kepekatan nitrat dalam daun dan akar tidak berbeza diantara kedua dua rawatan tersebut. Kajian menunjukkanurea dapat menggantikan nitrat sebagai sumber unsur nitrogen untuk tanaman dalam sistem hidroponik caraNFT.

ABSTRACTUrea as the source of nitrogen in the nutrient solution in NFT (nutrient film technique) hydroponic systemundergoes hydrolysis which results in the release of ammonium in solution. Urea hydrolysis was rapid from the7th day onwards and ended by the 20th day. At the same time, ammonium concentration in solution increasedand reached its maximum on the 20th day. Nitrification occurred simultaneously and peaked also on the 20thday. Plant dry matter weight at harvest was similar for both urea and nitrate treatments. Ammoniumconcentration in leaves and roots was higher in urea than in nitrate treatments. Nitrate concentration in leavesand roots was similar for both treatments. The study showed that urea can be substituted for nitrate as the nitrogensource in the NFT hydroponic system.

INTRODUCTIONThe source of nitrogen in the nutrient solution organisms (Skujins 1976; Vlek et al 1980). Therefor the growing of crops under the hydroponic is also evidence that soil urease activity may besystem has been in the forms of Ca(NOs)2 and derived from plants (Frakenberger and TabatabaiKNO? (Cooper 1975). Even though urea is in- 1 9 8 2 ) . i t has also been shown that floodwater ofcreasingly being used as the main source of t r o p i c a i lowland rice soils has measurablenitrogen fertilizer for crops grown on soil (Gould a m o u n t s o f u r e a s e a c t i v i t y (Sahrawat 1980), whichet al 1986), its use as the N source for crops c o u l d b , originate from the soil,grown under the hydroponic system has yet to o . « r i_ i? , -, ^ r , , . 1 1 , Since the presence of urease has alwaysbe evaluated. One of the reasons which could . ,; . . ' . /* • , „ ., r , , u been associated with soil, its presence in theninder the use of urea in hydroponics is that r

urea has to undergo hydrolysis with the release n u t r i e n t s o l u t i o n i n t h e Mropon ic system de-of N H ; and subsequent formation of N O ; void of soil therefore is of interest. Since bacte-through nitrificaton, forms that are utilised by r i a c a n proliferate in nutrient solution andthe plants. Hydrolysis of urea proceeds through urease originates from bacteria cells, the possi-enzymatic catalysis of urease (Vlek et al 1980). ble presence of urease in nutrient solution me-In soils urease activity is located in the soil dia and hydrolysis of urea in the solution there-biomass and solution originating from decaying fore is great.

AMINUDDIN HM KHALIP R., NORAYAH K. AND ALIAS H.

The objective of this study was to examinethe possibility of using urea as the nitrogensource in the nutrient solution in the hydro-ponic system by following its transformation inthe nutrient solution and the performance ofBrassica chinensis grown in it.

MATERIALS AND METHODS

The NFT (nutrient film technique) of soillessculture with plants grown in channels fed withrecirculating nutrient solution was used (Cooper,1975). The container which held therecirculating nutrient solution had a capacity of50 1. The channels were 3.7 m long, 30 cm wideand 5 cm high. These channels were constructedfrom 1 cm thick plywood. The inner surface ofthe channels was covered with polyethene sheetsalong which the solution flowed and bathed theplant roots. A submersible pump in the con-tainer delivered the solution through 2 cm (di-ameter) PVC pipes from the container to thetop of the channel, discharging the solutionthrough a 5 mm tube at the rate of 2 1/min. Thechannels were elevated at the top end (10 cmhigher) so that the solution would flow in a thinfilm on the surface of the channel to the bottomend and into the container and then recirculated.Each replicate of a treatment consisted of thechannel, a container and a pump. Three repli-cates were used for each treatment. The experi-ment was run in the greenhouse in a completerandomized design. Mini pak choy {Brassicachinensis var Ching Chiang -Taiwan ) seeds weresown on rockwool blocks 6 cm2 and 40 cm inlength for each block; nine blocks were used foreach channel.These blocks were placed at thecentre of the channels. Three seeds were placedat each point, 8 cm apart. Each rockwool blockhad 5 plants. Thinning was done on the tenthday leaving one plant per point. The totalnumber of plants per channel at the start of theexperiment was 45. Since the concentration ofurea in the solution had to be monitored con-tinuously for the whole period of the plantgrowth, elemental nutrients were added to thesolution for plant need, based on the electricalconductivity (E.C.) of the solution; the elementcontent of the solution was analysed on the 20thday to ensure that all elements with the excep-tion of N were adequate for plant need. Plantswere observed for deficiency symptoms. Cooperand Charlesworth (1977) concluded that addi-tion of nutrients under the NFT based on the

readings of the E.C. and the solution pH wasadequate to produce equivalent tomato yieldcompared to nutrient addition based on nutri-ent analysis.

Treatments

The standard nutrient formulation used wasbased on Cooper (1975) (Table 1). The otherthree treatments were 100% N-urea (No.2),mixture of 50%N-nitrate and 50%N- urea (No.3)and 75%N-urea and 25%N-nitrate (No.4). Anaccompanying treatment (No.5) similar to (No.2)but without plants grown was included. CaCl2,KOH and K,2SO4 were substituted for theCa(NO3)2 and KNO3 for treatments containingurea, resulting in some treatments having highersulphur and chloride contents which, however,did not exceed toxic levels. pH of the solutionwas maintained at between 5.5-6.5 by addingHC1 or NaOH. Electrical conductivity (E.C.) ofthe solution was maintained at 2.4 dS nr1 untilthe plants were harvested. Solution E.C. whichfell below this value was replenished with thestock solution of the respective treatments.Amounts added to each treatment are given inTable 2. Water loss through evapotranspirationwas replaced with tap water to the 50 1 markdaily.

Sampling

Solution samples were taken daily for the firstseven days and thereafter on the 10th, 20th,

TABLE 1Chemical make-up of the various treatments

Source

ureaKR2PO4

KNO,Ca(NO,),CaCl2MgSO4

KOH

MnSO4

H3BO,Na2MoO4

ZnSO4

CuSO4

EDTA-Fe

1

_

272808943

-492

—-5.273.03

.027

.22

.0519.48

Treatments (g/1000)

2

480272

_-

588492224348

5.273.03

.027

.22

.0519.48

3

240272808

-588492

__5.273.03

.02"

.22

.0519.48

4

360272404

-588492112174

5.273.03

.027

.22

.0519.48

5

480272

_-

5884922243485.273.03.027.22.05

19.48

PERTANIKAJ. TROP. AGRIC SCI. VOL. 16 NO. 2, 1993

UREA AS THE NITROGEN SOURCE IN NFT HYDROPONIC SYSTEM

TABLE 2Nitrogen supplement during the period of

plant growth

Treatments

T lT2T3T4T5

NO3-N (g)

14.79—

19.787.38-

Urea-N (g)

-

68.7

10.8-

30th and the 40th day. Plants were harvested onthe 40th day. PMA (phenylmercuric acetate) wasadded to samples to retard hydrolysis of ureabefore analysis of the samples was done. Sam-ples were analysed for NO3 and NH4

+ (Bremner1965) and urea (Douglas and Bremner 1970).

Plant samples were taken on the 20th, 30thand the 40th day. For the first two samplings, 9plants (one from each rockwool block taken atrandom) were harvested from each channel.The tops and roots were separated and weighed.The final harvest comprised 27 plants. Theywere oven dried, reweighed and ground. Foranalysis of NH4

+ and NO.,' in the samples, 1 gsample (oven dry) was shaken with 50 ml dis-tilled water for 1 hr and the extract filtered. 10ml of the extract was determined for NO3" andNH4

+ (Woolley et al 1960).

RESULTS AND DISCUSSIONResults of the analysis of the nutrient solutionon the 20th day (Table 3) indicated that themacronutrient element content was adequate(Cooper and Charlesworth 1977). The elementscontent in the leaves at harvest also indicatedthat their concentration in plants were adequate(Table 3). Visual observation of the plants indi-cated no apparent nutrient deficiency exceptthat the 100% N-urea treatment had darkergreen leaves than those in the other treatments.

Urea concentration in solution of treat-ments containing urea dropped rapidly fromthe 7th day onwards which snowed that it hadbeen hydrolysed. By the 20th day all the ureawas depleted from the nutrient solution (Fig. 1),In soils urea is found to be fully hydrolysedwithin 3 days after application mostly in theform of NH4

+ and by the 14th day most are inthe NO3 form (Gasser, 1964; Bundy and Bremner1974). Decrease in the urea concentration of

the solution in treatments containing urea wasrapid from the 7th day onwards. Loss of urea inthe 100% N- urea with plant treatment wasgreater compared to the same treatment with-out plants on the 10th day. It is possible that theurease activity was greater in solution whereplants were present. Elliott (1986) found hy-drolysis of urea in cropped media more rapidthan in uncropped media. Between the1st and 7th day the urea loss was about 5%.Between the 7th and 10th day, urea in solutionof cropped treatments decreased by 30%. Mostof the urea was hydrolysed between the 10thand 20th day. Ammonium in the solution wasdetected as early as the first day after the start ofthe experiment in all the treatments receivingurea, indicating that hydrolysis of urea occurredimmediately.

200 -

1 5 0 -

100-

50 -

0 -

\ / V \ \ © — OT1

V \ \ D —D T4

A-Fig. 1: Urea concentration in solution of the various treat-

ments

Ammonium in Solution

Ammonium concentration in the nutrient solu-tions increased until the 20th day, thereafter itsconcentrations began to decline except in thetreatment without plants (Fig.2). Plant uptakeand the transformation of NH4

+ to NO3' vianitrification are the possible reasons for thedecline after the 20th day. Furthermore, itssource, urea, had been depleted by that time.Ammonium concentration in solution dependedon the amount of urea in the treatments. Highurea levels resulted in high NH4

+ concentra-tions. The highest NH4

+ concentration was re-corded in the 100% N-urea treatments. Withplant growth the decrease was higher whichcould be due to plant uptake and increasednitrification. Decline in NH4

+ concentration inthe treatment without plants after the 20th day

PERTANIKAJ. TROP. AGRIC. SCI. VOL. 16 NO. 2, 1993 89

100046816SAMINUDDIN H., KHALIP R., NORAYAH K. AND ALIAS H.

was slight. 75% and 50% N- urea treatmentsattained maximum NH4

+ concentration on the20th day and the NH/ concentrations were inproportion to the amount of urea added. It cantherefore be said that rate of hydrolysis of ureawas not affected by the amount of urea presentwithin the amounts of urea used in the experi-ment. This showed that urease was a non-limit-ing factor. Several researchers have found thatin soils the rate of urea hvdrolysed by soil ureaseincreased with increase in urea concentrationuntil the amount of urea added is sufficient tosaturate the enzyme with the substrate (Douglasand Bremner 1971; Tabatabai and Bremner 1972;Dalai 1975).

Fig. 2: NH4* concentration in solution oj the various treat-ments

Nitrate in Solution

The extent of nitrification occurring in the solu-tion containing urea is shown in Fig, 3. Similarto the NH4

+ buildup, NO3 increase in the nutri-ent solution was observed in the 100% N-ureatreatments with and without plants; however thebuildup started later (from the 7th day on-wards). While treatments with 100% and 50% N-NO5 source declined for the period of 7- 10thday, treatments with 100%N-urea had NOS con-centration increased during the same period.

Maximum concentration of NO5 in thenutrient solution in the 100% N-urea treatmentswas attained on the 20th day. Subsequent peri-ods showed no increase in nitrate concentrationas shown by the 100% N- urea treatment withoutplants. At the 20th day the concentration ofNH4

+ was only one third of NOV

Ammonium buildup and nitrate formationin the treatments containing urea occurred si-multaneously. While 100% N-urea treatment with

plants had a NOS" concentration decrease afterthe 20th day, treatment without plants main-tained the NO3 concentration until the 40thday. Nitrate concentration in treatments whichcontained urea decreased more sharply untilthe 40th day compared to plants in the 100% N-NO3 treatment.

Fig. 3: NOf concentration in solution of the various treat-ments

Total N in solution

Summation of the various forms of N (NO3\NH4

+ and urea) in the solution of the treatmentsis presented in Fig. 4. Total N remained thesame for the first 7 days. From the 7th to 10thday total N of treatments 100% and 50% N-NO3"was lower compared to the others. From the10th to the 20th day , 100% and 75 % N- ureatreatments decreased in total N value. Furtherdecline in total N occurred in all treatmentsexcept the one without plants from the 20th tothe 30th day. At the 40th day, treatments whichcontained urea had lower values compared to100% N- NO^ treatment. From the 20th dayonwards the treatment without plants maintainedits total N to the end.

O — O T1A A T2a—a T3V V T4

O T5

big. 4: Total N in solution of the various treatments

90 PERTANIKAJ. TROP. AGRIC. SCI. VOL. 16 NO. 2, 1993

UREA AS THE NITROGEN SOURCE IN NFT HYDROPONIC SYSTEM

TABLE 3Macronutrient concentrations in the solution (ppm)

on the 20th. day and in the leaves at harvest (%)

Elements Treatments

sorption by plants was achieved when NH4+ and

NO3 were combined in the nutrient solution.The amount of N taken up by the plants ac-counts for 80% of the N supplied in the solu-tion.

K (solution-s) 306 332 249 263(leaves-1) 11.5 8.5 8.2 8.4

P (s) 29 52 32 39(1) L33 1.10 L04 1.22

Ca (s) 175 214 227 200(1) 3.70 3.08 3.09 3.25

Mg (s) 68 42 43 42(1) .63 .56 .43 .50

N (1) 4.97 5.51 5.18 4.4

Plant dry weight

Plant dry weight on the 20th day for 100% N-urea treatment was the lowest recorded and wassignificandy different (P<0.05) from those of 100%and 50% N-nitrate treatments (Table 4). Thisinitial difference in the dry weight can be attrib-uted to the time taken for urea to be hydrolysedbefore NH4

+ and the subsequent release of NO3"in the solution for plant use. On the 30th day, theplant dry matter weight of the 100% N- nitratetreatment was not significandy different from thatof 100%N-urea treatment. However, the two treat-ments with mixtures of urea and nitrate regis-tered the highest value. Studies by Cox andReisenauer (1973) have shown that mixtures ofNH4

+ and NO3 in solution give higher yield thanthose of NO3" or NH4

+ supplied singly. Moneeratet al (1982) reported that maximum dry weightwas achieved with a ratio of 60: 40 (NH4

+: NO3) inthe solution. Dry matter weight on the 40th dayshowed no significant difference among the fourtreatments (Table 4).

Total N Uptake by Plants

Total N uptake values normally follow dry mat-ter weight figures. Total N uptake values ofBrassica at harvest (40 days) were not signifi-cantly different among treatments. The 50% N-urea and 50% N-nitrate mixture recorded thehighest N uptake value of 3.16 g. Baker andMaynard (1972) reported that maximum N ab-

Ammonium in RootsThe percent coefficient of variation (%C.V.) ofthe NH4

+ content in the plant roots on the 20thday growth stage was high due to the problem ofseparating the roots from the rockwool media.At this stage the roots were very fine and werefirmly embedded in the media making the taskof separation difficult. At the 30th and 40th day,the roots were thicker and bigger and therockwool was softer making the task of separa-tion easier. This was reflected in the decreasingtrend in the C.V. value as the plants matured.No significant difference in ammonium concen-tration in the plant roots of the 20- and 30-day-old plants of the various treatments was ob-served (Table 4). Plants treated with 100% N-nitrate did contain NH4

+ in the roots. Thereduction and assimilation of NO3 in the rootsproduce NH4

+ as an intermediary product (Pate1973). NH4

+ concentration in the roots of the30- and 40-day old plants treated with 100%N-urea was higher than those treated with 100%N-nitrate. Pill and Lambert (1977) noted that ingeneral, plants supplied with NH4

+ have a higherconcentration of free NH4

+ than plants grown atcomparable levels of NO3-. NH4

+ accumulationin the roots of the 100% N-urea treatment couldalso be due to the slow conversion of NH3" intoglutamine which is a function of the carbohy-drate supply (Vickery et al 1936).

Ammonium in Leaves

Ammonium concentration in the leaves of 20-day-old plants treated with 100% N-urea washigher than that in plants in the 100% N- nitratetreatments (Table 4). Pate and Wallace (1964)found that the bulk of the NH4

+ absorbed byroots is converted to amino acids by the meta-bolic system of the roots and only a small per-centage of the NH4

+ is found in the xylemexudate presumably transported to the leavesfor assimilation. It is possible that in leafy vegeta-bles, the leaves play a major role in the assimila-tion of NH4

+ which thus results in concentrationof NH4

+ in leaves being comparable to that inthe roots. On the 30th day , NH4

+ concentrationin the leaves in all the treatments was similar.

PERTANIKAJ. TROP. AGRIC. SCI. VOL. 16 NO. % 1993 91

AMINUDDIN H., KHALIP R., NORAYAH K. AND ALIAS H.

TABLE 4Dry matter yield, total N, NH4

+ and NO3~ content of pak choy

Dry matter yield(g/9 plants)

Day

Total N uptake(g/9 plants)

Day

NH4 (roots) ppm

Day

NH4 (leaves) ppm

Day

NO3 (roots) %

Day

NO3 (leaves) %

Day

203040

40

203040

203040

203040

203040

CV. (%)

21.8930.7019.95

23.86

91.5829.7214.92

43.6628.1323.79

54.6047.9946.41

46.3033.7846.93

1

1.2616.5

135.18

2.20

aaba

a

824.6 a570.0 . b

2010.0 b

324.51260.01680.0

2.130.300.10

3.272.481.01

bab

aaa

aaa

2

0.7510.77

147.69

2.75

803.61247.82910.0

693.81620.02790.0

1.100.400.09

2.271.630.99

Treatments3

bba

a

aaa

aaa

abaa

aaba

1.2621.61

184.91

3.16

200.01470.02610.0

330.01680.02100.0

0.900.360.12

2.731.551.21

aaa

a

aaab

baab

abaa

aaba

4

1.2119.43

187.38

2.76

304.31320.02610.0

360.01350.02340.0

0.730.200.10

2.381.210.89

ababa

a

aaab

abaab

baa

aba

Means with the same letter in a row are not significantly different at 0.05 using DMRT.

This period coincides with the high concentra- NH4+ increased with plant growth, NO3- concen-

tion of NH4+ in the nutrient media. As the

concentration of NH4+ in solution started to fall

by the 40th day, NH4+ concentration in leaves of

100%N-nitrate plants decreased. Since 100% N-nitrate treatment plants had less than 5% NH4

T

content in the nutrient solution, we can deducethat the NH4

+ present in the the leaves was fromthe reduction of NO3 . The NH4 ' concentrationin the leaves increased as the plant matured.

Nitrate in Roots

Nitrate concentration in roots was in contrastwith that of NH4

+. Whilst the concentration of

tration decreased. NO3: NH4+ concentration ra-

tio was >20 in the 20-day-old plants and was <1in the 40-day-old plants. Increased NO3" assimila-tion to amino acids in plants as they maturecould be the reason for the fall in NO3" concen-tration. No significant difference in % nitrate inroots of plants treated with 100% N-nitrate and100% N-urea at the 20th, 30th and 40th day wasobserved.

Nitrate in Leaves

NO3* concentration in the leaves of pak choy washigher than in the roots. Pate (1973) found that

92 PERTANIKAJ. TROP. AGRIC. SCI. VOL. 16 NO. 2, 1993

UREA AS THE NITROGEN SOURCE IN NFT HYDROPONIC SYSTEM

the reduction of NO3' concentration in the leavesis relative to the roots and varies widely amongplant species. It is possible that NO3' assimilationin the leaves of pak choy is more intense than inthe roots. In maize plants, the roots were foundto reduce about 1/3 of the nitrate and thepercentage of NO3- reduction decreased as theplant aged (Raghuveer 1977). No significantdifference in % nitrate in leaves between treat-ments was observed at the three plant growthstages (Table 4). The amount of NO3 in theleaves decreased as the plant growth increased.Percent NO3 was ten-fold higher than NH4

+ inthe leaves, the ratio being greater when plantswere young.

CONCLUSION

Urea as the source of N used in nutrient solu-tion in the NFT hydroponic system undergoeshydrolysis with the release of NH4

+ and thesubsequent formation of NOV The urea wascompletely hydrolysed by the 20th day. Therewas no difference in either the plant dry weightin the 100% N-urea or 100% N-nitrate treat-ments at harvest. NH4

+ concentration in rootsand leaves was higher in the 100% N-urea treat-ment than in the 100% N-nitrate treatment. Nodifference in NO3 concentration in roots andleaves of 100% N-nitrate and 100% N-urea treat-ments was detected. The study showed the pos-sibility of using urea as the N source for the NFThydroponic system in crop production.

REFERENCES

BAKER, A.V and D.N. MAYNARD. 1972. Cation and

nitrate accumulation in pea and cucumberplants as influenced by N nutrition. / . Amer.Soc. Hort. Sci. 97: 27-30.

BREMNER, J.M. 1965. Inorganic Forms of Nitrogen.In C.A. Black (ed.). Methods of Soil Analysis,Part 2. 1179-1232. Madison, Wisconsin: Am.Soc. Agron.

BUNDY, L.G. and J.M. BREMNER. 1974. Effects of

nitrification inhibitors on transformation ofurea N in soils. Soil Biochem. 6: 369-376.

COOPER, AJ. 1975. Crop production in recirculatingnutrient solution. Scientia Hortic. 3: 251-258.

COOPER, AJ. and R.R. CHARLESWORTH. 1977. Nutri-

tional control of a nutrient-film tomato crop.Scientia Hortic. 7: 189-195.

Cox, WJ. and H.M. REISENAUER. 1973. Growth andion uptake by wheat supplied nitrogen as ni-trate, or ammonium, or both. Plant Soil 38:363-380.

DALAI., R.C. 1975. Urease activity in some Trinidadsoils. Soil Biol Biochem. 7: 5-8.

DOUGLAS, C.A. and J.M. BREMNER. 1970. Extraction

and colorimetric determination of urea. SoilSci. Soc. Am. Proc. 34: 859-862.

DOUGLAS, C.A. and J.M. BREMNER. 1971. A rapid

method of evaluating different compounds orinhibitors of urease activity in soils. Soil Biol.Biochem. 3: 309-315.

ELLIOTT, G.C. 1986. Urea hydrolysis in pottingmedia. / Am. Soc. Hort. Sci. 6: 862-866.

FRAKENBERGER, W. T. and M.A. TABATABAI. 1982.

Amidase and urease activities in plants. Plantand Soil 64: 153-166.

GASSER, J.K.R. 1964. Urea as a fertilizer. Soils Fert.27: 175-180.

GOULD, W.D., C. HAGEDORN and R.G.L. MCCREADY.

1986. Urea transformations and fertilizer effi-ciency in soil. Adv. in Agron. 40: 209.

MONEERAT, P.H., P.D. CASTALLANE, J.L.C. ZAMBON,

J.G. PANDUA and JJ.V. MULLER. 1982. Effect of

ammonium to nitrate ratio on initial develop-ment and mineral composition of tomato,Lycopersicon esculentum. Proceedings of Amer. Soc.Hort. Sci. 25: 451-455.

PATE, J.S. 1973. Uptake, assimilation and transportof nitrogen compounds by plants. Soil Biol.Biochem. 5: 109-119.

PATE, J.S. and W. WALLACE. 1964. Movement of

assimilated nitrogen from the root systems ofthe field pea (Pisum arvense L.) Ann. Bot. 28:83-99.

PILL, W.G. and V.N. LAMBERT. 1977. Effects of NH4

and NO3 nutrition with and without pH adjust-ments on tomato growth, ion compositionsand water relations. / . Am. Soc. Hort. Sci. 102:78-81.

RAGHUVEER, P. 1977. Characteristics of nitrate up-take from nutrient solution and root nitratereductase activity among corn genotype. Ph.DDissertation. Univ. of Illinois, Urbana-Champaign.

PERTANIKAJ. TROP. AGRIC. SCI. VOL. 16 NO. 2, 1993 93

AMINUDDIN H., KHALIP R., NORAYAH K AND ALIAS H.

SAHRAWAT, K.L. 1980. Urease activity in tropical rice VLEK, P.L.G., J.M. STUMPE and B.H.BYRNES. 1980.

soils and floodwater. Soil BioL Biochem. 12: 195- Urease activity and inhibition in flooded soil1 9 6 systems. Fert. Res. 1: 191-202.

SKUJINS, JJ . 1976. Extracellular enzymes in soils. WOOLLEY, J.T., G.P.HICKS and R.H. HAGEMAN. 1960.

CRC Crit. Rev. Microbiol 4: 383-421. Rap i d determination of nitrate and nitrite inplant material./. Agric. and Food Chem. 8: 481-

TABATABAI, M.A and J.M.BREMNER. 1972. Assay of 432.

urease activity in soils. Soil BioL Biochem. 4: 479-487.

VICKERY, H.B., G.W. PUCHEZ and H.E. CLARK. 1936.Glutamine metabolism of the beet. Plant PhysioL11: 413-420. {Received 27 August 1991)

94PERTANIKAJ. TROP. AGRIC. SCL VOL. 16 NO. 2, 1993