~ Nutrient Cycling in Agroecosystems 68: 1-11,2004." @ 2004 Kluwer Academic Publishers. Printed in the Netherlands.

Utilization of applied fertilizer nitrogen and irrigation water bydrip-fertigated squash as determined by nuclear and traditionaltechniques

Munir Jamil MohammadDepartment of Natural Resources and Environment, Faculty of Agriculture, Jordan University of Science andTechnology (JUST), P.O. Box 3030, [rbid, Jordan; (e-mail: mrusan@just.edu.jo; fax: +962-2-7095069)

Received 15 August 2002; accepted in revised form 29 April 2003

Keywords: Fertigation, Neutron probe, 15N labeled fertilizers, Squash, Water and N use efficiency

Abstract

Field experiments were conducted to evaluate utilization of applied nitrogen (N) and irrigation water to squash inresponse to method of fertilizer application and rates of fertigation N. The following treatments were studied ina randomized complete block design with four replications: zero N (NO), 50 (Ni), 100 (N2) and 150 (N3) mg NL - 1 in the irrigation water (IW). Additional soil application treatment (NS) equivalent to N2 was included. Thefertilizers were either injected into IW by means of an injection pump for the fertigation treatments or applieddirectly to the soil followed by irrigation for the soil application treatment. Squash was planted in plots with fourrows per plot. Each plant row had its own irrigation line and each plant its own dripper. Irrigation was applied toreplenish 80% of the Class A pan evaporation twice a week. Neutron probe readings were taken before and aftereach irrigation at 15, 30, 45, 60 and 90 cm soil depth. 15Nlabeled fertilizers were applied to microplots whichcontained four plants within each plot. At harvest, plant samples were taken from the microplots for the 15Nmeasurements and from the mainplot for yield determination and chemical analysis. Nitrogen uptake and % of Nderived from fertilizers (%Ndff) increased with fertigation compared to soil application treatment. N uptake dffhad a similar trend as the Ndff. Fertilizer N utilization efficiency (FNUE) by fruits as determined 15Nwas lowerwith the soil application than with fertigation treatments and tends to decrease with increasing N fertigation rates.FNUE values calculated by the difference method were higher than those determined by 15N data. All compo-nents of FNUE except the recovery efficiency decreased with the soil application method and with increasingfertigation N rates. FNUE correlated positively with squash yield in both seasons but was stronger in the secondseason. Water consumption increased with increasing N rates. Water use efficiency increased with N applicationand was higher with fertigation treatment. Water depletion was maximum in the 30 cm topsoil, suggesting thedepth of maximum roots growth. It was concluded that yield, FNUE and WUE increased with N fertigationcompared to soil application. The difference method tends to give higher FNUE values under relatively less fa-vorable climatic conditions compared to those measured by 15Ndata. Overall, the results indicate that fertigationis a more efficient technique for fertilizer application to squash.

Introduction good management and becomes crucial in arid andsemi arid regions where water resources are limited.In addition, in dryland irrigated agriculture, nitrogen(N) becomes the most limiting factor for cropproductivity. Under these conditions, the use effi-

Water and nutrient supply are the main factors con-trolling productivity of irrigated agriculture. Improv-ing the use efficiency of these factors is the target of

ciency of both ilTigation water (IW) and N is oftenlow and depends largely on the method of application(Bar-Yosef 1999).

Drip ilTigation as the most efficient ilTigationmethod (Sammis 1980; Sharmasarkar et al. 2001) israpidly expanding and highly recommended in theseregions. With this pressurized ilTigation method, con-ventional fertilization, which is still commonly prac-ticed by farmers (Bachchhav 1995; Bar- Yosef 1999),is not convenient nor efficient (Papadopoulos 1988).On the other hand, drip ilTigation is ideally suited forcontrolling the placement, time and rate of fertilizerapplication (Goldberg et al. 1976). By controllingthese N fertilization practices it is possible to enhancethe fertilizer nitrogen utilization efficiency (FNUE)and crop production and to minimize potential Nlosses (Papadopoulos 2000). The latter is of particu-lar importance in an arid environment where N lossesby volatilization are high (Mohammad et al. 1999).

The rate and time of fertilizer N application can bebetter controlled with fertigation (application of fer-tilizer N through ilTigation water; Papadopoulos2000). Therefore, fertigation of vegetable crops underpressurized ilTigation is increasingly expanding inmost countries of the arid and semi-arid regions. Inaddition, FNUE was improved with fertigationthrough simultaneous management of both IW and Napplication (Hargert et al. 1978; Starck et al. 1993;Muchow and Sinclair 1994; Mohammad et al. 1999).

Summer squash is considered one of the main veg-etable crops grown in most countries of the Mediter-ranean region. Scarcity of water in these countries ismore severe in summer, when summer squash is pro-duced. Therefore, high production and quality andthus high net economic return for the farmers bygrowing summer vegetable crops can be achievedonly through better use efficiency of IW and fertiliz-ers. Extensive fertigation research on most vegetablecrops is available. However, very few studies havebeen conducted on fertigation of summer squash. Be-sides, the use of 15N labeled fertilizers and neutronprobe provides accurate tools to evaluate the Nandwater utilization efficiency (Zapata 1990). The objec-tives of this study are to evaluate N and water utili-zation efficiency in response to method of fertilizerapplication and different rates of N fertigation using15N labeled fertilizer and neutron probe techniques.Additional objectives for this study were to analyzecomponents of FNUE and to compare the direct andindirect methods of determination of FNUE.

Materials and methods

This study was implemented at the Research Centerof Jordan University of Science and Technology(JUST). The research site is located in Irbid, Jordan,32Q30' N latitude and 35Q59' E longitude with an el-evation of 590 m above sea level. The area is char-

acterized by having an aridic moisture regime with awarm rainy winter and a hot long dry summer. Therainy season extends from October to April where thehighest amount of precipitation occurs during the pe-riod from January to March. The mean maximum andmean minimum temperatures during the growing sea-sons were 31.8 QC and 17.6 QC in 1999 (July-Sep-tember) and 27.4 QC and 11.2 QC in 2000 (April-June), respectively. The monthly mean maximum andminimum temperatures in 1999 were 32.8 QC and19.1 QCfor July, 34.1 QCand 19.2 QCfor August and31.5 QCand 17.3 QCfor September, respectively. Themonthly mean maximum and minimum temperaturesin 2000 were 26.5 QCand 10.5 QCfor April, 28.3 QCand 11.9 QC for May and 32.8 QC and 15.9 QC forJune, respectively. The soil is basic and alkaline, poorin organic matter, Nand P. Available K is consideredto be adequate for normal plant growth. The soil isfine textured with relatively high CEC. The soil at theexperimental site is classified as fine-loamy, mixed,thermic, calcic Paleargid (Khresat et al. 1998).

The following treatments were investigated in twofield experiments in a randomized complete blockdesign (RCBD) with four replications of each treat-ment: (i) zero N application (NO); (ii) 50 mg N L - 1

irrigation water (IW) (NI); (Hi) 100 mg N L - 1 IW(N2); (iv) 150 mg N L - 1 IW (N3); and (v) conven-tional soil application of solid N fertilizer at a rateequivalent to N2 (NS). The first four treatments wereapplied through the IW so that N was applied in eachirrigation except for the zero treatment. Nitrogen asammonium sulfate was applied in each ilTigation togive the required N concentration for each treatment.Phosphorus at a concentration of 50 mg P L - 1 in the

IW as phosphoric acid was added identically to alltreatments. Potassium was not applied to anytreatment due to the high soil K content. The fertiliz-ers were injected into IW by means of an injectionpump (1:100 dilution factor). The injection pump wasdriven by the pressure in the main line. Two injectorswere used for injecting the fertilizers into the IW; onefor application of N treatments (rates) and the otherfor the application of P.

Squash (cv. Hybrid Squash Scarlette) was plantedon June 26, 1999 and harvested on September 15,1999 in the first season and planted on April 2, 2000and harvested on July 7, 2000 in the second season.Squash was planted with 40 cm between plants and150 cm between rows. Plot dimensions were 6 mX 5

ID. Each plot contained 4 rows, each 6 m long. Eachrow had its own irrigation line positioned near theplants. Emitters were spaced 40 cm apart in the irri-gation line. Irrigation was applied to replenish 80%of the Class A pan evaporation twice a week.

Access tubes for neutron probe reading weremounted in the middle of the second row of each plotin three replicates. The neutron probe readings weretaken before and after each irrigation at 15, 30, 45,60 and 90 cm soil depths. The crop evapotranspira-tion (ETc) and soil moisture depletion with soil depthwere determined using the neutron probe readings.The crop coefficient was calculated as the ratio of theactual evapotranspiration to the potential evapotrans-piration (Doorenbos and Pruitt 1992) and water useefficiency was calculated by dividing fruit yield byETc. The 15N labeled N fertilizers were applied tomicro-plots which contained four plants within eachplot. The microplots were fertigated through invertedbottles with drippers which simulate the drippers ofthe original irrigation line (Papadopoulos 1995). Themain plots were fertigated with the drip-irrigationsystem.

At the final harvest, two middle whole plants ineach of the microplots, where the labeled fertilizerswere applied for the 15N measurements, werecollected and shoot dry matter was recorded. Thefruits from the microplots were collected, oven driedat 70 QC,weighed and stored. Oven dried shoot andfruit samples were ground to pass a 1 mm sieve andstored for tissue analysis. Plant samples werecollected and prepared according to the instructionsfor sampling for 15N analysis (Zapata 1990). Shootand fruit samples were taken from microplots andanalyzed for total N with the micro-Kjeldahl methodand for 15N atomic excess by the dry combustiontechnique with a mass spectrometer at the centrallaboratory of the International Atomic EnergyAgency, Seibersdorf, Vienna, Austria. The N derivedfrom fertilizer (Ndff) and FNUE were calculated ac-cording to Zapata (1990) using the following formu-las:

15%Ndff = (% N a.e. plant sample/

%15Na.e. labeled fertilizer) x 100

N updff=N uptakex%N dff

Fertilizer nitrogen utilization efficiency

= (NupdfflRate of N applied) x 100

where: Ndff=N derived from fertilizer; Nupdff=Nuptake derived from fertilizer, and FNUE=fertilizernitrogen utilization efficiency.

Fruit yield was determined from harvesting themiddle two rows of the main plot and the yield wascalculated per one hectare area. Plant shoot and fruitsamples taken from the main plots receiving the non-labeled N fertilizers were oven dried at 70 QC and

weighed to obtain the dry matter for each sample.Samples were then ground to pass a 2 mm sieve. Plantsamples were analyzed for total N using a modifiedmicro- Kjeldahl digestion procedure (Nelson andSommers 1980).

Fertilizer N use efficiency by agricultural crops canbe determined also using a non-isotopic differencemethod (Pandey et al. 2001). The high cost of 15Nla-beled fertilizers usually limits their use in determina-tion of FNUE, therefore, the difference method is

commonly used. However, both methods are comple-mentary. Nitrogen utilization efficiency (NUE) iscomposed of several components that are affecteddifferentially by soil moisture and method of fertilizerapplication (Huggins and Pan 1995; Paponov et al.1995). It has also been reported that NUE parameters(components) decreased as N rate increased (Pandeyet al. 2001). The NUE component analysis couldidentify better management practices for enhancingplant growth and NUE (Sowers et al. 1994; Cassmanet al. 1998). Therefore, using the main plot data theNUE and NUE components were calculated accord-ing to Pandey et al. (2001) using the non-isotopic dif-ference method and the following formulas:

Physiological FNUE (FNUEp)

= (FWf- FWc)/(TNf - TNc), kg kg-1

Agronomical FNUE (FNUEa)

= (FWf - FWc)/NA, kg kg-1

4

Table 1. Treatments investigated in both 1999 and 2000 experiments and the amount of water, N and P applied.

Concentration of N in irrigation water

Second season (2000)

Total amount applied (kg N ha -1)

First season (1999)

NO=O N mg L - 1Nl=50 N mg L - 1N2=1O0NmgL -1N3= 150 N mg L - 1NS=soil application

The following were also applied:Fertigation water (m3 ha - 1)Total water (m' ha - 1)P(kgha-l)

0

66132

198128

091

182273192

12101490

39.4

18202330

57.6

Recovery efficiency (RE)

= (TNf - TNc)/NA, kg kg-1

Partial factor productivity (PFP)

= FWf/NA, kg kg-1

FWf = fruit weight of fertilizer treatment

where: FWc =fruit weight of control treatment;TNf=total N uptake by fruit and shoot in fertilizertreatment; TNc=total N uptake by fruit and shoot incontrol treatment; and NA=nutrient (N or P) applied.

Amount of IW and N applied

The total amounts of N applied through the IW in1999were 0, 66, 132 and 198kg N ha - 1 for the NO,NI, N2, and N3 fertigation treatments, respectively,and 128 kg N ha - 1 for the base soil applicationtreatment. In 2000 the total amounts were 0, 91, 182and 273 kg N ha - 1 for the NO, NI, N2, and N3 fer-tigation treatments and 192 kg N ha - 1 for the basesoil application treatment (Table 1). The amounts offertigation water (IW with N fertilizers) were 1210and 1820 m3 ha - 1 for the 1999 and 2000 seasons,respectively. The total amounts of P applied as phos-phoric acid in the IW were 39.4 and 57.6 kg P ha - 1

for the 1999 and 2000 seasons, respectively. The soiltest values for K indicated the presence of adequateamounts of this nutrient in the soil for normal growth.Therefore K was not applied. The differences in Napplied with fertigation between the two seasons aredue to the differences in the amount of applied waterin the two seasons. The total amounts of IW applied

were 1490 and 2330 m3 ha - 1 for the first and second

seasons, respectively.

Statistical analysis

Analysis of variance (ANOVA) was used to deter-mine the effect of each treatment. When F ratio was

significant, a multiple mean comparison was per-formed using Fisher's Least Significance Test (0.05probability level). Statistical analyses including re-gression between fruit yield and FNUE componentsin 1999 and 2000 (n= 16) were performed with theSYSTAT statistical program (Wilkinson 1990).

Results and discussion

Fertilizer nitrogen utilization efficiency (FNUE)

Fertilizer N utilization efficiencies by fruits andshoots of squash growing in 1999 and 2000 as deter-mined by the isotopic 15N labeled N fertilizer arepresented in Tables 2 and 3, respectively. The N up-take by the fruits and shoots increased with fertilizerN application regardless of method of application.The highest values were obtained by the highest twofertigation treatments. Compared to the soil N appli-cation (Ns), fertigation of the equivalent amountthrough IW significantly increased the N uptake bothby the fruits and shoots. Similar results were obtainedin the second season.

It was also found that the amounts of total N taken

up by the squash receiving 66 and 91 kg N ha- 1 in

the 1999 and 2000 seasons, respectively, by drip fer-tigation were comparable to those obtained with 128and 192 kg N ha - 1 applied with conventional soil

aNO=O, N1 =50, N2= 100, N3=150 mg N L - 1; NS=soil application equivalent to N2; bN uptake in kg N ha - 1; Nup=nitrogen uptake;Ndff=nitrogen derived from fertilizer; FNUE=fertilizer nitrogen utilization efficiency.

aNO=O, N1 =50, N2= lOO, N3= 150 mg N L - 1; NS=soil application equivalent to N2; bN uptake in kg N ha - 1; Nup=nitrogen uptake;Ndff=nitrogen derived from fertilizer; FNUE=fertilizer nitrogen utilization efficiency.

application in the 1999 and 2000 seasons, respec-tively. On the other hand, the FNUE values of thesedrip fertigated treatments were 2.4 and 2.5 timeshigher in both seasons compared to the conventionalfertilizer application. This indicates that less fertilizerN can be applied by fertigation to obtain a level of Nuptake similar to that obtained with much higher ratesof fertilizer N applied by conventional soil applica-tion. In the 2000 season, the non-fertilized squashshoots and fruits extracted more N (70.36 kg ha - I)from the soil than in the 1999 season (39.33 kgha - I), indicating the positive impact of the 2000season climatic conditions on plant growth and utili-zation of the native soil N (Tables 2 and 3).

The Ndff% was also enhanced with fertigation ap-plication compared to soil application in both seasons.Consequently, the N uptake derived from fertilizer(Nup dff) was also higher with fertigation. The FNUEranged from 20.32 to 47.89% in 1999 and from 27.17to 68.93% in 2000. The values of FNUE obtained in

the 2000 season were relatively higher, suggesting apositive impact of the more favorable climatic condi-tions on enhancing N utilization by the crops. In ad-dition, and for the same reason, the fraction of theapplied N lost by volatilization is expected to behigher in the 1999 season, where the crops grew dur-

ing a hotter period (Mohammad et al. 1999). Nitro-gen utilization percentage by the crop (fruits+shoot)tends to decrease with increasing rates of fertigationtreatments in both seasons. This was in agreementwith the findings of other researchers (Pandey et al.2001; Sharmasarkar et al. 2001). In addition, FNUEwas enhanced with fertigation compared to the soilapplication. Enhancement of FNUE with fertigationhas been confirmed for other vegetable crops by otherresearchers (Kee Kwong et al. 1999; Mohammad etal. 1999). The lower FNUE with the soil applicationtreatment could be attributed to the possible losses ofN by volatilization and/or denitrification (Mohammadet al. 1999).

The FNUE for shoot and fruit were also determined

using the non-isotopic difference method application( Table 4). The values for FNUE showed a similartrend but were higher than those determined by the15Ndata in 1999. However, in 2000, both methods ofdetermination gave similar values for the FNUE.Other researchers reported higher values of FNUEwhen determined by the difference method (Harmsenand Moraghan 1988) or similar (Pilbeam et al. 2002)compared to that determined by the isotopic method.In 1999 the temperature was higher, which may havestimulated the mineralization of native soil N in the

Table 2. Fertilizer nitrogen utilization as determined by isotopic 15N labeled fertilizer to squash crops in 1999.

Trta Applied nitrogen (kg N ha -1) Nupb Ndff% Nup dff' FNUE (%)

Fruit Shoot Fruit Shoot Fruit Shoot Total

NO 0 28.24 11.09 - -N1 66 68.90 18.32 35.04 39.84 24.10 7.37 31.46 47.89N2 132 100.7 33.21 43.07 45.37 44.07 15.41 59.48 45.27N3 198 93.65 33.36 49.95 49.70 46.34 17.60 63.94 32.44NS 128 76.51 17.70 27.10 33.46 20.03 5.98 26.01 20.32

LSDo.O5 10.7 3.74 7.61 6.31 4.82 3.90 6.01 6.96

Table 3. Fertilizer nitrogen utilization as determined by isotopic 15N labeled fertilizer to squash crops in 2000.

Trta Applied nitrogen (kg N ha - 1) Nupb Ndff% Nup dff" FNUE (%)

Fruit Shoot Fruit Shoot Fruit Shoot Total

NO 0 49.11 21.25 - -N1 91 101.2 32.38 47.19 45.40 47.80 14.92 62.72 68.93N2 182 119.5 41.50 57.25 56.46 67.63 25.08 92.71 50.94N3 273 95.33 67.75 67.24 69.39 64.61 47.79 112.40 41.17NS 192 81.67 37.63 45.63 38.00 37.45 14.49 51.94 27.05

LSDo.o5 - 20.85 9.64 8.70 12.36 8.14 5.66 20.23 8.17

Table 4. Nitrogen uptake and fertilizer utilization efficiency as determined by non-isotopic difference method for the 1999 and 2000 growingseasons.

aNO=O, N1=50, N2=100, N3=150 mg N L -I; NS=soil application equivalent to N2; means within a column with different letters are

significantly different using the LSD test at the 0.05 probability level. FNUEp=physiological fertilizer nitrogen utilization efficiency;FNUEa=agronomical fertilizer nitrogen utilization efficiency; RE=recovery efficiency; PFP=partial factor productivity.

fertilized treatments; this could in part explain thehigher values of FNUE in 1999 (Harmsen and Mor-aghan 1988).

It should be mentioned that the highest fertigationN rate not only lowered the FNUE but also did notincrease the sugarcane yield, thus lowering the farm-ers' income and wasting the fertilizer N (Kee Kwonget al. 1999). Besides, the higher N uptake observedwith the highest fertigation N rates without a positiveeffect on the yield illustrates the luxury consumptionof N (Quawasmi et al. 1999). If such luxury

consumption took place during the fruiting period, itcould lead to nitrate accumulation in the fruits,reducing their quality (Gardner et al. 1985).

The component analysis of the nitrogen utilizationefficiency (NUE), presented in Table 5, indicates thatall components were affected similarly by the treat-ments except the recovery efficiency (RE). Namely,physiological NUE (NUEp), agronomic NUE (NUEa)and partial factor productivity (PFP) decreased withincreasing rates of fertigation N and with the soil ap-plication technique. Other researchers found that all

Trta Applied nitrogen Nup (kg N ha - I) FNUE (%)(kg N ha - ')

Fruit Shoot Total Fruit Shoot Total

1999NO 0 28.24 c 11.09c 39.33 c - -NI 66 68.90 b 18.32 b 87.22 b 61.60 a 10.95 b 72.55 aN2 132 100.7 a 33.21 a 133.91 a 54.87 b 16.76 a 71.63 aN3 198 93.65 a 33.36 a 127.01 a 33.03 c 11.25 b 44.28 bNS 128 76.51 b 17.70 b 94.21 b 37.71 c 5.16 c 42.87 b

2000NO 0 49.11 c 21.25 d 70.36 c - - -NI 91 101.2 ab 32.38 c 133.58 b 57.19a 12.23 b 69.42 aN2 182 119.5 a 41.50 b 161.00 a 38.69 b 11.13 b 49.82 bN3 273 95.33 b 67.75 a 163.08 a 16.93 c 17.03 a 33.96 cNS 192 81.67b 37.63 be 119.30b 16.96 c 8.53 c 25.49 d

aNO=O, N1=50, N2=1O0, N3=150 mg N L -I; NS=soil application equivalent to N2; FNUE=fertilizer N utilization efficiency; meanswithin a column with different letters are significantly different using the LSD test at the 0.05 probability level.

Table 5. Nitrogen utilization efficiency (NUE) components.

Trta kgNha-l FNUEp FNUEa RE for N PFP for N(kg kg-I) (kg kg - I) (kg kg-I) (kg kg - I)

1999NO 0NI 66 36.46 a 26.45 a 0.73 a 144.6 aN2 132 23.61 b 16.91 b 0.72 a 75.97 bN3 198 22.94b 10.17 c 0.44 b 49.55 cNS 128 21.98 b 9.42 c 0.43 b 40.33 c

2000NO 0 -NI 91 107.3 a 74.51 a 0.69 a 185.9 aN2 182 83.91 b 41.81b 0.50 b 97.53 bN3 273 52.74c 17.91 d 0.34 c 55.05 dNS 192 84.01 b 21.41 c 0.25 d 74.22 c

Table 6. Correlation coefficients between the ITuit yield and theFNUE components.

Fruit yield1999

Fruit yield2000

1999

FNUEp 0.12 ns 0.69 **FNUEa 0.49 * 0.70 **

Re for N 0.72 ** 0.80 ***PFP for N 0.33 ns 0.76 ***

FNUE by 15N 0.71 ** 0.79 ***

FNUEp=physiological nitrogen utilization efficiency; FNUEa=agronomical nitrogen utilization efficiency; RE=recovery effi-ciency; PFP =partial factor productivity; FNUE by 15N=FNUE

measures using 15N isotope. ns, *, ** and *** refer to non-signifi-cance and significance at the 0.1, 0.05 and 0.001 probability lev-els, respectively.

components of NUE including the RE decreased withincreasing N rates (Pandey et al. 2001). The RE val-ues in this study, on the other hand, were high for thelower rates. This could be attributed to the fact that at

lower rates the crops were under-fertilized and theadded N was also subjected to volatilization losses,especially in the hot first season. This probably ren-ders the crops to rely more on the mineralized nativesoil N and then increase the RE, which relates theyield to the applied N only.

All components of the NUE were positively andstrongly correlated with squash yield in the 2000 sea-son ( Table 6). In the 1999 season, however, squashyield was positively correlated only with the FNUEaand RE. The FNUE as determined using the 15Nstable isotope was positively correlated with squashyield in both seasons. However, the correlations werealways more closely in the 2000 season. The FNUEpwas correlated with the yield only in the 2000 sea-son, indicating that the crops in this season were moreefficient in assimilating the absorbed N, thus conse-quently increasing the yield. It could be inferred,therefore, that a significant fraction of the N taken upby the crops in the 1999 season was not assimilatedand rather accumulated as nitrate in the plant parts.This could lead to lowering fruit quality associatedwith nitrate accumulation (Gardner et al. 1985). In the1999 season, the crops grew under high temperaturesand heat stress. These stress conditions have been re-

ported to disturb nitrate metabolism due to damage ofenzymatic systems and stimulate its accumulation(Gardner et al. 1985). More research is needed to in-vestigate how the NUE components are affected byvarious fertilization techniques. This information is

400

J

1999350

I ~~~

j

'

~ 200~

150

1O0iI

50

a aa a

-',_..NO NI N2 N3 NS

Treatments

400

350 1 2000

300

1250

~ 200~

a

150

100

50

NO NI N2 N3 NS

Treatments

Figure 1. Crop evapotranspiration (ETc) as affected by the various

treatments (NO, NI, N2 and N3 = 0, 50, 100 and 150 mg N L - 1irrigation water; NS = soil application in amount equivalent toN2). Colunms with different letters are significantly different at the0.05 probability level.

essential to draw conclusions on fertilizer use

efficiency and the fertilizer effects on the environmentand yield quality.

Utilization of the applied irrigation water

The estimated crop evapotranspiration (ETc) valuesusing the neutron probe readings were affected by thetreatments only in the 2000 season (Figure 1). In thisseason, fertigation N rates increased the ETc com-pared to the control treatment. Soil N applicationtreatment was less effective in increasing the ETc thanthe fertigation treatments at the 10% probability level.

The values of Etc were generally higher in 1999than in 2000, although the amount of IW applied in1999 was lower. This could be attributed to the highertemperature that prevailed in 1999. In this season, thecrops required more IW to meet the increased demand

ab ab

,

NI N2 N3 NS

Treatments

Figure 2. Water use efficiency (WUE) as affected by the varioustreatments (NO, NI, NZ and N3 = 0, 50, 100 and 150 mg N L - 1irrigation water; NS = soil application in amount equivalent toNZ). Columns with different letters are significantly different at the0.05 probability level.

of evaporation losses under higher T Therefore, thecrops were not able to utilize the IW efficiently andthe evaporation component of the ETc was higher.Gardner et al. (1985) reported that the higher the at-mospheric temperature and solar radiation, the higherthe atmospheric demand (evaporative demand) andthe higher the ETc. The better water utilization in the2000 season was clearly illustrated by the higherWUE obtained (Figure 2). Al-Jamal and Sarnmis(2001) reported that under stress conditions, most ofthe applied water was lost as evaporation, which con-sequently reduces WUE. Zuraiqi et al. (2001)reported that fertilized crops showed higher Etc thannon-fertilized crops.

In spite of the pattern of ETc in both seasons, theWUE was affected by the treatments differently inboth seasons (Figure 2). The highest WUE value wasobserved with N2 in 1999 and with NI and N2 in

1 Ii

0.8~

~ 0.60\

'-'~ OA

0.2 ~rop devel-Initial opment Mid-season Late season

0 +-._H""-r- - -'-"-' ,-, ---,

0 W ~ ~ W 100

Days from planting

0.81

0.6 ~,,Ia

0

~ OAu~

0.2 , Crop devel-Initial apment Mid-season

0 + ""-'-r--- Hr

Late season

0 20 40 60 80 lOO

Days from planting

Figure ]. Squash crop coefficient (Kc) for each stage of crop de-velopment in both seasons-

2000. Compared to the NS, N2 improved WUE, in-dicating the advantages of the fertigation techniquefor this variable, and controlling N application withfertigation not only improves plant growth and yield,but also water utilization. Adequate fertilization hasbeen reported to increase WUE (Bar-Yosef 1999).Nitrogen (N) is generally the most limiting nutrientfor irrigated vegetables under arid and semi-arid con-ditions. It has been reported that an inadequate sup-ply of N not only limits the plant growth and yieldpotential (Mohammad et al. 1999) but also restrictsthe efficient use of soil moisture and other soil nutri-

ents (Muchow and Sinclair 1994). The positivewaterXN interaction on crop yield has been demon-strated for several vegetable crops, especially whengrown in arid environments (Clough et al. 1990,1992; Bar- Yosef 1999).

The crop coefficient (Kc) was measured at differ-ent stages of growth. The Kc ranged from 0.20 to 0.95in 1999 and from 0.24 to 0.71 in 2000 (Figure 3). Thehigher value of Kc in 1999 once again demonstratedthe higher value of the ETc of the 1999 season. Thelowest values of the Kc were obtained during the

4

.-3 j

1999 aab

'sc

S2

I

0'1----"'-,,-'-'

NO NI N2 N3 NS

Treatments

looo

- 6

'!§ 54

1

0

NO

1999

'""" 0-308 ,. , . , , . . . . , ,. .~..0:::b.. 30-60<1>

"0

'5{/) 60-90

-.. , -,

0% 40% 80%60%20%

Water consumption (%)

I_.~NO IINl DN2 [Si~~--~.~~J

2000

B 0-30~

"R 30-60<1>

"0r=

r55 60-90- 1

0% 20% 40% 60% 80%

Water consumption (%)

[ r:lNO EBN3 .NSJillN2t2!Nl

Figure 4. Water consumption with depth as affected by the various

treatments (NO, NI, N2 and N3 = 0, 50, 10O and 150 mg N L - 1irrigation water; NS = soil application in amount equivalent toN2).

early vegetative periods where a large proportion ofthe soil was bare, thus influencing the Kc. Under suchconditions, a significant part of the ETc will comefrom evaporation of water from bare soil. During thegrowing period, the Kc increased linearly followingthe increase in the canopy cover. When full cropcover was reached during the development growthperiod, the Kc reached the maximum values. After-wards, the Kc started to decline during maturity,where the senescence phase began and the Kc valuedeclined again but remained higher than at the initialstage. Such a pattern of changes of Kc values withtime during the crop growing period is welldocumented (Steduto 2000).

Soil moisture depletion as affected by the treat-ments and soil depths is shown in Figure 4. No sig-nificant differences were observed in the % of soil

moisture depletion among treatments. However,

moisture depletion with soil depths was markedlydifferent. About 70% of the soil moisture was

consumed from the top 30 cm, about 25% from the30-60 cm soil depth and about 5% from the 60-90cm soil depth. This pattern of soil moisture depletionsuggests that most of the active roots were concen-trated in the topsoil. The sharp wetting front of thewetted soil volume, created when dry soil is irrigated,limits and defines the boundary of the plant's root ac-tivity (Bresler 1977). Such shallow root systems havebeen commonly reported under drip irrigation (Bar-Yosef 1999; Mohammad 2000). This pattern of mois-ture depletion also reflects the vertical distribution ofthe active roots. This should be considered in the

management of drip fertigated crops. Controlling theplacement of fertilizers to match the root distributioncan be successfully achieved by fertigation tech-niques (Papadopoulos 2000). By increasing the ferti-gation frequency, the time fluctuation in nutrientconcentration in soil solution is reduced and, on theother hand, the root soil volume is reduced (Bar-Yosefet al. 1980; Martinez Hernandez et al. 1991). Undershallow root distribution conditions, scientists usuallyare concerned about leaching problems. However,with fertigation nutrient leaching from the root rhizo-sphere and ground water contamination are mini-mized (Hagin and Lowengart 1996). In addition, sucha pattern of root distribution made adoption of ferti-gation to crops grown on infertile and shallow soils asuccessful practice (Kafkafi and Bar- Yosef 1980).

Regression relationship between the ETc and thesquash yield and dry matter

A relatively good fit between the yield and the ETcwas observed in 2000 but they were poorly related inthe 1999 season (Figure 5). The linear relationshipbetween the yield and ETc observed in 2000 has alsobeen reported by other researchers (Battikhi and Hill1988). The poor relationship observed in the 1999season is not uncommon and it was attributed to fac-

tors such as hot climate (Viets 1962) and poor soilfertility (Battikhi and Hill 1988). The relationship be-tween the shoot dry matter and the Etc in both sea-sons had similar trends as those observed between the

yield and the ETc (Figure 6). It should also be men-tioned that in the first season the crop consumed morewater and produced lower yield and these variableswere poorly related, which clearly indicates the im-pact of the higher temperature in enhancing theevaporation component (E) of the evapotranspira-

. .......'..- .... .... .

.

y = 0.0031x + 8.2471

R2 = 0.009 (n=20)

350 450250

ETc (mm)

25 -2000

20-, .~

~ 15t:iJlO~

. r1.-.

. .

5

III

Y= 0.0313x + 7.2592

R2 = 0.226* (n=20)

0150

, ' '200 250 300 350

ETc (mm)

Figure 5. Relationshipbetween yield and crop evapotranspiration(ETc).

tional water consumption (ETc) and consequently theWUE. The relationship between the shoot DM andETc in general had a similar trend as that observedbetween the yield and ETc.

Conclusions

In conclusion, FNUE was enhanced with fertigationcompared to the soil application and decreased withincreasing rates of fertigation N. The values ofFNUE, determined using a non-isotopic differencemethod, had a similar trend but were higher than thatdetermined by the isotopic method in 1999 and weresimilar in 2000. Soil moisture depletions were mostlyfrom the surface soil layer (30 cm), suggesting ashallow root depth, a feature that is necessary to con-sider in N and water management. In addition, underheat stress conditions, the yield and dry matter werepoorly related to the ETc. Based on our results, ferti-

1.6 '

11.4 ~ 1999

.::-- 1.2j~ 1.0

C. 0.8 I:::EQ 0.6

0.4

0.2

0.0

150

... ... .- . iIr. .. ....

y= 0.0007x + 0.7435

R2 = 0.0169 (n=20)

250 350 450

ETc (mm)

.

y = 0.0058x + 0.1635

R2 = 0.386* (n=20)

'-'-- '-' . -"-'

250 300 350

ETc (mm)Figure 6. Relationship between dry matter and crop evapotranspi-ration (ETc).

gation is recommended as a more efficient method offertilizer application for squash crop.

Acknowledgements

This study was funded by the Research Deanship ofJordan University of Science and Technology (Project#1999/001) and partially by the International AtomicEnergy Agency. The technical assistance of S. Zuraiqiand W. Zayadneh is highly appreciated.

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