Can Low Nutrient Strategies Be Used for Pot Gerbera Production in Closed-Loop Subirrigation?

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1 Cn Low Nutrient Strtegies Be Used for Pot Gerber Production in Closed-Loop Subirrigtion? Youbin Zheng, Thoms Grhm, Stefn Richrd nd Mike Dixon CESRF, Deprtment of Environmentl Biology University of Guelph Ontrio Cnd Keywords: nutrient recircultion, slt ccumultion, chlorophyll, fertilizer input, stomtl conductnce, photosynthesis, photosynthetic cpcity Abstrct To determine if pot gerber (Gerber jmesonii, Vr. Shogun) production could be mintined under lowered (reltive to commercil) nutrient concentrtion regime, in closed-loop subirrigtion systems, plnts were grown under one of four nutrient regimes for four weeks. The four tretments comprised of 100%, 50%, 25% nd 10% of the nutrient concentrtions typiclly used in commercil pot gerber production. The verge electricl conductivities (EC) of the solutions were 1.7, 1.0, 0.5 nd 0.2 ds m -1, respectively. There were no differences mong the four tretments with respect to lef net CO 2 ssimiltion rte, stomtl conductnce, photosynthetic cpcity (in terms of light- nd CO 2 -sturted rte of CO 2 ssimiltion nd mximum rte of Rubisco crboxyltion), lef re, flower number nd flower ppernce. Lower concentrtion nutrient tretments did not reduce totl plnt dry weight. There were no significnt differences in lef chlorophyll content between plnts receiving the 50% nd 100% strength nutrient solutions. During the finl week of the experiment, slight interveinl chlorosis, ccompnied by reduced lef chlorophyll content, ws observed in some juvenile leves of plnts irrigted with the 25% nd 10% strength nutrient solutions. It is suspected tht the chlorosis in the 25% nd 10% tretments ws due to iron nd/or mngnese deficiency. It is concluded tht nutrient solution concentrtions typiclly used in commercil greenhouse opertions, cn be sfely reduced by t lest 50% without dversely ffecting pot gerber production. Nutrient slts ccumulted in the top section of the growth substrte under ll tretments; however, levels were significntly lower in the reduced concentrtion tretments. No differences in wter consumption were observed mong tretments. Fertilizer usge ws drmticlly reduced t lower nutrient feeds. The 50%, 25% nd 10% tretments used only 36%, 23.5% nd 8.5% s much fertilizer, respectively, s the 100% tretment. The mjor ionic rtios of ll the nutrient solutions in the solution reservoirs were still within n cceptble rnge fter four weeks of recycling. INTRODUCTION Sub-irrigtion hs become incresingly populr in the greenhouse industry over the lst decde. This increse in populrity is ttributed to lower nutrient nd wter requirements, greter uniformity in nutrient delivery, voidnce of folir wetting during the wtering process (disese control) nd greter flexibility in pot sizing nd spcing compred to conventionl over-hed wtering. These fctors trnslte into svings in lbour, mteril input, nd product losses (Biernbum nd Versluys, 1998; Blom, 1991; Uv et l., 1998). Although flexible nd cost effective production strtegy, subirrigtion does hve some ssocited problems. Slt ccumultion t the substrte surfce is one of the mjor concerns with respect to subirrigtion (Argo nd Biernbum, 1996; Morvnt et l., 1997). Slt ccumultion is cused by the evportion of wter from the surfce of the growth substrte, leving behind fertilizer slts in the top lyers of the growth medium (Argo nd Biernbum, 1995). In this fshion, slt concentrtions cn quickly rech toxic levels, resulting in observble dmge to the crop. This problem is excerbted by the Proc. IC on Greensys Eds.: G. vn Strten et l. Act Hort. 691, ISHS

2 typiclly high levels of nutrient slts present in commercilly pplied nutrient solutions. Commercil greenhouse growers typiclly use unnecessrily high nutrient concentrtions in n ttempt to mximize crop yield. This prctice does not represent n economiclly, or biologiclly optimized production strtegy. Reserch hs shown tht mcronutrient concentrtions, commonly used by commercil greenhouse tomto growers, cn be reduced by 75% without hving ny dverse effect on growth, fruit yield nd fruit qulity (Siddiqi et l., 1998). Similrly, for NFT lettuce production, the current nutrient concentrtions in use cn be reduced by up to 99% without hving ny dverse effect on growth nd rtes of nutrient uptke (Chen et l., 1997). The objectives of this study were to; 1) test the hypothesis tht pot gerber cn be grown using lower concentrtion nutrient solutions thn re currently used in commercil subirrigtion systems, without hving negtive influence on plnt qulity nd production; 2) profile the slt distribution in the pot substrte under subirrigtion. MATERIALS AND METHODS Gerber disy seedlings were potted in 1.9-liter (15 cm dimeter x11 cm height) pots on 12 Februry The substrte used ws BM6 Corse (Berger/Berger Pet Moss, Quebec, Cnd) contining 20% perlite nd 80% Cndin sphgnum petmoss. The EC nd ph of the potting substrte determined prior to the subirrigtion tretments were 1.4 ± 0.05 ds m -1 nd 5.96 ± 0.05, respectively; nd there were no significnt differences (P > 0.05) mong top, middle nd bottom sections of the growth substrtes. Commercil pot gerber greenhouses typiclly use overhed fertigtion for severl weeks before switching to sub-irrigtion. Subirrigtion is mintined for 4-5 weeks until plnts re redy for mrket. To mintin this prctice in our study, plnts were overhed fertigted with full strength nutrient solution for 5 weeks in commercil greenhouse, before being moved to subirrigtion system in the reserch greenhouse t the University of Guelph (Guelph, Ontrio, Cnd). The full strength nutrient solution used in this study consisted of the following mcronutrients (in mm): 11.2 NO 3 -N nd 1.5 NH 4 -N (totl N of 178 mg L -1 ), 1.2 P, 3.0 C, 5.5 K, 1.0 SO 4 2- nd 1.0 Mg; nd micronutrients (in µm): 2.2 Mn, 4.0 Zn, 29.6 B, 0.5 Mo, 1.0 Cu nd 53.9 Fe s Fe-EDTA. The experiment ws rndomized complete block design with four nutrient level tretments in three blocks. In ech experimentl unit (trough), there were 10 plnts. Plnts in ech trough were fertigted s required (2-4 times per week) by filling troughs with nutrient solutions from reservoir tnk contining 70 L of nutrient solution. Troughs were filled so tht the bottom 2.5 cm of the pots were submerged for 1-2 minutes. The four nutrient levels were 10, 25, 50 nd 100% of full strength nutrient solution. After ech fertigtion, the ph of the solutions ws restored to the trget vlue of 5.6 with HNO 3. Additionlly, the EC ws restored to the originl trget vlues (1.7, 1.0, 0.5 nd 0.2 ds m -1 for the 100, 50, 25 nd 10% solutions respectively) by dding 100 times concentrted stock solution. During the djustment, the volumes of dded wter nd nutrient stock solutions were recorded for clcultion of wter nd nutrient inputs. Nutrient solution smples were collected nd nlysed (by HPLC) fter ech ph nd EC djustment. The ir temperture nd reltive humidity set points in the greenhouse were dy/night 23/19 C nd 70% respectively. Tretments were strted on 5 Mrch, 2004 nd plnts were hrvested on 3 April, 2004 when they reched mrketble size. Crop performnce, in terms of symptoms of slt toxicity nd nutrient deficiency, ws observed dily. Lef chlorophyll content index mesurements were conducted once week using SPAD-501 meter (Minolt, Osk, Jpn). Mesurements were mde on both the youngest fully expnded lef t the time of mesuring, nd the youngest fully expnded lef present t the onset of the experiment (representing old lef tissue). Lef gs exchnge prmeters [net CO 2 ssimiltion rte (A), stomtl conductnce (g s ), intercellulr CO 2 concentrtion (c i )] were mesured on the youngest, helthy nd fully expnded leves prior to the finl hrvesting. Mesurements were mde using LI-COR 6400 (LI-COR, Lincoln, NE, USA) t CO 2 concentrtions of 400 µmol mol -1, vpour pressure deficit of 0.95 kp, PAR of 500 µmol m -2 s -1 nd lef temperture of 25 o C. The 366

3 responses of lef net CO 2 ssimiltion rte to CO 2 concentrtions (A/c i curves) were mesured t lef temperture of 25 o C, under sturted photosynthetic ctive rdition (PAR) level of 1500 µmol m -2 s -1. The light- nd CO 2 -sturted rte of CO 2 ssimiltion (A mx ), nd mximum rte of Rubisco crboxyltion (V c,mx ) were clculted from the A/c i curve ccording to Zheng et l. (2002). At hrvest, flowers of ech plnt were counted. Ech plnt ws divided into flowers, leves, crown nd roots. Lef re of ech plnt ws mesured using LI-3100 re meter (LI-COR, Inc., Lincoln, NE). Plnt prts were dried to constnt weight in forced ir oven t 65 o C. Totl dry weight ws clculted s the sum of dry weights for ll the plnt prts. Root/Shoot rtio ws clculted by dividing root dry weight by the sum of bove ground plnt prts dry weights. Specific lef re (SLA, cm 2 g -1 ) ws clculted by dividing lef re by lef dry weight of ech plnt. At hrvest, the substrte (with roots) of 5 rndomly selected pots from ech experimentl unit were evenly divided into top, middle nd bottom sections. The ph nd EC of ech section ws determined ccording to the Sturtion Medi Extrct (SME) method (Wrncke, 1986). Anlysis of vrince (ANOVA) ws conducted using SAS (version 8, SAS Institute Inc., Cry, USA, 1999). Multiple comprisons mong the tretments of ech experiment were conducted using Tukey s Studentized rnge (HSD) test. RESULTS Growth There were no tretment effects on plnt lef re or the root/shoot rtio (Tble 1). The 50% tretment showed n increse in totl dry weight when compred to the 100% tretment. Lef thickness, s indicted by specific lef re (SLA), decresed slightly s nutrient levels decresed, reltive to the 100% tretment (Tble 1). Flower Numbers nd Dry Weight Flower numbers for ech pot were counted weekly from the strt of the experiment. Results showed tht there were no significnt (P > 0.05) differences in flower production mongst the tretments (dt not shown). At the finl hrvest, there were no significnt (P > 0.05) differences in flower number (3.5) nd flower dry weight (4.7g/pot) mong ll the tretments. Visible Slt Toxicity nd Nutrient Deficiency Symptoms No slt toxicity symptoms were observed during the experiment. There were no visible symptoms of nutrient deficiency on ny of the plnts within the first 3 weeks in ll of the tretments. However, in the 4 th week, some plnts in the 25% nd 10% tretments strted to show signs of interveinl chlorosis in some juvenile leves. Lef Chlorophyll Content Index SPAD mesurements mde on the youngest fully expnded leves on the dy of the mesurement showed tht there ws no lef chlorophyll content index (CCI) difference mongst ll the tretments before the third week (Fig. 1). Three weeks fter the strt of the tretment, however, the 25% nd 10% tretments showed significntly (P < 0.05) lower CCI thn the 100% nd 50% tretments (Fig. 1). Dt for the youngest fully expnded lef on dy one of the experiment (representing old leves), indicted tht there were no CCI differences mong tretments through to the end of the experiment (dt not shown). Lef Gs Exchnge nd Photosynthetic Cpcity Low nutrient tretments did not significntly (P > 0.05) ffect plnt lef net CO 2 ssimiltion rte (A, 12.2 µmol m -2 s -1 ), stomtl conductnce (g s, 316 mmol m -2 s -1 ), or intercellulr CO 2 concentrtion (c i, µmol mol -1 ) under greenhouse growth conditions. Lef photosynthetic cpcity, in terms of A mx (21.1 µmol m -2 s -1 ) nd V c,mx (µmol m -2 s -1 ), ws lso unffected by nutrient solution concentrtion. 367

4 Verticl Distribution of EC nd ph within the Growth Substrte Subirrigtion resulted in significnt ccumultion of nutrient slts within the top third of the growth substrte (Fig. 2). The EC vlues of the top third of the substrte were 3 to 13 times higher thn those of the bottom two-thirds. Sttisticl nlysis showed tht there were significnt (P < 0.001) tretment nd section effects on the EC nd ph of the substrtes. In generl, lower level nutrient tretments tended to increse the ph vlues of the nutrient solution extrcted from the growth substrtes (Fig. 2). Nutrient Usge, Wter Consumption nd Nutrient Solution Composition Chnges There were no sttisticlly significnt differences in wter consumption mongst the tretments during the entire experimentl period. The verge dily wter consumption ws 115±3 ml per plnt. Fertilizer usge ws drmticlly reduced t lower nutrient feeds. The 50%, 25% nd 10% tretments used only 36%, 23.5% nd 8.5% s much fertilizer, respectively, s the 100% tretment. Regression nlysis (Fig. 3) of nutrient ion (NO 3 -, H 2 PO 4 -, SO 4 2-, N +, NH 4 +, K +, Mg 2+ ; mesured fter ech nutrient djustment) concentrtions in the irrigtion nutrient solutions, s function of time (dys), indicted tht there were no significnt chnges in NO 3 - nd K + levels; however, there were depletion trends for NH 4 + nd Mg 2+ throughout the durtion of the experiment, in ll tretments. DISCUSSION The reduction of chemicl fertilizer inputs, by even smll mounts, cn hve mjor impcts on the mount of chemicls used in greenhouse opertion. In ddition to svings relized through lowered nutrient inputs, there would lso be significnt svings in terms of environmentl impct nd the hndling of spent nutrient solutions. This study clerly demonstrtes tht even t 50% reduction in nutrient solution concentrtion, production remined comprble to tht of typicl full strength commercil nutrient solution. At the 50% concentrtion level there were no differences in key plnt performnce prmeters (chlorophyll content, flower number, flower qulity, lef re, nd dry weight), s compred to the full strength tretment. These results suggest tht nutrient solutions, currently utilized in the commercil production of pot gerber, could be reduced by t lest hlf of their current concentrtion without cusing ny dverse effects on production or product qulity, when utilizing subirrigtion systems, for 4 weeks prior to shipping to mrket. The fesibility of reducing nutrient concentrtions, while mintining production nd qulity stndrds, is crop dependent. Kng nd vn Iersel (2002) subirrigted seedlings of Lobulri mritim, Celosi spp., Dinthus Chinensis, Gomphren globos, Mtthiol incn nd Zinni elegns, grown in cells (160 ml), with nutrient solutions of vrying concentrtions. Their results showed tht Zinni production ws mximized when wtered with hlf-strength Hoglnd solution. However, the other species studied hd mximum production levels t the full strength solution (15 mm N). Yelnich nd Biernbum (1990) grew Euphorbi pulcherrim (poinsetti) under 3 concentrtions of N (7.1, 14.3 nd 28.6 mm) nd K (2.6, 5.1 nd 10.3 mm), nd found tht the growth ws mximized with the lowest concentrtions in both top irrigtion nd subirrigtion systems. Bs nd vn der Berg (2004) reported tht the stem weights of cut roses, fertigted with two strengths of nutrient solutions (EC of 0.9 or 1.9 ds m -1 ), were incresed in lower EC tretments. Additionlly, they showed tht the number of stems/plnt nd totl fresh weight/plnt were not significntly ffected by the EC levels pplied. The results shown in this study suggest tht commercil nutrient solutions, currently used in pot gerber production, re within the concentrtion rnge tht would be considered luxurious consumption. This is supported by the fct tht, even with 50% reduction in concentrtion, production levels were not reduced. Additionlly, there ws 64% reduction in fertilizer usge t the 50% nutrient tretment, rther thn the expected 50% reduction. 368

5 Plnt gs exchnge, including lef stomtl conductnce nd crbon exchnge, is good indictor of environmentl stress, including nutrient deficiency (vn Iersel nd Kng, 2002; Zheng et l., 2000). Schuch et l. (1995) reported tht the stomtl conductnce of 6 poinsetti cultivrs ws incresed when grown with low concentrtion nutrient solution (5.7 mm N compring with 11.4 nd 17.1 mm N). Schuch et l. (1995) suggested tht these plnts suffered from loss of stomtl control in response to the nutrient deficit. Our study hs shown tht, for gerber, the lower level nutrient tretments did not ffect stomtl conductnce, lef net CO 2 ssimiltion rte or lef photosynthetic cpcity. This my suggest tht there were no mjor nutrient deficiencies t the 50% level. The mesurement of the old lef chlorophyll content showed tht, even t the end of the experiments, there ws no pprecible decrese in chlorophyll levels; this effectively ruled out N-deficiency s N-deficiency normlly mnifests itself in older leves first. Since s much s 50% of lef nitrogen is in the form of Rubisco (Ellis, 1979), reduced N supply would cuse decreses in lef Rubisco protein nd CO 2 ssimiltion (Pul nd Driscoll, 1997). The lck of ny decrese in the mximum rte of Rubisco crboxyltion (Tble 3) indictes tht there ws no nitrogen deficiency in the lower nutrient concentrtion tretments in the present study. Jmes nd vn Iersel (2001), nd vn Iersel (1999) lso reported no chnges in plnt tissue mcronutrient elements when growing petunis nd begonis t low nutrient concentrtions under subirrigtion conditions. Interveinl chlorosis nd decresed chlorophyll content in some of the young leves in the 25% nd 10% tretments, in the finl week, my suggest tht iron (Fe) nd/or mngnese (Mn) were deficient. The pprent Fe/Mn deficiency might be due to elevted ph levels in the growth substrte receiving the low concentrtion nutrient tretments nd/or the reduced fertilizer supply. Further reserch is needed to confirm which ws the primry cuse. An increse of substrte ph under lower nutrient levels hs lso been observed by other reserchers (Jmes nd vn Iersel. 2001; Kng nd vn Iersel. 2002). The results presented in this pper vlidte concerns over slt ccumultion in growth substrtes, in subirrigtion systems. The presented experiment ws conducted over 4-week period, reltively short time spn compred to commercil production timelines for mny other crops. Even during this short durtion study, significnt slt ccumultion occurred in the top section of the substrte, especilly in the full strength tretment. The reduced concentrtion tretments still showed n ccumultion, but to much lesser extent. The ccumultion of slts in the surfce lyer of growth medi hs lso been reported in numerous other studies (Argo nd Biernbum, 1995; Argo nd Biernbum, 1996; Kent nd Reed, 1996; Mk nd Yeh, 2001; Morvnt et l., 1997). Although nutrient slts ccumulted in the top section of the growth substrte, no detrimentl effect ws observed during this study. Presently, there re no relible on-line individul nutrient ion sensors tht cn be used in feedbck control system to djust specific nutrient ion levels. Almost ll nutrient solution composition djustments re bsed on EC nd ph in nutrient solution recycling systems. Long term nutrient solution recycling cn cuse nutrient ionic rtio imblnces. Anlysis of reservoir nutrient solution composition indicted tht fter recycling the nutrient solution for 4 weeks, with djustment bsed on EC nd ph fter ech irrigtion, the ionic rtios of ll the nutrient solutions were still within cceptble rnge (Fig. 3). In conclusion, our study demonstrted tht current nutrient ppliction rtes t the finl stge (4-5 weeks) of greenhouse pot gerber production, in closed-loop subirrigtion systems, cn be reduced by t lest 50% without ny detrimentl effect on plnt growth or qulity. However, when the concentrtion of the nutrient solution ws lowered to 25%, plnts strted to show chlorosis symptom on some juvenile leves. There were significnt ccumultions of slts in the top section of the growth substrte; however, the ccumultion did not rech phytotoxic levels under lower nutrient level tretments. Tretments did not ffect plnt wter consumption; however, there were significnt fertilizer input svings under lower nutrient tretments. After four weeks of recycling, the ionic rtios of the nutrient solution in the reservoir were still within n cceptble rnge. 369

6 Further reserch is needed to investigte the responses of longer term crops, such s miniture roses nd cyclmen, to low nutrient fertigtion nd fertilizer slt ccumultion in the top section of substrtes. ACKNOWLEDGEMENT This work ws finncilly supported by FCO, CRESTech nd OMAF. We thnk Vos Florl Ltd. for providing plnt mterils nd Dnut for her excellent technicl support. Literture Cited Argo, R.W. nd Biernbum, A.J The effect of irrigtion method, wter-soluble fertiliztion, preplnt nutrient chrge, nd surfce evportion on erly vegettive nd root growth of poinsetti. J. Amer. Soc. Hort. Sci. 120: Argo, R.W. nd Biernbum, A.J Avilbility nd persistence of mcronutrients from lime nd preplnt nutrient chrge fertilizers in pet-bsed root medi. J. Amer. Soc. Hort. Sci. 121: Bs, R. nd vn der Berg, D Limiting nutrient emission from cut rose closed system by high-flux irrigtion nd low nutrient concentrtions? Act. Hort. 664: Biernbum, J.A. nd Versluys, N.B Wter Mngement. HortTechnology. 8: Blom, T Subirrigtion: Wtering plnts from the bottom up. Highlights of Agriculture nd Food Reserch in Ontrio. 14:2-5. Chen, G.X., Gstldi, C., Siddiqind, Y.M. nd Glss, M.D Growth of lettuce crop t low mbient nutrient concentrtions: strtegy designed to limit the potentil for eutrophiction. J. Plnt Nutr. 20: Ellis, R.J The most bundnt protein in the world. Trends in Biochem. Sci. 4: Jmes, E.C. nd vn Iersel, M.W Fertilizer concentrtion ffects growth nd flowering of subirrigted petunis nd begonis. HortScience. 36: Kng, J.G. nd vn Iersel, M.W Nutrient solution concentrtion ffects growth of subirrigted bedding plnts. Journl of Plnt Nutrition. 25: Kent, M.W. nd Reed, D.W Nitrogen nutrition of New Guine imptiens Brbdos nd Spthiphyllum Petite in subirrigtion system. J. Amer. Soc. Hort. Sci. 121: Mk, A.T.Y. nd Yeh, D.M Nitrogen nutrition of Spthiphyllum senstion grown in sphgnum pet- nd coir-bsed medi with two irrigtion methods. HortScience. 36: Morvnt, J.K., Dole, J.M. nd Allen, E Irrigtion systems lter distribution of roots, soluble slts, nitrogen nd ph in the root medium. HortTechnology. 7: Pul M.J. nd Driscoll, S.P Sugr repression of photosynthesis: the role of crbohydrtes in signlling nitrogen deficiency through source:sink imblnce. Plnt Cell Environ. 20: Schuch, U., Redk, A.R. nd Bethke, J Whole-plnt response of six poinsetti cultivrs to three fertilizer nd two irrigtion regimes. J. Amer. Soc. Hort. Sci. 121: Siddiqi, Y.M., Kronzucker, J.H., Britto, T.D. nd Glss, A.D.M Growth of tomto crop t reduced nutrient concentrtions s strtegy to limit eutrophiction. J. Plnt Nutr. 21: Uv, W.L., Weiler, T.C. nd Millign, R.A A survey on the plnning nd doption of zero runoff subirrigtion systems in greenhouse opertions. HortScience. 33: vn Iersel, M.M Fertilizer concentrtion ffects growth nd nutrient composition of subirrigted pnsies. HortScience. 34:

7 vn Iersel, M.M. nd Kng, J Nutrient solution concentrtion ffects whole-plnt CO 2 exchnge nd growth of subirrigtion pnsy. J. Amer. Soc. Hort. Sci. 127: Wrncke, D.D Anlyzing greenhouse growth medi by the sturtion extrction method. HortScience. 21: Yelnich, V.M. nd Biernbum, A.J Effect of fertilizer concentrtion nd method of ppliction on medi nutrient content, nitrogen runoff nd growth of Euphorbi Pulcherrim V-14 Glory. Act. Hort. 272: Zheng, Y., Lyons, T. nd Brnes, J.D Effects of ozone on the production nd utiliztion of ssimiltes in Plntgo mjor. Environ. Exp. Bot. 43: Zheng, Y., Shimizu, H. nd Brnes, J.D Limittions to CO 2 ssimiltion in ozoneexposed leves of Plntgo mjor. New Phytol. 155: Tbles Tble 1. Growth of gerber plnts subirrigted with different strength nutrient solutions. Tretment Lef re (cm 2 ) SLA (cm 2 g -1 ) Root/Shoot Totl dry weight (g) 100% 1241± ± ± ±0.86 b 50% 1461± ±5.1 b 0.09± ± % 1497± ±6.1 b 0.10± ±0.75 b 10% 1470± ±4.7 b 0.11± ±0.72 b P >0.05 <0.001 >0.05 <0.001 Mens within the sme column followed by different letters re significntly different t P<

8 Figurese Chlorophyll content index % 50% 25% 10% Dys fter the strt of tretment Fig. 1. Lef chlorophyll content index of the youngest fully expnded leves, mesured t 1 week intervls, fter the strt of the experiment. The 100%, 50%, 25% nd 10% refer to the pplied nutrient levels. EC (ds m -2 ) b b Top Middle Bottom b b b b Tenth Quter Hlf Full b b ph b b Top Middle Bottom Tenth Quter Hlf Full Fertigtion Nurrient Soluton concentrtion b Fig. 2. Substrte EC nd ph verticl distributions fter 4-week subirrigtion under four different strength (100, 50, 25 nd 10% of full strength) nutrient solutions. Brs in the sme tretment bering different letters re significntly different t P 0.05 level. NO 3 H 2PO 4 SO 4 N NH 4 K Mg C 25% % 7 50% % (mm) 2 (mm) (mm) 6 3 (mm) Dys fter the strt of tretment Fig. 3. Reservoir nutrient solution ion concentrtion chnges with time under recycling conditions. Where time effects were significnt (P 0.05), lines indicte the clculted regression, otherwise no lines re shown. 372