Scientia Horticulturae 99 (2004) 279–288

Effect of potassium magnesium chloride in thefertigation solution as partial source of potassium

on growth, yield and quality of greenhouse tomato

Bishnu P. Chapagain, Zeev Wiesman∗Institute of Agriculture and Applied Biology, The Institutes for Applied Research,

Ben-Gurion University of the Negev, P.O. Box 653, Beer Sheva 84105, Israel

Accepted 17 June 2003

Abstract

The effect of partial replacement of KCl in the fertigation by KCl·MgCl2 on growth, yield andquality of greenhouse tomato (cv. Durinta) was studied in a soil-less system. Forty-seven days afterplanting (DAP), three treatment solutions were applied to the plants using different K sources: (1)KNO3, (2) KCl, and (3) KCl·MgCl2 + KCl (25%:75% in terms of K supplied). In both treatments 2and 3, NH4NO3, Ca(NO3)2 and HNO3 were added as source of N. Plant height and total chlorophyllwere the highest in the KCl+ KCl · MgCl2 treatment. Leaf Mg content was significantly lower in theKCl treatment, whereas highest in the KCl+KCl ·MgCl2 treatment. Both KCl and KCl+KCl ·MgCl2led to a significantly higher leaf Cl content as compared with the KNO3 treatment, but no Cl toxicitywas observed in either treatment. Total yield was not different among treatments. Fruit firmness andfreshness of the calyx were significantly improved by KCl and KCl+KCl ·MgCl2, and the number ofrotten and blotchy fruits were significantly reduced by both these treatments. KCl+KCl ·MgCl2 alsoled to significantly higher levels of glucose, Mg and dry matter content in the fruit. Lower NO3 andhigher Fe contents were measured in both treatments 2 and 3. Although KCl as sole K source showedlower foliar Mg level as compared to KNO3, the use of the KCl in tomato fertigation improved tomatofruit appearance and qualities. However, 25% replacement of KCl by KCl+ KCl · MgCl2 increasedthe foliar Mg level and improved fruit qualities even further.© 2003 Elsevier B.V. All rights reserved.

Keywords: Tomato; Potassium magnesium chloride; Potassium nitrate; Fruit quality; Fertigation

1. Introduction

The appearance of the tomato fruit is generally considered to be an index of quality andoften determines consumer choice. Great efforts have recently been focused in producing

∗ Corresponding author. Tel./fax:+972-8-6477184.E-mail addresses: bishnu@bgumail.bgu.ac.il (B.P. Chapagain), wiesman@bgumail.bgu.ac.il (Z. Wiesman).

0304-4238/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0304-4238(03)00109-2

280 B.P. Chapagain, Z. Wiesman / Scientia Horticulturae 99 (2004) 279–288

a good appearance and quality tomato through the utilization of inexpensive and environ-mentally friendly resources. Production of quality fruits is controlled by the interaction ofgenetic, environmental and cultural factors, including plant nutrients (Dorais et al., 2001).Among essential plant nutrients, potassium is the one that is absorbed by the tomato plant inthe largest amounts and it is considered to be the key to production of quality fruits (Mengeland Kirkby, 1987; Marschner, 1995; Usherwood, 1985). Most of the tomato studies have sofar shown that increasing the electrical conductivity (EC) of the nutrient solution improvesfruit quality (Adams, 1987, 1991; Adams and Ho, 1989; Gough and Hobson, 1990) andincreases shelf life (Mizrahi, 1982).

In Israel, most soil-less greenhouse tomatoes are fertilized with KNO3, despite the factthat the use of this K source leads to high NO3 leachate and is costly. Use of KCl asalternative potassium source is avoided due to the fear that its anion, Cl might damagethe plant. However, in a comparative study that was conducted recently by our group, KClenhanced fruit appearance and improved fruit quality as compared with KNO3 (Chapagainet al., 2003); however, a significantly lower foliar Mg content (Chapagain et al., 2003) andMg deficiency (Chapagain, 2001) was observed during plant growth while using KCl as asole K source in tomato fertigation.

A level of less than 0.4% Mg in the dry matter of young fully expanded leaves (YFEL)is generally considered critical in greenhouse-grown tomatoes (Snyder, 1992). Hao et al.(2000)report that 50–80 ppm Mg in the nutrient solution results in the best overall yieldand fruit quality in rockwool-grown tomato. However, so far little work has been done onthe interaction between magnesium and tomato production and its fruit quality.

Based on our previous studies and known facts about the effects of Mg in crop produc-tion, we argued that if we were to boost Mg concentrations at the same time as we reducedNO3 and raised Cl levels in the nutrient solution—through the use of KCl as potassiumsource rather than KNO3—then the tomato plant would maintain high foliar Mg, leadingto an improvement in the quality parameters over and above the previously mentioned ben-efits of KCl. Accordingly, we chose potassium magnesium chloride (KCl·MgCl2·6H2O,quiK-Mag), a fertilizer derived from carnallite (a natural mineral from the Dead Sea), andsubstituted it for 25% of the KCl. Potassium magnesium chloride (KCl·MgCl2) contains15% K2O, 14% MgO and 39% Cl, with 4% NaCl (maximum), and is recommended fororganic agriculture (DSW, 2000).

The aim of this work was to compare the growth, leaf mineral content, fruit yield andquality of greenhouse-grown-tomato with KCl as sole K source or 25% substituted byKCl·MgCl2.

2. Materials and methods

2.1. Plant culture and design

One-month-old tomato (Lycopersicon esculentum Mill. cv. Durinta) seedlings wereplanted in 10 l plastic pots filled with perlite in a semi-controlled greenhouse at Ben-GurionUniversity of the Negev, Beer Sheva, Israel on 13 October 2001. The pots were planted witha single plant each and spaced 0.4 m between, in double rows with 1 m between rows to

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approximate 25,000 plants ha−1. The experimental design consisted of randomized blocks,replicated four times in groups of eight plants per replicate (total of 32 plants per treatment).Each plant was fed by a single dripper (2 l h−1). As the plant grew, all lateral shoots wereremoved manually, and the resulting single stem was trained up a string according to thehigh wire system. Plants were headed back after eight trusses (at 107 DAP). Fruits werethinned to no more than six fruits per cluster. Bumblebees (Bombus sp.) were introducedinto the greenhouse for better pollination. The oldest leaves (i.e. those at the bottom of thestem) were periodically removed. Harvesting was carried out at the early red stage (stages7–8 according to the Carmel Israel tomato color chart). Final harvesting was carried outon 18 April 2002 (188 DAP). The greenhouse was heated at night to maintain a minimumtemperature of 11◦C. Monthly maximum temperatures ranges from 27 to 32◦C, while min-imum temperatures were 11–17◦C over the course of study. No additional light or CO2 wasprovided in the greenhouse.

2.2. Plant nutrition and treatment application

From the day of planting to 46 DAP (before treatment initiation), all the plants were sup-plied the same nutrient solution (100:44:42 mg l−1 NPK) by using 20:20:20 formula (HaifaChemicals, Israel), admixed with 0.1 ml l−1 Koratin+ B, a mixture of the micronutrientsMn, Cu, B, Fe, Zn, and Mo (FCCI, Israel). From 47 DAP, three different solutions weresupplied in conformity with the experimental design: (1) KNO3 (Haifa Chemicals, Israel)as sole K source; (2) KCl as sole K source; (3) 25% of K from KCl·MgCl2, remaining75% from KCl (hereafter KCl·MgCl2). The concentration of NPK was the same in all threetreatments. Average concentrations of Ca, Mg, and Cl, EC, and pH of the nutrient solutionwere as inTable 1. NH4NO3, Ca(NO3)2, and HNO3 were added as N sources in treatments2 and 3, whereas phosphoric acid (H3PO4) and Koratin+ B were used as phosphorus andmicronutrient supplements in all three treatments. The proportion of 25% KCl·MgCl2 and75% KCl was chosen to assure the recommended level of K. The amount of irrigation wasadjusted to the age of the plant. Each of the different treatment solutions was placed in aseparate tank, and the irrigation was controlled by a computerized system. The irrigation

Table 1Mineral concentration of the water used in the experiment and the fertigation solutions applied to the tomato plants

Treatment Mineral concentration (mg l−1) pH EC (dS m−1)

N P K Ca Mg Cl

Water used in the experimenta

0.76 0.36 9.8 47 27 227 7.8 0.98

Fertigation solutionb

KNO3 75.5 49.8 179 74.1 40.4 249 7.2 1.8KCl 74.7 50.8 181 86.2 41.4 395 6.8 2.2KCl·MgCl2 76.1 51.3 182 87.5 69.0 420 7.0 2.4

a Properties of water of Israel National Water Carrier. Also content 140 mg l−1 HCO3, 59.5 mg l−1 SO4 and1.2 mg l−1 Br (Saurhaft, 1997).

b Values are means of measurements at 1-week intervals during the treatment period.

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regimes were scheduled as to achieve about 50% leaching fractions (drainage/inflow). Theirrigation water used in this experiment was the Israel National Water Carrier’s fresh water.A micro-level description of the irrigation water is presented inTable 1.

2.3. Measurements and statistical analysis

Plant height and contents of chlorophyll and minerals in the leaf were measured justbefore treatment initiation (46 DAP) and on two other occasions during plant growth: at1 month from the initiation of treatment (76 DAP), and 2 months from that date (106DAP). The procedure as described byMoran (1982)was followed for determination ofleaf chlorophyll content. P, K, Ca and Mg contents in leaf were analyzed by ICP-AES(Perkin-Elmer OPTIMA-3000) after grinding of oven dried (70◦C, 72 h) YFEL followedby acid digestion (Lichter et al., 2002). Leaf chloride (Cl) was determined by titration with0.5 N AgNO3.

Total fruit yield; fruit number and fruit weight were assessed at harvest for each cluster.Fruits were selected for further analysis and kept in storage (at 12◦C for 12 days and 20◦Cfor 3 days) in order to simulate export conditions. Percentages of firm or rotten fruits andfruits with calyx, calyx freshness and blotchiness (blotchy fruits) were assessed immediatelyafter removal from storage as described byMizrahi et al. (1988). The homogenized fruittissue was centrifuged and the supernatant measured for juice EC using an EC Meter (TH2400, EL-Hama Instruments, Israel), pH using a pH Meter (EcoMet P15, MRC, Israel),total soluble solids (TSSs) using a digital refractometer (PR-100, ATAGO, Japan), titrableacidity (TA) by titration with NaOH (0.05 N), and glucose by with an Elite Glucometer(Bayer, UK) as described byDorais et al. (2000). Similarly, juice NO3 was also measuredusing the Reflectoquant Nitrate Test (MERCK-Rqflexz). P, K, Ca, Mg, Fe and Cl contentsof fruits were analyzed following the same procedures as for the leaves.

Data were statistically analyzed with JMP software (SAS, 2000), using the Tukey–KramerHSD test for determining significant differences among treatments atP ≤ 0.05.

3. Results

3.1. Plant growth and leaf analysis

3.1.1. Plant heightNo significant differences in plant height were found among the treatments at 46 or

76 DAP, whereas at 106 DAP this parameter was significantly higher in the KCl·MgCl2treatment as compared with KCl and KNO3 (Table 2).

3.1.2. Leaf chlorophyll contentLeaves exhibited a uniform level of chlorophyll(a+b) before treatment initiation. At 76

DAP total chlorophyll was significantly higher in the KCl·MgCl2 treatment as comparedwith KCl, while KNO3 did not differ significantly from the other treatments. At 106 DAPKCl·MgCl2 induced a significantly higher total chlorophyll(a + b) than the other twotreatments, while KCl registered the lowest chlorophyll content (Table 2).

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Table 2Effects of potassium source in fertigation solution on plant height, leaf chlorophyll content and leaf mineralcomposition of greenhouse-grown tomatoa

Treatment Plantheight(cm)

Leaf analyses

Chlorophyll(�g cm−2)

P (mg g−1

dry matter)K (mg g−1

dry matter)Ca (mg g−1

dry matter)Mg (mg g−1

dry matter)Cl (mg g−1

dry matter)

46 DAPb

KNO3 76.22 a 21.46 a 10.16 a 50.87 a 16.25 a 9.21 a 17.55 aKCl 74.95 a 21.29 a 9.92 a 48.25 a 16.65 a 9.37 a 18.04 aKCl·MgCl2 75.24 a 21.27 a 9.87 a 47.33 a 15.97 a 8.86 a 17.85 a

76 DAPKNO3 164.25 a 23.94 ab 10.1 a 43.45 a 16.56 a 7.02 a 15.68 bKCl 162.01 a 23.40 b 9.76 a 44.21 a 14.06 a 5.47 b 21.14 aKCl·MgCl2 168.85 a 25.41 a 8.75 a 45.23 a 14.91 a 7.49 a 22.20 a

106 DAPKNO3 223.71 b 24.14 b 11.51 a 38.55 a 21.12 a 7.43 ab 20.46 bKCl 219.25 b 22.31 c 9.28 b 43.25 a 20.11 a 5.75 b 26.01 aKCl·MgCl2 231.47 a 25.46 a 9.12 b 44.21 a 19.10 a 8.25 a 28.18 a

146 DAPKNO3 11.58 a 30.28 a 22.45 a 7.21 ab 21.3 bKCl 9.89 a 31.58 a 21.58 a 5.10 b 26.20 aKCl·MgCl2 10.21 a 32.24 a 19.12 a 8.31 a 28.00 a

a Means in each column followed by different letters are significantly different atP ≤ 0.05 (n = 32 for plantheight andn = 8 for leaf analyses).

b Days after planting.

3.1.3. Leaf mineral compositionAnalyses of leaf mineral composition showed that leaf K, Ca, and Fe contents were not in-

fluenced by potassium source (Table 2). P was lower in KCl-treated and KCl·MgCl2-treatedplants only at 106 DAP; at the other analysis dates, P content was similar in all three treat-ments. Mg was significantly lower under KCl as compared with KNO3, but significantlyhigher than in the nitrate treatment when KCl was supplemented with KCl·MgCl2 (treat-ment 3). Leaf Cl content was always higher under KCl and KCl·MgCl2 as compared withKNO3; however, no Cl toxicity was observed.

3.2. Yield and quality

3.2.1. Fruit yieldNo significant difference was observed in total fruit yield, number of fruits, or average

fruit weight among the treatments. However, 2.3 and 0.5% higher yield was recorded in theKCl·MgCl2 treatment as compared with KCl and KNO3, respectively (Table 3).

3.2.2. Fruit appearanceThe percentage of firm fruit and calyx freshness were significantly higher under KCl

and KCl·MgCl2 as compared with the KNO3 treatment. In addition, percentages of rottenand blotchy fruits were significantly lower in both the KCl and the KCl·MgCl2 treatments.

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Table 3Effect of potassium source in fertigation solution on yield and appearance of greenhouse-grown tomato fruitsa

Treatment Yield(g perplant)

No. offruits (perplant)

Averagefruit weight(g)

Firmfruit (%)

Rottenfruit (%)

Fruit withcalyx (%)

Calyxfreshness(1–3)b

Blotchyfruit (%)

KNO3 4848 a 40.4 a 120 a 50.51 b 2.77 a 97.77 a 2.01 b 5.01 aKCl 4760 a 40.0 a 119 a 58.33 a 0.43 b 98.27 a 2.18 a 3.33 bKCl·MgCl2 4873 a 41.3 a 118 a 56.23 a 0.45 b 98.12 a 2.21 a 3.21 b

a Values of the yield, number of fruits and average fruit weight are the means of 32 plants from trusses 1–8.Values of the firm fruits, rotten fruits, fruit with calyx, calyx freshness and blotchy fruit percentage are the meansof 96 samples from trusses 1–8 (eight fruits per sample). Means in each column followed by different letters aresignificantly different atP ≤ 0.05.

b Calyx freshness: (1) low, (2) medium and (3) high.

No significant difference among treatments was observed in percent fruit with calyx afterstorage simulation (Table 3).

3.2.3. Fruit glucose, TSS, TA and DMFruit glucose and DM content were found to be significantly higher in the KCl·MgCl2

treatments as compared with KCl and KNO3. However, there were no significant differencesin fruit TSS and TA among the treatments (Table 4).

3.2.4. Fruit mineral compositionSignificantly lower fruit NO3 and significantly higher fruit Fe and Cl contents were found

in the KCl and KCl·MgCl2 treatments as compared with KNO3. Mg content was higher infruits treated with KCl·MgCl2. However, no significant differences were found in P, K andCa content (Table 4).

4. Discussion

The main aim of this experiment was to investigate the effects on fertigated greenhousetomatoes of KCl as potassium source with or without partial substitution of KCl·MgCl2.The results of this study coincide with our earlier finding that KCl in the fertigationsolution enhances fruit appearance and improves fruit quality as compared with KNO3(Chapagain et al., 2003), but reduces leaf Mg content. Replacement of 25% of the KCl byKCl·MgCl2 prevents loss of foliar Mg (Table 2). The impact of this result could be seenin plant heights and leaf chlorophyll levels (Table 2): both were significantly higher in theKCl·MgCl2-treated plants. The influence of foliar Mg levels on chlorophyll and ultimatelyplant vigor is well-documented in the literature (Mengel and Kirkby, 1987). Our leaf anal-yses also showed significantly higher Cl accumulation in KCl-treated plants, the highest Cllevels being recorded in KCl·MgCl2-treated plants; however, Cl concentrations remainedbelow the toxic level for tomato described by earlier studies (Marschner, 1995; Xu et al.,2000), and no Cl toxicity was detected in either treatment. Cl increases the incidence ofgold speck injury (Nukaya et al., 1991), but this phenomenon was not observed here.

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Table 4Effect of potassium source in fertigation solution on glucose, TSS, TA, DM content of greenhouse-grown tomato fruitsa

Treatment Glucose(mg g−1)

TSS(◦Brix)

TA(meq g−1)

DM (%) NO3

(mg g−1)Cl(mg g−1)

P(mg g−1)

K(mg g−1)

Ca(mg g−1)

Mg(mg g−1)

Fe (mg g−1)

KNO3 7.09 b 3.51 a 0.061 a 4.85 b 0.014 a 3.37 b 8.32 a 66.25 a 2.75 a 2.13 a 0.64 bKCl 7.21 b 3.56 a 0.060 a 4.92 b 0.008 b 4.30 a 7.26 a 67.54 a 2.87 a 2.08 b 0.077 aKCl·MgCl2 7.50 a 3.62 a 0.062 a 5.03 a 0.009 b 4.85 a 7.10 a 68.11 a 2.84 a 2.36 a 0.081 a

a Values are means for 10 samples from trusses 1–8 (eight fruits per sample). Glucose, TSS, TA, DM and NO3 are based on fresh weight and all minerals are basedon dry weight. Means in each column followed by different letters are significantly different atP ≤ 0.05.

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There are differences about the optimum concentration of Mg in the nutrient solutionin greenhouse tomato fertigation.Carvajal et al. (1999)suggest 0.5 mM Mg in the nutri-ent solution is adequate for optimum fruit yield for lower EC level and 1.0 mM is forhigher EC level. A 40–50 ppm Mg has been recommended for greenhouse tomato pro-duction for Israeli growers (Zaidan, 1999). However, the positive results achieved in thisexperiment by fertigating with KCl·MgCl2 support, the findings of an earlier study byHaoet al. (2000), in which the addition of 50–80 ppm Mg to the nutrient solution reportedlyimproved overall yield and quality in rockwool-grown tomatoes. Owing to the additionof NH4NO3 as nitrogen source to the nutrient solution, the KCl treatment actually had ahigher level of NH4-N than the KNO3 treatment (data not shown). The high foliar Mgrecorded in KNO3-treated plants, as well as the low Mg in the leaf tissue of KCl-treatedplants (Table 2), could therefore be related to cationic competition between NH4 and Mg, asreported byMengel and Kirkby (1987). On the other hand, the higher leaf Mg concentrationin the KCl·MgCl2 treatment may be attributed to the high Mg level in the nutrient solu-tion as compared with the KCl treatment (Table 1). Grimme (1983)has reported a higherMg uptake for high-pH fertigation but in our experiment there was no big variation of pHin both nutrient solution (Table 1) and leachate (data not shown) over the entire growingperiod.

In both KCl and KCl·MgCl2 treatments, the nutrient solution contained slightly height-ened levels of Ca due to supplementation with Ca(NO3)2 as source of N. However, the othermajor elements, including K, were present in similar concentrations in all three treatments.Kafkafi et al. (1982)has reported a low P uptake by tomato plant with increase salinity, thisphenomena was seen only in 106 DAP in our experiment and at 146 DAP no differenceof leaf P content was measured among treatments. The effect of higher concentrations ofcationic nutrients (K, Ca, Mg) in the nutrient solution in promoting higher contents of therespective cations in tomato as reported byRijck and Schrevens (1998)is only partiallysupported by our study.

Replacement of KNO3 by KCl as potassium source confers a number of benefits—highpercentage of firm fruits, fresh calyx, low incidence of rotten and blotchy fruits (Table 3).Since new trend for marketing clusters of tomatoes with calyx instead of single fruits isgaining popularity, these parameters which are basically related to good appearance andgood appearance of the tomato fruit is generally considered to be an index of quality andoften determines consumer choice (Dorais et al., 2001); improvements of these parameterscould play active role in tomato marketing. KCl also increases the Fe and decreases NO3content in tomato fruit (Table 4). These benefits are directly connected with human healthsince NO3 is considered as carcinogenic and also a cause of methemoglobinemia (Wrightand Davidson, 1964; Craddock, 1983). Higher Cl along with lower NO3 in the nutrientsolution might have played the vital role to improve these parameters. However, the reasonfor the incensement of Fe concentration in the tomato fruit in KCl-treated plants is unknownand will require to clarifying. Substitution of KCl·MgCl2 for 25% of the KCl does notdetract from the benefits conferred by the KCl and leads to significantly higher levels offruit glucose, magnesium and dry matter content (Tables 3 and 4). This interaction betweenmagnesium and glucose is well understood due to the facts shown that magnesium increasedthe photosynthetic capacity and carbon fixation stimulation that directly leads to productionof higher glucose levels. Glucose level in the fruit is considered to be the key component

B.P. Chapagain, Z. Wiesman / Scientia Horticulturae 99 (2004) 279–288 287

of tomato taste, and it is also associated with a higher concentration of dry matter (Adamsand Ho, 1989; Dorais et al., 2001).

The positive results of KCl treatment might be argued from the influenced by the elevatedlevel of EC in the nutrient solution due to higher salt index of KCl as compared to KNO3treatment but this EC level (2.2 and 2.4 dS m−1 for the KCl and KCl·MgCl2 treatment, re-spectively) lies in the lower range with in the EC range known to benefit the quality and shelflife of tomato in other studies (Adams, 1987, 1991). Most of these studies show a reductionof fruit yield of tomato by decreasing the fruit size along increasing level of EC but our studydoes not show any significant affect on fruit yield and size among treatments (Table 3). Inmost of earlier studies, Cl level was increased along with Na by applying NaCl but in ourstudies Na level was remained constant to all treatments (data not shown). The combinedeffect of high Cl and low NO3 with changes of NO3–N to NH4–N ratio in the nutrient so-lution might have played the vital role for the positive results of KCl treatments and uptakeof Mg might have interfered by such combination. This study shows a major role playedby Mg when supplying higher Cl and reducing NO3 concentration in tomato fertigation.

5. Conclusion

The results of this study support our previous finding that KCl as sole source of potassiumenhances fruit appearance and improves quality in greenhouse tomato without affectinggrowth or yield. Furthermore, replacing 25% of the KCl by KCl·MgCl2 can boost foliar Mglevels, which declines when KCl is used alone. Use of KCl·MgCl2 also improves the qualityof the fruit by increasing its glucose, magnesium and dry matter contents, while retainingthe benefits conferred by the sole KCl. The findings of this study help to improve the qualityof tomato fruits as well as reduce the NO3 emission to the environment. However, this studyalso realizes the need of further study about the interaction of Mg in greenhouse tomatofertigation in these circumstances.

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