INFLUENCE OF CLOSED SYSTEMS ON THE

INFLUENCE OF CLOSED SYSTEMS ON THE DEVELOPMENT OF CUCUMBERS Böhme, M., Department of Horticulture, Humboldt University of Berlin, Königin-Luise-Str. 22, D-14195 Berlin, Germany

INTRODUCTION When growing plants in greenhouses, there is a growing necessity of switching from open hydroponic systems to closed ones. Some countries, such as The Netherlands, already enacted legislation demanding a swift transition (Ammerlaan, 1993). Advantages of closed systems are seen in their better environmental compatibility along with a more efficient use of inputs, particularly fertilizers and water. Currently, by 'closed system' we chiefly mean the recycling of surplus nutrient solution (drainwater). In future, however, other factors besides substrates - e.g., energy, carbon dioxide and other resources - should be included as well. From the nutrient film technique (NFT), a method with recirculating nutrient solution that has been practiced already for quite a while, some problems - for example, disinfection of the solution - are already known which need to be taken into account in substrate culture as well. But there are also specific problems. Several aspects have to be considered with regard to water and nutrient supply in drip-irrigated substrate culture: the amount of drainwater may vary widely from day to day, the nutrient concentrations in the surplus solution may vary, and there may be a certain inflow of harmful amounts of chlorine, sodium and sulphate, particularly from poor-quality water (Ohta et al., 1991; Böhme, 1994, 1995). Against this background, several questions have to be answered when switching over to the closed system: - How much nutrient solution should be applied? - Large or small amounts of drainwater - which is the better choice? - Influence of the kind and service life of substrates on the drainwater quantities produced; - Effects of closed system on water and nutrient balances; - How to allow for the inflow of recycled plant nutrients and other chemical elements when calculating the solution for drip irrigation; - Are there significant effects on crop yield? Trials so far have been reported mainly with tomato (Guillaumin, 1992), but they do not yet allow a satisfactory evaluation of recycling techniques. That is why cucumber, known to be a more sensitive crop, was used for the experiments reported here. MATERIAL AND METHODS In 1994 and 1995, experiments for comparison of open and closed systems with hydroponic substrate culture of cucumber were carried out in a 400 sq.m. greenhouse. Two separate nutrient solution cycles were set up. In both test facilities, slabs (rockwool) or containers (perlite) were put in channels for drainwater collection. In the open system the drainwater was collected by each channel and its quantity was recorded daily. In the closed system the drainwater from all channels was collected in a pipe and put into a tank. This drainwater was then admixed to the "new" solution, using the EC value of the drainwater as a basis.

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An average EC target value of 1.2 was set for admixing the mixture of rainwater (or tap water) and drainwater to the solution. Hence, at an EC average of 2.3 in the solution, an amount of concentrated solution was admixed that complied with an EC of 1.1. The space available for the root system was between 8 and 9 litres per plant. NETAFIM drip irrigation was used to supply the solution to the test plants. To eliminate substrate-specific effects on the drainwater, inert substrates, rockwool and perlite, were used exclusively. The solution was calculated by means of a nutrient and irrigation unit based on physiologically active radiation (PAR, J * cm-2 * d-1), a separate radiation value having been set for each variant. The radiation values were between 50 and 140 J * cm-2 * d-1. Moreover, in 1994 comparisons were also made with other irrigation control systems: a drain meter and a moisture sensor were tested. Due to the wide variations particularly in substrate water capacities, the radiation value for the various substrates had to be continuously adjusted, and dripping times had to be modified. In each variant the drainwater contents were determined daily, while the concentrations of the plant nutrients NO3-N, P, K, Mg and Ca and of the elements Na, SO4 and Cl as well as the pH and EC values were analysed every month or two. The new nutrient solution was recalculated after each analysis of the substrate solution (in slabs and in containers). The 'Hydrofer' computer program was used for calculating the amounts of fertilizers, salts and acids required. With this program it is possible to consider the substrate nutrient contents, light conditions, the developmental stages of the crops and the composition of the water (Böhme, 1993). If possible, rainwater was used, but sometimes well water with high Ca, Na, Cl and SO4 concentrations had to be used for irrigation. Culture dates were different in the two test years (Table 1). RESULTS AND DISCUSSION In the first test year, crop yields obtained in a total of 198 vegetation days were between 19.82 and 26.32 kg * m-2 (Fig. 1). Significant differences were found between the substrates, between the irrigation variants and between the open system and the closed one. Crop yields were highest in the variant where the dripping intensity had been controlled via radiation (RA) and a moisture sensor (water measurements - WM). Control by drainwater measurement (DWC) seems to be less efficient as to adjustment to the plants' demand for solution. Compared with WM, the result was even significantly inferior in both variants (open system and closed system). Almost all variants showed adverse effects of the closed system on crop yields, particularly in the cucumber culture in perlite. In the second test year with somewhat modified variants, only in rockwool (used for the second year) the open system was significantly superior to all the other variants (Fig. 2). Unlike in experiments carried out in previous years, in this trial to the yields were somewhat though not significantly - lower. Another criterion in the comparison of open and closed systems was the amounts of nutrients applied to the cucumber plants during the culture period. In theory, less nutrient solution would be required in the closed system with drainwater recycling. In practice, however, in the first test year the closed system consumed 80 litres more of the concentrated solution, but in the second year the hypothesis proved true as the closed system used 430 litres less of the concentrated solution. More clarification about the two systems was expected from a nutrient balance. A comparison of the macronutrients - nitrogen, phosphorus, potassium, magnesium and calcium - applied to the cucumber plants during the 131-day vegetation period essentially proved the above assumption to be true (Fig. 3). Only magnesium and calcium consumption was somewhat higher in the closed system. The significantly lower potassium consumption was a

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surprise. This fact may, of course, also indicate that the target values in the 'Hydrofer' fertilizer calculation program (Böhme, 1993) need to be brought better in line with the specific conditions of the closed system. Tables 2 and 3 show the nutrient quantities used for each new concentrated solution in the two systems. As from the fifth date (26 April), the type and quantity of nutrients applied began to differ more strongly. Thanks to computer-aided calculation based on nutrient analysis in the rhizosphere, beginning with 28 May only small amounts of nitrogen, potassium and phosphorus were still required in the closed system, mainly for pH adjustment with phosphoric acid. To shed more light upon the differences in nutrient balance between the two systems, macronutrient concentrations were determined in the rhizosphere and in the drainwater collection tank of the closed system (Fig. 4). The following conclusions can be drawn:  Sulphate concentrations increase continuously in the rhizosphere in the closed system, but in the drainwater they decline after very high initial levels.  Potassium concentrations were well above the target values, particularly in the closed system, but potassium concentrations in the drainwater declined after potassium application had been reduced and finally discontinued.  As can be seen from the drainwater analysis, excessive nitrogen and calcium concentrations were lowered if the application of these elements had been reduced.  In the closed system there is a risk that ballast elements (sulphate, sodium, chlorine, etc.) would accumulate, which means that there may be a need for limiting the service life (recycling period) of the drainwater, particularly in case of poor water quality. In the second test year again the amounts of macronutrients applied during the 175-day vegetation period were recorded for the two systems (Fig. 5). Trends were similar to those in the first year. Nitrogen and phosphorus quantities differed but insignificantly between the two systems. Potassium concentrations were also lower in the closed system, but calcium contents were somewhat higher. Compared with the amounts recorded by Geissler et al. (1991), somewhat more potassium was applied in both systems, and calcium amounts were significantly lower. However, when comparing the results, allowance should be made for the fact that the test data pertained to the overall amounts applied and that losses were not considered. The nutrient quantities applied in the second test year each time a new solution was prepared (Tables 4 and 5) clearly showed a downward trend with the later dates. New solution was prepared at longer intervals but with larger nutrient quantities. The development in terms of time of the nutrient quantities applied to the concentrated solution shows a trend that is somewhat different from the one in first test year, particularly for the closed system. Particularly notable is the higher demand for nitrogen, potassium and calcium in the period between 7 April and 12 May. Solution recycling in the closed system is controlled by the EC value. This is why in both test years EC values were measured over a longer period of time (Fig. 6). In the first year salt concentrations were somewhat unstable but quite high. In 1995 the EC values were balanced quite well, with a significant rise as from May. As can be seen, the trends in salt concentration differ greatly between the test years. Likewise, EC values may vary also within one vegetation period. Hence, EC values need to be continuously adjusted at the control unit in order to ensure both a steady emptying of the drainwater collection tank and suitable proportions of drainwater, fresh water and concentrated solution.

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CONCLUSION No major differences in crop yield were found in a comparison of cucumber culture in open and closed systems, but there were significant yield differences between some rockwool variants and perlite variants. The assumption that less water and nutrients would be needed in a closed recycling system was corroborated particularly in the second test year. Potassium in particular was required in much smaller quantities in the closed system, the reasons of which still need to be clarified. Nutrient accumulation in the drainwater can be kept low if the target values in the new solution get continuously adjusted. The best strategy for the admixing of drainwater has still to be determined. The conception of drainwater recycling via EC value needs reconsideration and perhaps has to be put into a more concrete form, for control may be either by quantity or by EC value. Control by EC value may be problematic above all in those cases where the tap or well water contains plenty of sulphate, sodium, chlorine and/or other elements, and rainwater is not available in sufficient quantities. Under the conditions of this experiment, the high amounts of calcium, sodium, chlorine and sulphate in the drainwater are another major problem. The continuous use of such drained nutrient solution may well lead to concentrations that may either harm the plants or have adverse effects on fruit quality (Schacht et al., 1992; Burg, 1994). Consequently, the amount of drained solution should be minimized by a more efficient adjustment of the solution quantities to the needs of the plants, particularly in such sensitive crops as cucumber.

REFERENCES Ammerlaan, J.C.J, 1993. Environment-conscious production systems in Dutch glasshouse horticulture. ISHS International symposium on new cultivation systems in greenhouse, Cagliari, Acta Horticulturae, 361 Böhme, M., 1993. Parameters for calculating nutrient solution for hydroponics. Eighth international congress on soilless culture, Hunters Rest, Proceedings, Wageningen, 8596 Böhme, M., 1994: Effects of closed systems in substrate culture for vegetable production in greenhouses XXIV th International Horticultural Congress, Kyoto Japan, Acta Horticulturae, 396, 45-54 Böhme, M., 1995: Evaluation of organic, synthetic and mineral substrates for hydroponically grown cucumber; Symposium on growing media & plant nutrition in horticulture, Naaldwijk The Netherlands; Acta Horticulturae 401, 209-217 Burg, A. van der, 1994. Vooral natriumcijfer loopt op. Groenten + Fruit /Glasggroenten 11 -18 March: 20-21 Geissler, Th. et al. (1991) Gemüseproduktion unter Glas und Plasten- Produktionsverfahren. DLV Berlin 1991, p. 101 Guillaumin, A., 1992. Recycling nutrient solution in soilless tomato culture on a biodegradable substrate, Revue Horticole, 331: 31-33

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Ohta, K.; Ito, N.; Hosoki, T. and Higashimura, H., 1991. Influence of the concentrations of nutrient solution salt supplement on quality and yield of cherry tomato grown hydroponically. Journal of the Japanese Society for Horticulture Science, 60: 89-95 Schacht, H.; Exner, M. and Schenk, M., 1992. Influence of N supply on the Ca nutrition of greenhouse cucumber in soilless, closed culture systems. Gartenbauwissenschaften 57: 238-242

SUMMARY To minimize environmental pollution from plant cultivation in greenhouses, possibilities were examined for the continuous exploitation of water and nutrients in closed systems. An open system was compared with a closed one, using different technological solutions and cucumber as test plant. The effects of open and closed systems were examined on the following plant parameters: development of shoots and roots, crop yield, water and nutrient consumption, and plant health. According to the results obtained, less fertilizer was consumed when using the closed system, the effect on crop yield is uncertain, and there are still open questions as to the control of the mixing of drainwater and nutrient solution. Table 1. Details of experiments with cucumber for check of drainwater nutrient concentrations in different substrates. Plant density - 2.1 * m-² ; number of plants - 10 with 4 replications each 1994 1995 cv. Primera DR cv. Tyria EZ cv. Primera DR cv. Tyria EZ Planting: 18 February 21 July 20 January 22 July First harvest: 30 March 11 August 28 June 11 August Last harvest: 27 June 26 September 06 July 02 November

Table 2. Amount of nutrients (in g) applied to the concentrated solution for cucumber in open system, and service life of the solution, 1994

01 Mar 29 Mar 19 Apr 26 Apr 04 Mai 09 May 20 May 28 May 31 May 08 Jun 15 Jun 17 Jun 20 Jun

Service life [days] 28 21 7 8 5 11 8 3 8 7 2 3 7

N

P

K

Mg

Ca

504 0 504 504 0 0 504 79 308 150 150 150 308

129 154 146 146 188 188 146 88 67 133 67 133 133

1063 188 1063 1063 188 188 1063 300 925 458 458 458 925

138 0 138 138 0 0 138 67 0 0 0 0 0

288 0 288 288 0 0 288 0 0 0 0 0 0

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Table 3. Amount of nutrients (in g) applied to the concentrated solution for cucumber in closed system, and service life of the solution, 1994

01 Mar 29 Mar 19 Apr 26 Apr 04 May 09 May 20 May 28 May 31 May 08 Jun 15 Jun 17 Jun 20 Jun

Service life [days] 28 21 7 8 5 11 8 3 8 7 2 3 7

N

P

K

Mg

Ca

504 0 504 204 504 350 504 79 79 79 38 38 79

129 46 146 79 146 67 146 88 67 133 67 133 133

1063 54 1063 429 1063 463 1063 300 0 0 0 0 0

138 0 138 54 138 0 138 67 0 0 0 0 0

288 0 288 113 288 288 288 0 0 0 0 0 0

Table 4. Amount of nutrients (in g) applied to the concentrated solution for cucumber in open system, and service life of the solution, 1995

26 Jan 17 Feb 07 Apr 27 Apr 12 May 29 May 2 Jun

Service life [days] 29 28 21 15 17 25 14

N

P

K

Mg

Ca

1140 3020 1430 1350 1380 1340 940

580 64 28 540 270 23 16

4080 6218 2844 3930 2950 2588 1800

400 870 370 700 350 310 220

0 1420 0,71 360 730 710 500

Table 5. Amount of nutrients (in g) applied to the concentrated solution for cucumber in closed system, and service life of the solution, 1995

26 Jan 17 Feb 17 Mar 07 Apr 27 Apr 12 Mai 29 Mai 23 Jun

Service life [days] 29 28 21 20 15 22 25 14

N

P

K

Mg

Ca

1140 1340 770 1250 1350 3220 1340 400

580 230 16 18 540 540 23 7

4080 2588 1550 2322 3930 6550 3 768

400 310 220 250 700 0,7 13 90

0 710 350 710 360 1820 29 210

Yield [kg x m-²]

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30.00 27.50 25.00 22.50 20.00 17.50 15.00 12.50 10.00 7.50 5.00 2.50 0.00 Rockw.RA

Rockw.DWC Open system

Rockw.WM

Perlite RA

Closed system

Fig. 1. Cucumber yields in open and closed substrate culture systems; overall result of one spring culture and one autumn culture (131 and 67 vegetation days, respectively), 1994 RA = radiation, DWC = drainwater measurement, WM = water measurement

60.00

Yield [kg x m-²]

50.00 40.00 30.00 20.00 10.00 0.00 Rockw.1st y

Rockw.2nd y

Open system

Perlite 1st y

Perlite 2nd y

Closed system

Fig. 2. Cucumber yields in open and closed substrate culture systems; overall result of one spring culture and one autumn culture (175 and 83 vegetation days, respectively), 1995

Nutrient quantities [g per plant]

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45.00 40.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00 N

P

K

Mg

Ca

Nutrients Open system

Closed system

Fig. 3. Nutrient quantities applied per one cucumber plant in substrate culture -open and closed systems (1994)

900

'Closed system'

-1

Elements in [mg * l ]

800 700 600 500

Drainwater 'Open system'

400 300 200 100 0 Apr NO3-N

Mai

Jun P

Jul

Apr K

Mg

Mai

Jun Ca

Jul

Apr Na

Mai

Jun

SO4

Fig. 4. Substrate and drainwater nutrient concentrations - open and closed systems with cucumber, 1994

Jul Cl

Nutrient quantities [g per plant]

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110 100 90 80 70 60 50 40 30 20 10 0 N

P

K

Mg

Ca

Nutrients Open system

Closed system

Fig. 5. Nutrient quantities applied per one cucumber plant in substrate culture - open and closed systems (1995)

-1

EC values [mS * cm ]

4 3 .5 3 2 .5 2 1 .5 1 0 .5 0 06. A pr

11. A pr

14. A pr

20. A pr

06. M ay

03. Jun

07. Jun

10. Jun

12. Jun

14. Jun

16. Jun

18. Jun

20. Jun

-1

EC values [mS * cm ]

D a te s o f m e a s u r e m e n t, 1 9 9 4

4 3.5 3 2.5 2 1.5 1 0.5 0 17. 08. 11. 14. 18. 21. 25. 29. F eb M ar M ar M ar M ar M ar M ar M ar

10. A pr

21. A pr

30. 09. 14. 31. A pr M ay M ay M ay

D ates of measurement, 1995

Fig. 6. EC values in drainwater of closed system with cucumber, 1994 and 1995

04. Jul