and quality of strawberries

Transcription

1 Australian Journal of E.~perimental Agricrtlture, 1994,34, Effects of hydroponic solution composition, electrical conductivity and plant spacing on yield and quality of strawberries R. A. ~a~-ooshi* and G. C. ~~.esswell* A NSW Agriculture, Horticultural Research and Advisory Station, PO Box 581, Gosford, NSW 2250, Australia. NSW Agriculture, Biological and Chemical Research Institute, Rydalmere, NSW 21 16, Australia. Summary. The influence of nutrient solution adjustment and replacement (management), its electrical conductivity (EC) and plant spacing on yield and quality of strawbeny fruit (var. Torrey) produced in a recirculating hydroponic system was studied at Gosford, New South Wales. Four ways of managing the nutrient solution were examined: (i) ph and EC adjusted daily and the solution replaced every 8 weeks (current grower practice); (ii) as above except that a topping up solution with lower potassium to nitrogen (K: N) ratio was used for EC adjustment; (iii) no daily adjustment of EC or ph and one-third of solution replaced every 2 weeks; and (iv) no daily adjustment of solution volume, EC or ph and full replacement after 8 weeks. Compared with grower practice, these alternative solution management strategies provided no advantage in yield, fruit number, or in the OBrix, citric acid, sweetness or flavour of fruit. Use of a topping up solution supplemented with ammonium nitrate (NH4N03) and calcium nitrate [Ca(N03)2] to reduce the K : N ratio from 1.7 : 1.0 to 1.4: 1.0 had no effect (P>0.05) on yield but significantly increased (P<0.05) berry weight and improved fruit aroma. Regardless of which method of nutrient solution adjustment and replacement was used, the chemical composition of the recirculating solution changed markedly over 53 days. The method of nutrient solution management significantly (P<0.05) affected leaf phosphorus (P), calcium (Ca), magnesium (Mg), manganese (Mn), and zinc (Zn) but only Mg fell to a suboptimal level for growth of strawberries. Reducing the EC of the nutrient solution from 3 to 2 ds/m at early fruit set gave heavier (P<0.05) berries compared with constant EC of 2 ds/m. Increasing the EC from 2 to 3 or reducing it from 3 to 2 at early fruit set resulted in sweeter (P<0.05) berries and reducing the EC from 4 to 2 improved fruit aroma. Yield declined (P<0.05) when EC was increased from 2 to 4 ds/m. Solution EC bad significant effects (Pc0.05) on leaf P, Mg, Mn and Zn. An increase in planting density from 5.35 to 9.35 plants/m2 lowered (P<0.05) marketable yield per plant and fruit acidity but gave 41% higher (P<O.O5) yield on an area basis. Introduction The nutrient film technique (NFT) and channel systems employing rockwool, perlite or gravel as media have all been used for hydroponic strawberry production in Australia (Weir 1987; McCrae 1988). In these systems, the recycled nutrient solution is managed in 1 of 2 ways. In the first, water and nutrients removed by the crop or lost through evaporation are added each day ('topping-up') to maintain a constant electrical conductivity (EC) and the nutrient solution is replaced with a fresh solution every 6-8 weeks. In the second, no daily adjustments are made and fresh solution is added only when the nutrient solution is almost depleted. Although the latter approach affords little control over the relative availability of nutrients to the crop, it has been used successfully for commercial strawberry production. Nutrient solutions used for growing strawberries in Australia typically have relatively high levels of K but in other respects are similar in composition to those used for growing lettuce. The K: N ratio in these strawberry nutrient solution mixes are usually around 1.7: 1.0. For maximum chemical stability of the nutrient solution in a recirculating (closed) system, the quantitative proportions of all nutrients added during daily adjustment must closely reflect crop usage patterns. For strawberries, the available evidence (Albregts and Howard 1979; Welch and Quick 1981; Chow 1988; Lieten 1993) indicates that a K : N ratio near to 1.4 : 1.0 is ideal. In media-based strawberry production systems, the nutrient solution is normally maintained at an EC of 2 ds/m. Lower conductivities (about 1.3 ds/m) are becoming popular in NFT systems but concerns have

2 530 R. A. Sarooshi and G. C. Cresswell been raised about the keeping quality of this fruit. Both yield and quality of fruit can be significantly affected by the salinity or EC of the soil solution (Lieten 1993). Hunter and Morgan (1989) in Ireland found that lowering the EC of the nutrient solution from 1.4 to 0.7 increased strawberry yield but reduced soluble solids (OBrix) and acidity in fruit. Increased OBrix and acidity have been recorded in tomatoes irrigated with more saline water around fruiting (Shalhevet and Yaron 1973; Pasternak et al. 1986; Adams 1991). The possibility of improving the flavour and the keeping quality of strawberries by varying solution EC during the season has interested hydroponic growers for some time but little has been published on the subject. Strawberries grown in a 90% peat and 10% perlite medium produced smaller fruit when the EC of the nutrient solution was reduced from 6.0 and 4.75 ds/m to 3 ds/m after commencement of the first harvest compared with plants receiving a constant 3 ds/m. Reducing solution EC from 3 to 1 dslm lowered fruit sugar (OBrix) and acid content (Anon. 1988). The planting densities used for hydroponic strawberry production in Australia are normally from to plants/ha. Much higher densities have been tried with soil-grown strawberries. Freeman (1981) obtained a 56% increase in fruit yield by increasing planting density from to plantstha. Given that better control of nutrition and moisture are possible in soilless systems, higher planting densities than are currently used commercially should be achievable. We report the results from 3 trials which examined the effect of nutrient solution adjustment and replacement on yield, fruit quality and nutrient status of strawberry plants and on the chemical composition of the recycled nutrient solution. The trials also examined the influence of changing and higher solution EC and plant density on fruit yield and quality and the effect of changing and higher solution EC on plant nutrient status. Materials and methods At the Horticultural Research and Advisory Station, Gosford, strawberry runners (var. Torrey) were planted in plastic wrapped slabs of Growool (Rockwool) measuring 750 x 100 x 75 mm. Drainage slits were provided at the lower end of the slabs which were supported in 110 mm white PVC plastic pipes laid horizontally on steel supports. A 60-mm-wide strip was cut from the top of the pipe to accept the Growool slabs. There were a total of 24 pipe rows, each 3 m long holding 4 slabs of Growool in each row. The 3 pipe rows from each treatment were supplied from and drained into a separate 200 L tank holding the nutrient solution. Pipes were spaced 1 m apart, 1.2 m from ground level, with a slope of 2 in 100 and oriented N-S. The Growool slabs were thoroughly soaked in water and leached before planting (Hangar 1982). Each slab of Growool had three, 4 L/h emitters. Plants were given a 15 min irrigation with nutrient solution 4 times a day, and the excess solution was recycled. Runners were planted in early May and the trial was run until the end of December. The experiments were conducted in the open, under bird netting. The mean daily temperatures ranged from 17.8OC in June-July to 25.9OC in December and night temperatures from 7.1 C in July to 18.6OC in December. The experiment was a split-plot design with randomised complete blocks used in the whole plot. There were 8 treatments in the whole plot and 2 plant spacings in the subplots, replicated 3 times. The basal nutrient mixture for all treatments was a commercial hydroponic formulation (Simple Grow strawberry mix). This powder concentrate was also used to adjust the EC in all except treatment 3 (T3). Its composition (mg/l) at EC 2.0 ds/m was: nitrate-nitrogen (NO3-N), 138; ammonium-nitrogen (NH4-N), 35; phosphate-phosphorus (PO4-P), 36; potassium (K), 292; calcium (Ca), 95; magnesium (Mg), 30; chloride (Cl), 17; sodium (Na), 14, manganese (Mn), 0.45; copper (Cu), 0.17; zinc (Zn), 0.2; iron (Fe), 6; and a potassium to nitrogen (K : N) ratio of 1.7 : 1.0. Treatments Effect of nutrient solution adjustment and replacement. Four management strategies (treatments) were compared. Except in treatment 2 (T2), water was replenished daily to maintain the volume of the nutrient solution at 200 L. When necessary, ph was adjusted using calcium carbonate (CaC03) or sulfuric acid (H2S04). Treatments were as follows. TI, nutrient solution adjusted daily with dry basal nutrient mixture (Simple Grow) to EC 2.0 ds/m, ph 6.0 and replaced every 8 weeks -current grower practice. T2, nutrient solution replaced every 8 weeks. Freshly made solution had an EC of 2.0 dslm and a ph of 6.0. Water and nutrient losses were not replaced within the 8 weeks. ph was not adjusted and ranged from 3.4 to 6.0 during the experiment. T3, nutrient solution adjusted daily with a modified topping-up mix to EC 2.0 ds/m, ph 6.0 and replaced every 8 weeks. The modified topping-up mix was made by adding 250 g of Ca(N03)*. 4H20 and 25 g of NH4N03 to 2 kg of the Simple Grow mixture. This had no effect on ammonium (NH4+) concentration which remained at 35 mg/l and increased Ca concentration from 95 to 121 mgl The K:N ratio of this treatment was lowered to 1.4 : 1.0 compared with a ratio of 1.7 : 1.0 in the topping-up solutions used for all other treatments. T4, nutrient solution not adjusted daily. One-third of total solution was replaced every 2 weeks with fresh solution having an EC of 2.0 ds/m and a ph of 6.0.

3 Nutrient solution management of hydroponic strawberries 53 1 Effect of conductivity (EC). Five EC regimes, Ca, Mg, Mn, Cu, Zn and boron (B) using the methods of including TI, were compared. The volume of the Leece and Short (1967). The nutrient composition of the solution was maintained at 200 L and the ph adjusted solution obtained from the 8 treatment tanks was also daily to 6.0. Treatments were as follows. monitored weekly for 53 days (7 November- T.5, EC 3.0 ds/m maintained up to early fruit set (mid 30 December 1988). August) then 2.0 ds/m for rest of season (up to end Marketable and cull fruit yield and marketable fruit of December). number were recorded 3 times per week during the T6, EC 2.0 ds/m maintained up to early fruit set, then fruiting period from which mean marketable berry 3.0 ds/m for rest of the season. weight for the season was calculated. Fruit which was T7, EC 4.0 ds/m maintained up to early fruit set, then deformed Or or weighed less than 2.0 ds/m for rest of the season. 5 g/berry was considered unmarketable. T8, EC 2.0 dslm maintained up to early fruit set, then OBrix and percentage citric acid of fruit (acidity) were 4.0 ds/m for rest of the season. recorded. Total soluble solids (OBrix) was determined with a temperature compensating hand refractometer. Effect of plant Over the treatments, Berries were crushed, strained and 10 mj- of the strained half the plants in each row were spaced 187 mm apart extract was titrated against 0.1 mol/l NaOH to obtain (4 plants/slab or 5.35 plants/m2) and the other half 107 mm percentage citric acid. apart (7 plants/slab or 9.35 plants/m2). Spac' ings were Taste testing of fully coloured mature fruit was done by randomised within each row. Measurements The daily fall in the level of solution in each of the 200 L treatment tanks was measured over 4 weeks from early November-early December. The ph and EC of the nutrient solution in each tank were recorded daily using portable ph and conductivity meters. Samples of recently matured leaves (blade and petioles) were collected in early November corresponding with the second major flush of fruit from each row, combining the 2 plant spacings. The leaves were then oven dried, ground and analysed for N, P, K, 24 staff members of NSW Agriculture, Gosford. Tasting commenced 2 h after tasters last had food or drink. pane? members were given a plate divided into 8 sectors, each sector containing 1 berry from each of the 8 treatments. Each taster was on a separate table and the testing procedure was repeated 3 times. Fruit was rated for colour, sweetness, aroma, flavour, acidity, texture and general acceptability. Responses were recorded on a continuous linear scale ranging from 0 to 15 and these ratings were then converted to a score out of 10. Data were analysed using analysis of variance and treatment differences were compared using the least significance test. Table 1. Effects of nutrient solution adjustment, conductivity and plant spacing on fruit yield and quality of strawberry fruit (var. Torrey) See text for full description of treatments TI-T8 * Treatment Fruit yield No. of Beny weight OBrix Citric acid Cull yield (glplant) (tha) fruitiplant (9) (glplant) Nutrient adjustment TI A 303 (21.6) T2 261 T3 322 T4 270 Electrical conductivity TIA 303 T5 286 T6 278 T7 280 T s.d. (P = 0.05) 45 Plant spacing 187 mm mm s.d. (P = 0.05) 17 A Current grower practice. I

4 532 R. A. Sarooshi and G. C. Cresswell Results and discussion Effect of nutrient solution adjustment and replacement The average yield of berries (303 gtplant) from the current grower practice (TI) was within the range obtained in commercial plantings in coastal New South Wales. This was despite the lateness of the planting which is known to reduce yield of strawberries in this area (Cox 1976). Compared with the grower practice, the 3 other strategies for managing the nutrient solution gave no advantage in yield, fruit number, or in the OBrix, citric acid, sweetness or flavour of fruit (Tables 1 and 2). However, T2 and T4 whose nutrient solutions were not adjusted daily, yielded less than T3 which was adjusted daily with modified topping-up mix. Treatment 3 also had heavier bemes (1 1.9 glberry) than that of the other 3 treatments and its fruit aroma was better than that of T1. Calcium nitrate and ammonium nitrate (NH4N03) additions made to the topping-up mix used in T3 altered the ratio of K : N from 1.7 : 1.0 (in the current grower treatment) to 1.4: 1.0. This new ratio is closer to the proportions of these nutrients removed by a productive strawberry crop. Nutrient uptake data presented by Lieten (1993) indicate a K : N ratio of 1.3 : 1.0 is optimal around the green fruit stage. Welch and Quick (1981) report removal of K : N by fruit in ratios of 1.2 : 1.O and 1.4 : 1.O for first and second year harvests from soil- grown Tioga. According to Albregts and Howard (1979) K and N removed by 3 soil-grown strawberry varieties including Tioga over a full season was in the ratio of 1.1 : 1.O. The method used to maintain the chemical composition of the recycled nutrient solution significantly affected the leaf content of P, Ca, Mg, Mn and Zn, but only leaf Mg was at a suboptimal level for growth of strawberries (Table 3). Using a critical value Table 2. Effects of nutrient solution adjustment and conductivity on the sweetness and aroma of strawberry fruit (var. Torrey) See text for full description of treatments TI-T8 Treatment Sweetness Aroma Nutrient adjustment ~1~ T T T Electrical conductivity TI* 4.3 T T T T s.d. (P = 0.05) A Current grower practice. of 0.2% (Ulrich et al. 1980; Jones et al. 1991), leaf Mg was deficient in T4 and marginal (<0.25%) in all treatments except T3. Roorda Van Eysinga and Van Caem (1977) suggest a level of 0.4% Mg is required by strawberries for highest yields. When strawberry plants are deficient in Mg, fruit are pale and soft but size and flavour are not normally affected (Ulrich et al. 1980). Leaf Ca was marginal in all treatments but the characteristic tip burn symptom of deficiency was not observed. Increasing the proportions of N and Ca in the topping-up solution used in T3 had no effect on leaf N but increased (P<0.05) concentrations of leaf Ca (although the lev& was still marginal) and Mg. Table 3. Effect of nutrient solution adjustment and conductivity on the nutrient composition of strawberry leaves (var. Torrey) See text for full description of treatments TI-T8 Treatment N P K Ca Mg Mn Cu Zn B (%) (%) (%) (%) (%I (PgIg) (PW (Pm (P&) Nutrient adjustment T ~ A T2 T3 T4 Electrical conductivity T1A T5 T6 T7 T8 1.s.d. (P = 0.05) 1.s.d. (P = 0.001) * Current grower practice.

5 Nutrient solution management of hydroponic strawberries 533 The average water use per plant over the 4 weeks from early November-early December was 270 mllday. This is comparable with ml/plant.day estimated by McCrae (1988) as the peak summer requirement for strawberries on the North Coast of New South Wales. The nutrient composition of the growing solution changed appreciably over a 53-day monitoring period, regardless of which method of nutrient solution management was used. Figure 1 shows trends for some of the major nutrients. Taking the average percentage variation from the initial concentration of each nutrient recorded at the 8 sampling points as an index of the degree of change or buffering (Table 4), the method of nutrient solution management did not alter the degree of buffering appreciably. However, the analysis does highlight differences in the buffering of individual nutrients which are independent of the solution management treatments. For example, concentrations of NO3-N, PO4-P and Mg varied less over the monitoring period than concentrations of K and Ca. Although the solution management treatments were very different, the changes in the nutrient composition of the growing solutions followed similar trends. This suggests that factors like plant growth stage, evaporation, I I Time (days) Figure 1. Effect on nutrient management treatments on concentration of: (a) nitrate-nitrogen (NO3-N); (6) phosphate-phosphorus (PO4-P); (c) potassium (K); (d) calcium (Ca); and (e) magnesium (Mg) in the nutrient solution over a 53-day monitoring period. No topping-up (A); daily topping-up using basal nutrient mixture (current practice) (0); daily topping-up using modified mix with N supplement (0); one-third of solution replaced every two weeks (A).

6 534 R. A. Sarooshi and G. C. Cresswell Table 4. Effect of nutrient solution adiustment and reolacement on the percentage variations in the concentrations of nutrients in the growing solutions compared with the levels in the fresh solution Readings are the mean of 8 measurements over 53 days See text for full description of treatments T1-T4 Treatment NO,-N P0,P K Ca Mg (%I (%I (%I (%) (70) T T T T and light intensity had an overriding influence on nutrient removal, and within limits, on the composition of the recycled nutrient solution. Effect of conductivity Varying the EC of the nutrient solution, significantly (P<0.05) affected fruit yield, berry weight and the sweetness and aroma of berries (Tables 1 and 2) but not fruit number, OBrix, citric acid, flavour, colour, texture and general acceptability of berries. Solution EC had significant effects (P<0.05) on concentrations of P, Mg, Mn and Zn in leaves (Table 3). Magnesium was deficient in both high EC treatments (T7 and T8). All other nutrient levels were adequate for strawberries. Compared with the current grower practice (TI), heavier, sweeter berries were produced when the EC was reduced from 3 to 2 at early fruit set. Berries were also sweeter when the EC was increased from 2 to 3 at this time. A higher score for aroma was obtained when the EC was reduced from 4 to 2 (T7) and yield was depressed when EC was increased from 2 to 4. Lieten (1993) when discussing the effects of constant EC treatments on strawberries grown on substrate in run-towaste systems reports that yield, plant and root weight all decline while fruit become smaller but firmer as the EC is increased over the range 2-8 ds/m. At low EC (<1.4 ds/m), average berry size increases but dry matter content, sugar and acid level decreases. Fruit is also softer and more susceptible to rotting. Lieten (1993) concludes that a minimum EC of 1.2 ds/m is necessary during harvest to maintain good fruit quality. Hunter and Morgan (1989) obtained higher yields at an EC of 0.7 ds/m due to increased fruit numbers but fruit had lower soluble solids (OBrix) and acid levels compared with control (EC of 1.4 ds/m). In an experiment where EC was varied during crop development, Lieten (1993) reported highest yields were obtained when the EC was increased from 1.3 to 1.7 ds/m at harvest. High yields were also obtained with a regime of 0.45 ds/m increased to 1.3 ds/m at harvest but this treatment gave the smallest berries. The largest berries were obtained when the EC was maintained at 1.3 ds/m (vegetative), 1.7 ds/m (flowering) and 1.3 ds/m (harvest) but yield was less than half that of the best treatment. De Bruyn and Voogt (1989) found that both yield and fruit size increased when the EC was raised from 1.0 to 1.6 ds/m 1 month after planting compared with constant EC treatments of 2 and 3 ds/m. The results of this trial show that changing the EC of the growing solution during the development of the crop can influence the quality of strawberry fruit. However, crop responses proved very complex and no 1 regime was superior to the others in maximising all aspects of fruit quality. Effect of spacing Spacing plants 187 mm apart, as is current practice, gave significantly higher marketable yield, fruit number, cull fruit yield and citric acid level in the fruit on a per plant basis than the higher density of 107 mm plant spacing (Table 1). However, because of greater plant numbers per unit area, the higher planting density yielded more on a per hectare basis. Plant spacing had no effect on mean fruit weight or OBrix. Lower fruit acid levels for plants spaced 107 mm compared to those 187 mm apart, resulted in higher Brix/acid ratios for the plants spaced 107 mm apart. By increasing planting density from to plantsha, Freeman (1981) increased marketable yield of soil-grown Torrey from 23.4 to 27.4 tha. In our trial, marketable yields increased from 16.4 to 23.4 tka when density was increased from to plantsha. The results show that increasing plant density from 5.35 to 9.35/m2 gives higher production from a unit area but this is obtained at the expense of slightly lowered fruit acid and reduced yield per plant. Acknowledgments We wish to thank Ms R. Holbrook for valuable field and laboratory assistance and Ms L. Spohr for providing help and advice on statistical analyses. Growool slabs were provided by Growool Horticultural Systems, 2 Wiltona Place, Girraween, NSW 2145, and the pumps and irrigation equipment by Berjax pumps, West Gosford. References Anon. (1988). Feed and water for bucket crops. Grower 110 (lo), Adams, P. (1991). Effects of increasing the salinity of the nutrient solution with major nutrients or sodium chloride on the yield, quality and composition of tomatoes grown in rockwood. Jourrzal of Horticultural Science 66 (2), Albregts, E. E., and Howard, C. M. (1979). 'Nutrient Accumulation by Strawbeny Plants.' Dover ARC Research Report SV (Agricultural Research Center, IFAS, University of Florida: Dover, Florida.)

7 Nutrient solution management of hydroponic strawberries 535 Bruyn, J. M. de, and Voogt, W. (1989). Strawberries which ECvalue is the most satisfactory in autumn culture? Groenten en fruit 45(3), 30. Chow, K. K. (1988). Studies of the production of winged bean and strawberries in hydroponic systems. M.Sc. Thesis. La Trobe University. Cox, J. E. (1976). Effect of time of planting on fruit yield and runner production of cold stored and freshly lifted strawberry plants. Australian Journal of Experimental Agriculture and Animal Husbandry 16, Freeman, B. (1981). Response of strawberry fruit yield to plant population density. Australian Journal of Experimental Agriculture and Animal Husbandry 21, Hangar, B. (1982). Rockwool in Horticulture-a review. Australian Horticulture 80(5), Hunter, R. R., and Morgan, J. V. (1989). Nutrition of strawberries in rockwool. Acta Horticulturae 238, Jones, J. B., Wolf, J. B., and Mills, H. A. (1991). 'Plant Analysis Handbook.' (Micro-Macro Publishing: Georgia, USA.) Leece, D. R., and Short, C. C. (1967). A routine procedure for the nutrient element analysis of peach leaves utilising atomic absorption spectroscopy. Chemistry Branch Bulletin No. F 78. (N.S.W. Department of Agriculture: Sydney.) Lieten, P. (1993). Nutrition of strawberries in hydroponic and substrate culture. In 'Proceedings of the 7th National Benyfruit Conference, Perth.' (Department of Agriculture, Western Australia: Perth.) McCrae, M. (1988). Hydroponic strawberry production. In 'Proceedings of Hydroponics Seminar' March Salamander Bay. (Ed. C. R. Beckingham.) pp (Department of Agriculture, New South Wales: East Maitland.) Pastemak, D., De Malack, Y., and Borovic, I. (1986). Irrigating with brackish water under desert conditions. VII. Effect of time of application of brackish water on production of processing tomatoes (Lycopersicon esculentum Mill.). Scientia Horticulturae 12, Roorda Van Eysinga, J. P. N. L., and Van Caem, H. E. (1977). Nutrition of glasshouse strawberries with nitrogen, phosphorus and potassium. Scientia Horticulturae 7, Shalhevet,.I., and Yaron, B. (1973). Effect of soil and water salinity on tomato growth. Plant Soil 39, Ulrich, A., Mostafa, M. A. E., and Allen, W. W. (1980). 'Strawberry Deficiency Symptoms: A Visual and Plant Analysis Guide to Fertilisation'. Publication No (University of California Division of Agricultural Science: Berkeley.) Weir, R. G. (1987). Hydroponic systems and soilless culture. In 'Proceedings of Hydroponics Workshop' March Salamander Bay. (Ed. C. R. Beckingham.) pp (Department of Agriculture, New South Wales: East Maitland.) Welch, N. C., and Quick, T. (1981). Fertilising summer-planted strawberries in California's central coast. California Agriculture 35(9, lo), 267. Received 27 August 1992, accepted 10 December 1993