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Soilless crop cultivation has become a preferred practice in the greenhouse industry ( Van Os and Benoit, 1999 ); the most widely used soilless system is growing crops on rockwool ( Sonneveld, 1991 ). In comparison with the common soil

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Abstract

Rockwool is an inert medium for use in greenhouses. It is reported to contribute negligible nutrients to plants. However, Rosa multiflora ‘Burr’ rootstocks grown in Grodan rockwool exhibited no visible Fe chlorosis with an Fe-free nutrient solution. Leaf chlorophyll content was 2.65 mg·g-1 with Fe and 2.85 mg·g-1 without Fe. Available Fe concentrations of three commercial materials (Hortiwool, Grodan block, and Grodan loose), estimated by using diethylenetriaminepentaacetic acid (DTPA) extraction (2 DPTA : 1 rockwool, v/w), were 43.0, 0.33, and 3.95 mg Fe/liter, respectively. With long-term DTPA extractions (20:1), Fe extracted from Hortiwool and loose Grodan increased for ≈3 days before leveling off, while Fe extracted from Grodan block increased for 6 days. Measurable levels of Mn, Cu (348 mg·liter-1), and Zn were found in DTPA extracts of Hortiwool; measurable levels of Mn and Cu were extracted from loose Grodan and measurable levels only of Mn from Grodan block.

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°F (19.4 °C) average day and night temperature. Producers use rockwool as growing media in large commercial operations around the world ( Papadopoulos, 1991 ). Most Mississippi growers use locally produced pine bark that has been finely shredded and

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production often relies on germination blends or mixtures containing high proportions of peat or coir to facilitate high water retention characteristics. An alternative substrate, rockwool is primarily composed of basaltic rock heated to ≈1600 °F and then

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foam, and rockwool ( Fonteno and Nelson, 1990 ; Handreck and Black, 2002 ; Milks et al., 1989a ). Fine particle sizes of these components are often required to evenly fill small container cells, resulting in decreased pore size and higher water

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processed minerals (rockwool, perlite, vermiculite, expanded clay, zeolite). Organic substrates include natural organic matter (pine sawdust, pine bark, wood chips, peatmoss, coconut coir, and rice hulls) ( Papadopoulos et al. 2008 ). Since the

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Loose rockwool had a total porosity similar to peatmoss (92%, by volume) but with water retention capabilities similar to sand. Root media formulations containing loose rockwool were tested with seven plant species for plant response and nutrient uptake. The volume percent formulation, 20 rockwool : 10 peatmoss : 20 vermiculite : 45 pine bark : 5 perlite, was superior to formulations containing 10% or 30% rockwool. Plant response in this rockwool medium in bedding plant flats was superior to that in two high-performing commercial media for impatiens (Impatiens sultanii Hook), marigold (Tagetes patula L.), and petunia (Petunia hybrida Vilm) and equal to one commercial medium for tomato (Lycopersicon esculentum Mill.). However, response of chrysanthemum (Chrysanthemum × morifolium Ramat.), geranium (Pelargonium × hortorum Bailey), and poinsettia (Euphorbia pulcherrima Willd. ex Kl.) in 1.58-liter pots was inferior to both commercial media in one-half of the trials. Differential plant responses in the root media treatments did not relate directly to differences found to occur in plant nutrient composition. The high initial pH level of rockwool necessitated reduced application of limestone and increased application of calcium sulfate to offset Ca deficiency.

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In rockwool-grown greenhouse vegetables, unsatisfactory spatial root development, rapid root collapse, and increased incidence of root diseases are very common. Improved water management could alleviate these problems to some extent, because this could favorably modify water-air distribution in the slab, thereby improving greenhouse vegetable yield and quality. The present study tested six irrigation strategies on the productivity of rockwool-grown tomatoes (cv. Rapsodie) during Jan.o–Aug. 2004. The four treatments, based on electronic Grodan? water content meters (WCMs), received irrigations when the slab water content (SWC) was ≈60%, 70%, 80%, and 90% while the other two treatments, based on balances, applied irrigations after a 700- or 500-g loss in the daily-adjusted slab weight (LDASW). Initially, we noticed a heterogeneous EC build-up in the slabs across various treatments, which probably distorted the expression of treatment effects (if any) on plant growth, yield, and water use. To minimize this problem, an EC control strategy of applying extra irrigation was devised and adopted in two sequential phases: 1) application of a 30-minute-long extra irrigation twice a week (for 7 weeks); and 2) extra irrigation(s) using the irrigation control routine of an Argus computer when the slab EC was ≈3.5 mS/cm (for 5 weeks). Slab EC was well controlled in both these phases and we observed significant treatment effects on root growth and marketable yield. Analyzing the results, we concluded that irrigating at 70% to 80% SWC was best for maximum root growth as well as marketable yield. The two irrigation treatments based on the 700- and 500-g LDASW were well maintained and performed equally well, producing marketable yields comparable to those produced by irrigating at 70% and 80% SWC.

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Experiments were conducted to determine the optimum levels of N and P for use in greenhouse cucumber (Cucumis sativus L. `Vetomil') production. Plants were grown in rockwool slabs using a double-stem pruning method. Treatment 1 plants were fed 90 ppm N until N in the growing slabs was depleted (averaged <10 ppm); N was then increased to 175 ppm. Treatment 2 and 3 plants were given a constant 175 or 225 ppm N, respectively. Plants in all treatments depleted N in the slabs by three to four weeks after transplant (WAT); N remained low in Treatment 1, but recovered to adequate levels in Treatments 2 and 3. Phosphorus was provided at a constant 50 ppm and was depleted to <10 ppm in the slabs of all three treatments by four WAT. Fruit yield increased significantly with each increase in solution N. Similar results in a second trial indicated that N and/or P may have been limiting factors even at the highest levels tested. Research will continue to determine optimum levels of N and P for maximizing yield.

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Four different granulated rockwool (RW) aggregates were combined with peat at 15, 30, or 45 percent (v/v) RW resulting in twelve different peat:RW media. The RW aggregates used were either fine or coarse textured and absorbent or repellent to water. A soil based medium was used as a control. Bulk volume, bulk density, total porosity, water porosity (WP), and aeration porosity (AP) were determined for all media. Hybrid lily, cvs. `Enchantment' and `Jamboree', growth in these media were compared by measuring the dates of visible bud and anthesis, flower number, leaf number and area, plant height and dry weight of stems, leaves, and flowers. Physical properties of the RW media varied significantly from the soil based medium. Increasing the volume percent RW had a negative linear effect on WP but a positive linear effect on AP for all RW aggregates. Lily growth in the soil based medium was statistically similar to all RW media. The dates of visible bud and anthesis, as well as leaf number and area decreased linearly as the volume percent repellent coarse RW increased.

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