Chapter Cropping in rockwool and other inert media

Rockwool is a fibrous material produced from a granite-like rock known as diabase, or basalt. During manufacture the minerals are melted at a temperature of around 1600°C and transformed into fibers bonded together with resins. Initially manufactured for the building trade as an insulator, this spongy material has recently become available in cubes or slabs, to which a wetting agent is added to make it water-absorbent for horticultural use. Other inert products that have been used as growing substrates, singly or in combination, include perlite, vermiculite, polyurethane (Oasis), and polystyrene beads. Vermiculite cannot be considered entirely inert as it contains some potassium and magnesium, which gradually become available to plants when it breaks down. All these inert materials are manufactured in a way similar to rockwool, and they all share physical and chemical characteristics. They are all sterile (free of pathogens and weed seeds), and they offer a low cation-exchange capacity (vermiculite is an exception 1 and a high water-holding capacity They permit adequate root aeration and a high degree of control over watering and feeding. Furthermore, because of their light weight, they are easy to handle, making the interval between crops shorter than usual. Finally, they are conducive to energy savings, first because they eliminate soil steaming and second because their use makes root heating practical, which allows for more precise control of air temperature on the basis of minimum temperature requirements of the shoots rather than the roots.

Rockwool is by far the most important inert medium because of the extent to which it is used commercially around the world and because of the wealth of information available from experienced growers and plant scientists (Plate 3). However, with proper management, all the media mentioned have similar yield potential. Since most of the technology in inert media used in the production of greenhouse tomatoes is similar, a detailed management procedure on rockwool, with references to other media where necessary, is applicable to all the media.

Horticultural-grade rockwool is manufactured in several countries (UK, Denmark, Holland, Germany, France, United States, and recently, Canada) under various trademarks (e.g., Basalan, Capogro, Grodan, Pargro). Although the chemical composition of rockwool varies with the manufacturer, the ingredients making up the fibers are not available to the plants, and so all nutrients must be added regularly to the crop in a liquid feed.

Rockwool is available in the form of slabs, blocks, or granules. The slabs are rectangular pieces of varying lengths and widths, but they are usually 7.5 cm deep for raising long-season crops such as tomatoes, cucumbers, peppers, and eggplant, among others. Typical dimensions are 90 x 15 x 7.5, 90 x 20 x 7.5, and 90 x 30 x 7.5 cm. The blocks are also available in a variety of sizes and are used for seed germination and transplant raising. The granular form is used for soil or for a soilless mix amendment and can also be used in bags as a partial or complete substitute for peat.

Oasis is also available in block form, for transplant raising, and in granular form, for making bags; it is not readily available as a slab. An important difference between the two media is their pH. The pH of new rockwool is about 7.0-8.5, which must be corrected with a slightly acid fertilizer solution to about 5.5 before use. The pH of new Oasis is on the acid side, and a slightly alkaline solution, usually potassium bicarbonate, has to be added before use. In both cases the exact concentration of acid or base required can be determined by trial and error tests on a small scale, or the necessary information can be obtained from the manufacturer. The lack of a significant cation exchange capacity in inert media makes adjusting their pH simple and inexpensive, because low amounts of chemicals are required. Before the crop is started the growth medium should be watered thoroughly, to allow for pH adjustment, to fill the capillary tubes, and to ensure that the irrigation water added later will spread uniformly in the growth medium. Approximately 0.8 L of water should be added to each litre of rockwool to ensure complete saturation, which indicates its high water-holding capacity (80%) combined with adequate aeration (17%), even when fully wet.

A crop should be propagated and grown in the same type of medium, thus ensuring that the capillary connections between the transplant pot and cropping media are established quickly at transplanting and that no excessive drying out or water saturation occurs around the stems. Rockwool blocks are available in many sizes, but for tomato propagation the most commonly used size is the 7.5cm cube, individually wrapped in polyethylene to prevent excessive drying out. Raise seedlings in vermiculite or perlite and then prick them out into rockwool blocks with a cavity at the top. Alternatively, raise seedling by placing individual seeds into very small rockwool blocks (plugs), specially made to fit into the cavity of the transplant blocks, and cover them with fine vermiculite. Before using the rockwool blocks, place them on polyethylene and wet them with an acidic nutrient solution, to adjust the pH. After pricking out the seedlings, apply nutrient solution at each watering. Some form of bottom heat to raise the substrate temperature to 22-24 °C is beneficial and always holds some promise for energy savings.

The nutrient composition of the fertilizer solution used in rockwool culture depends on the chemical composition of the existing irrigation water, the stage of plant growth, and the season. Once the original water has been analyzed, fertilizer and acid application rates can be calculated on the basis of a most desirable nutrient formula as determined by research and experience. The quality of the irrigation water is more important in rockwool culture than in soil and should always be considered when establishing a feeding program. For example, if the water contains a large amount of calcium or magnesium the rates of calcium nitrate and magnesium sulfate should be reduced accordingly, and the nitrogen lost in these adjustments should be made up by increasing the amount of another nitrogen-containing fertilizer. Other nutrients, such as potassium and nitrogen, are rarely present in significant quantities to necessitate an adjustment to the nutrient formula. The water supply sometimes contains a large amount of certain trace elements such as iron, zinc, and manganese, in which case some correction to the fertilizer feed is needed. Avoid saline water that contains more than 50 ppm sodium or 70 ppm chloride; when the concentration of these two ions reaches 100 and 140 ppm, respectively, the water cannot be used easily in rockwool culture. When using rainwater, raise the usually low level of bicarbonate by adding potassium bicarbonate to the final solution, not to the stock solutions, to increase the buffering capacity of the solution for a more stable pH in the rockwool slabs. On the other hand, when the bicarbonate in the water supply is higher than 60 ppm, add phosphoric or nitric acid (or both) to neutralize it. For a proper solution to these special problems, seek a second opinion, preferably that of an experienced grower or a horticultural adviser. Although rockwool systems may be used with either recirculating or nonrecirculating nutrient solutions, the use of a nonrecirculating system is preferred because of its simplicity and dependability. However, even a nonrecirculating, open-ended system has to be checked and repaired regularly, and the pH and electrical conductivity (EC) of the solution checked daily, given the inert nature of the substrate and the quick response of the crop to human error and mechanical failure.

Once transplant raising is complete, stand the plants on the rockwool slabs, through precut holes on the plastic liner, and ensure good contact between the propagation blocks and the slabs. Place one or more drippers in the irrigation system on each propagation block. It might also be advisable to stand the transplants on the rockwool slabs for several days before cutting holes in the plastic liner. This procedure limits root growth within the transplant block and slows down growth by holding back water at the early part of the spring crop, when light is limited. After the plants have established a good root system in the slabs, make slits for drainage on the sides of the plastic wrapping near the bottom of the slabs. The distance of the slits from the bottom of the rockwool slabs determines thevolume of nutrient solution on reserve and plays a major role in establishing the specifications of the irrigation system and the irrigation regime: the lower the slits the smaller the size of the nutrient solution reservoir in each slab and the more frequent the irrigation needed; however, the lower the location of the slits the less the volume of saturated rockwool and therefore the greater the efficiency in using the rockwool as a rooting substrate, which theoretically should result in higher productivity. An inexperienced rockwool grower with a drip irrigation system of modest performance is well advised to start with cutting drainage holes at some distance (1-3 cm) from the bottom of the slabs. Once the grower has gained experience he/she could progressively extend the slits downward to maximize the utilization of the available rockwool.

The rows of rockwool slabs should be as level as possible and should stand on boards of polysterene, which help level off small imperfections in the soil surface. The boards also form part of a substrate heating system, which is based on the circulation of warm water in polyethylene tubing set into grooves carved in the polysterene. In addition, the polysterene boards act as insulators between the warm rockwool slabs and the cold soil underneath.

Make at least two stock solutions from readily available fertilizers, to avoid precipitation in the concentrate storage tank that results when calcium- or magnesium-containing fertilizers are mixed with those containing sulfates or phosphates. Dilute the stock solutions and combine them in a mixing tank before applying them to the crop, thus providing a complete nutrient solution at every watering. Monitor the total concentration of nutrients in the irrigation water continually by a salt sensor (electrical conductivity cell) and, if necessary adjust the mixing ratio of the fertilizer diluter to achieve an optimum feeding strength for the crop; automatic adjustment is technically available. Similarly, monitor, with a pH meter, the pH of the irrigation water after adding all fertilizers and any acid,. Adjust the rate of acid application to achieve a desirable pH

for the nutrient solution; automatic adjustment is technically available. Many alternative feed recipes can be used, depending on the cultivar grown, the water source, the stage of crop growth, and the season. The fertigation recommendations in Table 19 are based on using rainwater and should be treated only as a starting point in the search for finding the optimum for a given operation.

Daily checks of the pH and salt concentration of the slab solution are necessary and fortnightly analysis for all essential nutrients is highly recommended. Correct serious nutrient imbalances by making appropriate changes to the nutrient formula, but the changes should deviate as little as possible from the normal solution. The alterations should be double-checked by an experienced person and should be implemented only until the imbalance is corrected. Changes in the nutrient formula based on crop growth and appearance are also possible, but such changes should be made only by persons with experience in rockwool culture. To reduce costs, use rockwool slabs for more than one season, provided they are effectively steam-sterilized in between crops. Thoroughly flush out accumulated salts with plain water for 1 or 2 h before sterilization. Methyl bromide can also be used to sterilize rockwool between crops, but steaming is more effective over a greater variety of pathogens and is preferred when available. After the slabs are sterilized, rewrap them with polyethylene film so that they are ready again for use. Reused slabs do not require further pH adjustment and are easier to rewet than new ones. Rockwool slabs can be reused only a limited number of times, usually once, as some breakdown in the fiber structure occurs with handling and sterilization, and as a result the air pore space in the slabs decreases with every reuse. An interesting alternative to the reuse of rockwool slabs for reducing production cost is the recent introducion of a low-density, low-cost rockwool slab that is used for a single cropping season.

Other nearly inert media such as sand and gravel have also been used as growing substrates for greenhouse vegetables. Considerable information on sand and gravel culture is available, mostly from other countries; on occasion, Canadian growers have experimented with them. However, these media are heavy, difficult to handle, difficult to sterilize in between crops, and usually require extensive, permanent modifications to the greenhouse floor. Like most other media, sand and gravel have an equally high yield potential when managed properly and can be the best choice under certain conditions.

Table 19 Fertigation schedule for tomato production in rockwool (in kilograms of fertilizer per 1000 L of stock solution)

Stock A

Stock B

Recommended irrigation

Application time

Calcium nitrate

Potassium nitrate

Monopotassium phosphate

Potassium sulfate

Magnesium sulfate

L/day

Saturation of slabs 130.0 0

For 4-6 weeks 100.0 64.0 after planting

Normal feed 100.0 35.0

Heavy fruit load 100.0 37.0

2500

gradually increase to 3500

1800-2300

1800-2500

. Traceelements (e.g. 0.7kgofSTEM)and0.5kgofironchelate(13%iron) must also be added to all the above fertilizer feeds; a typical trace element mix, e.g. Peters soluble trace element, mix (STEM) contains 1.45% boron, 3.2% copper, 7.5% iron, 8.15% manganese, 0.046% molybdenum, and 4.5% zinc. . Dissolve given amount of each fertilizer, including trace elements, in 1000 L of water and add to the irrigation water in equal doses, ideally with a multihead fertilizer injector. Start injection at a very low rate and increase progressively until the desired EC is achieved. Adjust, the pH of the fertigation solution to 5.5 by injecting a dilute solution of phosphoric, nitric, or sulfuric acid. . The recommended strength of the stock solutions is within the working range of a fertilizer injector with a 1:100 mixing ratio. Tf a fertilizer injector with a 1:200 mixing ratio is used, double the amount of each fertilizer. Similar adjustments can be made for fertilizer injectors with other mixing ratios. In case the solubility limit of a fertilizer (e.g., potassium sulfate) is exceeded, more than one stock solution of the same fertilizer can be prepared and the amount of the fertilizer divided equally between the stocks. Note: When a stock solution is mixed at a concentration of 1 part A, 1 part B, and 98 parts water, it supplies all essential nutrients to the crop at the following concentrations, in parts per million.

N

P

K

Ca

Mg

Fe

Mn

Zn

Cu

B

Mo

Saturation of slabs

201

62

253

247

36

0.8

0.57

0.32

0.22

0.1

0.005

For 4-6 weeks after planting

238

62

370

190

36

0.8

0.57

0.32

0.22

0.1

0.005

Normal feed

200

62

370

190

36

0.8

0.57

0.32

0.22

0.1

0.005

Heavy fruit load

203

62

390

190

36

0.8

0.57

0.32

0.22

0.1

0.005

How To Can Tangy Tomatoes

How To Can Tangy Tomatoes

Interested In Canning Juicy Tomatoes? Here's How You Can Prepare Canned Tomatoes At Home. A Comprehensive Guide On Tomato Canning. The process of canning tomatoes at home has been a family tradition with many generations. Making home canned or home tinned tomatoes is something that is remembered by families for years! You must have surely seen your granny canning tomatoes at home in order to prepare for the approaching winters. In winters, one is usually unsure of getting fresh tomatoes.

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