Greenhouse Tomato Handbook
Contents
Greenhouse tomato production has attracted much attention in recent years, partly because of a new wave of interest in “alternative crops.” The attraction is based on the perception that greenhouse tomatoes may be more profitable than the more conventional agronomic or horticultural crops. The popularity may also be due to misconceptions about how easily this crop can be grown. While the value of greenhouse tomatoes is high on a per unit basis, the costs are also high. The following points are outlined to clear up misconceptions you might have. Keep these in mind before pursuing greenhouse tomatoes, either as a livelihood or as a crop for supplemental income:
These comments are not meant to discourage prospective growers. If you are preparing to invest time and money into growing greenhouse tomatoes, however, you should be fully aware of the pitfalls as well as the benefits before proceeding any further. If you are willing to spend the necessary time to learn how to grow this crop, you can be successful if you follow the basic guidelines in this and other publications. Plan for SuccessThe best way to learn is not by your own mistakes, but by other people’s mistakes. Visit as many other greenhouse tomato growers as possible and ask questions. Most growers are happy to share information.
Plant PopulationWhen growing greenhouse tomatoes, it is important to use the proper planting density. Greenhouse tomatoes need at least four square feet per plant or 10,000 plants per acre. In fact, recent research at the Truck Crops Branch Experiment Station shows that using a planting density of 5 sq. ft/plant produced the same per unit area, while reducing the plant population. To determine how many plants can be grown in your greenhouse, multiply the length by the width and then divide by four or five. For a 24- by 96-foot greenhouse, about 460 to 576 plants can be grown; for a 30- by 96-foot greenhouse, 576 to 720 plants will fit, depending on planting density. Note: If you will use some of the floor space for other purposes (for example, storage, packing, grading), subtract this area from the total before dividing by four or five. Using a higher planting density will cause the yield per plant to decrease, while the yield per greenhouse will stay about the same. This is due primarily to plants shading each other. The costs and the amount of labor required, however, increase with more plants. Also, crowding plants tends to promote disease development, since foliage does not dry as readily, and sprays cannot easily penetrate the thick foliage. Arrange plants in double rows, about 4 feet apart on center. Within a row, plants will average 14 to 16 inches between stems. VarietiesThe first step in raising any crop is to choose the best variety. Growing a variety that is not the best choice, or using seed that are not of the best quality, reduces your potential for success at the outset. It is smart to start off with the greatest potential rather than limiting yourself by using inferior seed, even if it saves a few dollars. Hybrid tomato seed is expensive. It now costs 10 to 30 cents per seed, depending on the variety and quantity that you buy. This cost reflects the laborious process of hand pollination required to produce the hybrid seed. Although this seems rather expensive, it is still one of the lowest costs of production. After the heating, labor, and fertilizer costs are incurred, the extra expense of using the finest seed is relatively small. There are thousands of tomato varieties available on the market, but only a few are acceptable for greenhouse production. If you plan to grow tomatoes in a greenhouse, you need to use a greenhouse variety. These are almost exclusively Dutch hybrid indeterminate varieties, bred in Holland specifically for greenhouse production. Field varieties are typically adapted to higher light and lower humidity conditions and probably would not yield well in the greenhouse. A glass or plastic greenhouse has about 20 percent less light than outdoors, and many field types do not tolerate this reduction. There are many companies in Holland and other European countries that deal in greenhouse varieties; however, only a few have distributors in the United States. You can buy seed from a greenhouse supply catalog, or directly from the seed companies, which are shown in the list of suppliers at the end of this publication. Base variety selection on these criteria:
In Mississippi, as in most of the United States, the market preference is for a red tomato. In Ohio and southern Canada (Leamington, Ontario), the preference is for pink tomatoes. The only physical difference is in the skin color. There are no flavor or biochemical differences. The varieties most worth considering at the time of this printing are Trust, Match, Switch, and Blitz. Tropic cannot be recommended to commercial growers because of its lack of size uniformity, intolerance to high nitrogen fertilizer, and lack of resistance to Tobacco Mosaic Virus (TMV) and other diseases; however, Tropic is fine for a hobby greenhouse. Jumbo may be the largest fruited variety available, but it lacks resistance to TMV and most other diseases, is not tolerant to higher levels of nitrogen fertilizer, and is not as uniform in size as some of the other varieties. Serious growers should not use outdoor varieties such as Celebrity, Better Boy, Travellers, etc., in the greenhouse, although these are fine for the garden. You can buy seed by the piece with lower costs per unit for larger quantities and higher costs for smaller quantities. Tomato seeds are very small; one-fifth of an ounce contains about 1,200 seeds. If you have a two-bay greenhouse (4,500 square feet) with about 550 plants per bay, this is enough seed. Always plant a few extra seeds (10 - 20 percent) since germination will not be 100 percent. This also gives you the opportunity to discard any seedlings that do not meet your high quality standards. Store extra seed in unopened containers or in zip-locked bags in the freezer. Table 1. Varieties of greenhouse tomatoes and their characteristics
*
DR = De Ruiter; B = Seminis (Bruinsma/Asgrow); S = Stokes; RZ = Rijk Zwaan;
HZ = Hazera. Table 1 lists some of the common greenhouse tomato varieties and some of their characteristics. Note that only some of the varieties have resistance to Fusarium Crown and Root Rot (FR). This disease has been in Mississippi greenhouses since 1990. The variety Trend is not recommended for Mississippi greenhouses. Growing in Aggregate MediaMany types of growing systems for greenhouse tomatoes are available. These systems include NFT (nutrient film technique), PVC pipes, sand, ground culture (in the soil), troughs, rock wool slabs, and various types of aggregate media. This latter group includes peat moss and peat-lite mixes, perlite, rock wool aggregate, glass wool, pine bark, and many others. Most of the multi-acre greenhouse ranges in the U.S. use rock wool. This inert, highly porous material is made by melting volcanic rock, limestone, and coke at 292°F and spinning it into fibers. In Mississippi, pine bark is the leading growing medium due to its suitable properties, availability, and low cost. For growers using pine bark, it is recommended that the bark be composted fines. Fines are particles less than 3/8 of an inch in diameter. Initial composting is necessary so the bark does not damage roots of tomato transplants from high temperature, and so that there is no nitrogen depletion caused by the decomposition process. If you cannot determine the age of bark, purchase the bark at least 3 months before it is to be used; this allows time for composting at your site. The recommended volume of aggregate medium is 1/2 cubic foot per plant. With pine bark, this is easily achieved by using 2 cubic-foot bags and transplanting three or four plants per bag. You can buy these pine-bark filled, perforated, polyethylene bags from Mississippi suppliers (see list at end of this publication). Alternatively, two plants can be grown in a 7 1/2-gallon bag or bucket, or you can grow one plant in a 3- or 4-gallon container (1 cubic foot equals 7 1/2 gallons). Planting ScheduleThere are two principal cropping systems for growing greenhouse tomatoes: two crops per year and one crop per year. With the one-crop system, set plants in mid-September or later and grow until mid- to late June. For Mississippi, the two-crop system, with a spring crop and a fall crop, is preferred. This is because plants held over the winter are more likely to be infected with diseases such as Botrytis (gray mold) and Cladosporium (leaf mold) thatthrive in damp greenhouse conditions during December, January, and February. Plants that have been growing for several months have dense growth by winter, reducing airflow and aggravating humidity problems. These conditions are favorable to disease development. Also, plants that have been growing since September do not have the same vigor as young plants transplanted in January. Alternatively, one, shorter crop can be grown just in the spring, or through the late winter and spring. Marketing conditions should determine the cropping season. July and August are too hot to have producing tomato plants in greenhouses in the Midsouth. Summer fruits are plagued with splitting, cracking, blotchy ripening, and generally poor quality and yield. In addition, the market price is down because field tomatoes are available. For the two-crop system, seed the fall crop after the middle of July in trays, with one seed per cell. If you plant earlier, young plants will be subject to high summer temperatures and will be off to a poor start. Allow plants to grow for 4 1/2 to 5 weeks. Transplant during mid-August into bags, buckets, or rock wool. The first harvest will be late October or early November. Seed the spring crop in late November, transplanting this crop soon after the first of January, when transplants are about 6 weeks old. This means you will have young seedlings at the same time you have mature plants. It is preferable to have a separate propagation house for new seedlings. If seedlings are grown in the same greenhouse with mature plants, it is likely that insects (whiteflies, leafminers, etc.) and diseases will contaminate the new crop. The first harvest should occur in March. The spring crop can be grown until late June, or until the temperature becomes too extreme for quality production (see section on Greenhouse Cooling). Top the plants 6 weeks before the intended termination date. Termination should be close to the end of December for a fall crop and mid- to late June for a spring crop. When topping, remove the tops of plants from marble-sized fruit and higher. Such small fruit will not have time to mature before termination of the crop. Leave one or two leaves above the highest cluster of fruit that remains. This helps shade the fruit and prevents sun scald. Pruning and TrainingFor best production, prune tomato plants to a single stem by removing all lateral shoots, commonly referred to as “suckers.” One sucker will form at the point where each leaf originates from the main stem, just above the leaf petiole (stem). Allowing all suckers to grow and bear fruit would increase the total number of fruit, but they would be small and of poor quality. It is better to have one main stem that bears fruit, as this will produce larger, more uniform, and higher quality fruit. Removing suckers once per week will keep them under control. It is advisable to leave one or two of the smallest suckers at the top of the plant. Then, if the plant becomes damaged and the terminal breaks off, one of these suckers can be allowed to grow and become the new terminal. Generally, remove any sucker longer than one inch. Rig a support system of wires above the crop. Use 9-gauge or 3/32-inch galvanized wire or stronger (a 100-pound box contains about 1,700 linear feet). These wires should run parallel to the direction of the rows and 7 feet off the ground. Cut strings 14 feet long, so there is enough slack to allow the plant to be leaned and dropped when it reaches the height of the wire. Tie one string loosely (not a slipknot) or clip it to the bottom of each plant, throw it over the wire, and then tie it to the wire with a slip knot. Plastic clips are commercially available (see Appendix 1 for list of suppliers). Note: Be certain the greenhouse frame can support the weight of fruit-laden plants, or that a sturdy frame is constructed at the ends of rows to support the wires. Plan on the wire bearing a load of 15 pounds per linear foot, or 3 tons for 600 plants. When the plant reaches the wire height, it should be leaned and dropped. Hold the string securely with your left hand just above the plant; loosen the knot with your right hand. Simultaneously let the plant down about 2 feet below the wire and slide the string to the right. The plant must be leaned over while it is dropped; otherwise the stem will probably break. Always lean in the same direction. Do not lean some to the right and some to left or they will shade each other. Lower all plants to the same height so they don’t shade each other. Repeat this operation each time plants grow higher than the wire. This is another reason the two-crop-per-year system is preferable&endash;there is less labor needed for leaning and dropping the plants. As you prune the plant to one main stem, wrap it around the support string. You can prune and wrap in one operation, doing both to a plant before moving on to the next plant. Always wrap in the same direction&emdash;if you start clockwise, continue clockwise; otherwise, when the plant gets heavy with fruit, it may slip down the string and break. Some growers prefer to use plastic clips to secure the plant to the string, either in combination with wrapping or to replace wrapping. Cluster pruning will also improve size and uniformity. This involves removing small fruit from some clusters, leaving three, four, or five of the best ones. Remove misshaped or deformed fruit first. Otherwise, remove the smallest fruit, which is usually the last one formed on each cluster. PollinationPollination of the female flower part must occur before fruit will set. Any activity or inactivity that prevents thorough pollination reduces the number of fruit set per plant. Several problems can result from poor pollination: off-shaped fruit if seeds do not develop uniformly throughout the fruit, smaller fruit, and fruit that are rough (ridged) along the tops. Pollination can be prevented by various stresses such as cold or hot temperatures, drought, high humidity, nutrient deficiencies, nutrient toxicities, etc., as well as lack of pollen transfer. Tomato flowers have both male and female parts within every flower. Botanically, these are termed “perfect” flowers. Field tomatoes are pollinated primarily by wind rather than by bees, which pollinate many other types of vegetables. Most of a flower’s pollen fertilizes the ovary within the same flower, although some of the pollen reaches surrounding flowers. Wind shakes the flower so that pollen leaves the anther and travels to the stigma. In the greenhouse, wind is not strong enough to shake the flowers sufficiently to transfer the pollen. Even though the greenhouse is ventilated with fans, on cooler days when the fans are not operating, the air is relatively motionless. The optimum temperature for pollination is within the range 70 to 82 °F. Optimum relative humidity is 70 percent. Above 80 percent relative humidity, pollen grains stick together and are not dispersed well. With relative humidity less than 60 percent for extended periods, the stigma may dry out so that pollen grains will not stick to it. With ideal conditions, fertilization occurs 48 hours after pollination. Serious greenhouse tomato growers should use an electric pollinator to ensure good fruit set. (What is a “serious” grower? &emdash; one who grows tomatoes for a profit.) In a hobby greenhouse, the expense of a pollinator is probably not necessary. You can purchase an electric pollinator from most greenhouse supply distributors. These can be powered with a disposable alkaline battery or with a rechargeable 6-volt battery. Purchasing a rechargeable battery with a charger may be less expensive than continually buying alkaline batteries. Also, you should never be without power. The rechargeable battery can be plugged in after use to be sure it is ready for the next pollination. How much time does it take to pollinate? Vibrate each cluster (not each blossom) for about half a second. Touch the wand to the top side of the pedicel (flower stem). Do not touch individual blossoms, as this will damage them, causing damaged fruit. One acre (10,000 plants) is estimated to take 5 to 6 hours to pollinate. For a 24- by 96-foot greenhouse, this would be about 20 minutes. New growers may take an hour or more until they gain experience. If you have 10,000 square feet or more under one roof, seriously consider using bumblebees for pollination. Purchase your bumblebee hives from commercial suppliers. Use an electric pollinator if you have one or two gutter-connected bays, or more bays that are not under one roof. For ranges between 5,000 and 10,000 square feet under one roof, you need to compare the cost of the bee hives to the cost of labor to achieve pollination. Generally, a hive will last for about 8 weeks before you need to replace it with a new hive. It is a good idea to allow for some overlapped time between the old and new hives. Note: Even if bumble-bees are used, an electric pollinator will still be needed to pollinate the first few flowers that open. Hives should be introduced when 50 percent of plants have open blooms. Questions About PollinationHow often should
I pollinate? Does the time
of day matter? How do I know if pollination is actually taking place? You can see it happen. If the air is relatively dry and the light is good, you can see a small cloud of yellow powder around the flower when the cluster is vibrated. These are the pollen grains. This is your insurance that conditions are excellent for pollination. How does cloudy
weather affect pollination? What if it is
cloudy for a long period of time? Is the expense
of an electric pollinator really necessary? TemperatureA day temperature of 70 to 82 °F is optimum, while night temperature of 62 to 64 °F is optimum for greenhouse tomatoes. During cloudy weather, a temperature closer to the lower end of these ranges is preferred, while in sunny weather, temperatures closer to the higher end are better. Below 60 °F, nutrient deficiencies may occur because plants cannot absorb some elements at cool temperatures. The first sign of cool temperature stress is purpling of the leaves, indicating lack of phosphorus uptake (even though there may be adequate phosphorus in the nutrient solution). One or two nights of 56 or 58 °F temperature can cause a considerable number of rough fruit several weeks later when fruit exposed to the cold temperature reach full size. You should maintain a minimum temperature of 64 °F. Ideally, the thermostat should be located at blossom height rather than above the tops of plants. Avoid temperatures over 90 °F if at all possible. At 86 °F, lycopene (the pigment that makes tomatoes red) no longer develops. See the section Greenhouse Cooling for help in managing high temperatures. Locate thermostats near the center of the greenhouse for good temperature control. Also, enclose the thermostat in an aspirated box, or shade it so that it indicates the air temperature correctly. If the sun is allowed to shine directly on the thermostat, it will read a higher temperature than the air surrounding it. Relative HumidityThe optimum relative humidity for greenhouse tomatoes is 60-70 percent. This is very difficult to obtain in Mississippi greenhouses and is given for your information only. Greenhouse CoolingGreenhouse tomato growers in Mississippi have to deal with high temperatures. When greenhouse temperatures go into the high nineties or even higher, poor-quality fruit, especially fruit splitting, is the usual result. The splitting in the skin is caused by the surface of the fruit reaching a higher temperature than the layer just beneath the surface. This causes a rupturing between adjacent cells, showing up as a split or burst skin. The mechanism is similar to that of the bimetallic coil in a thermostat, except that the thermostat coil can tighten or loosen as the temperature changes, while the tomato skin cannot; therefore, the skin tears. When the sun shines through the plastic greenhouse covering onto the plants, it is common for leaf temperature to be 30 degrees higher than the air temperature in the greenhouse. As leaf temperature increases, leaves lose more water to the atmosphere and make up for this loss by taking up more water through the roots. This is how the plants keep from overheating. As long as there is ample water surrounding the roots, this system works fine, up to a point. If there is ever a water shortage, plants wilt at the tops, reducing the water loss. If the water loss is very severe, plants wilt completely from top to bottom. Plants revive from a mild wilt condition when water becomes available or the stress situation is removed (as it is at night). But, if plants reach the “permanent wilting point,” tissue is permanently damaged and they will not recover, even with plenty of water. Also, if the temperature climbs into the upper nineties, plants may not be able to retrieve enough water through their roots to make up for the rapid loss from their leaves, even if there is plenty available in the growing medium. At this point, leaves become burned or scorched. They look as if they have been touched with the flame of a propane torch. Remember &emdash;so long as the sun shines, the leaf temperature is considerably higher than the air temperature. There are several ways to lower the air and plant temperature in the greenhouse. None are 100 percent successful by themselves in this climate because of the extreme heat in Mississippi. However, any one of these methods will lower the temperature to some extent. All of the following methods are used as additional cooling after exhaust fans have already been installed:
Other cooling techniques:
IrrigationExcept in small hobby greenhouses, irrigation should be controlled automatically, with the use of time clocks or electronic controllers. The volume of water will vary depending on the season and the size of plants. New transplants need only about 2 ounces (50 ml) per plant per day. At maturity on sunny days, however, plants may need up to 3 quarts (2.7 liters or 2,700 ml) of water per plant per day. Generally, 2 quarts per plant per day are adequate for fully grown or almost fully grown plants. Monitor plants closely, especially for the first couple of weeks following transplanting, so that the volume of water can be increased as needed. Water should be delivered to each plant. This is usually done with “spaghetti tubing” and emitters that carry water from main lines to the base of each plant. Each watering should include fertilizer (see section on Fertility); therefore, the process is more appropriately referred to as “fertigation.” Most growers use from 6 to 12 waterings per day once plants are established. In a medium that drains extremely well, such as rice hulls, 12 or more waterings per day may be needed to keep plants from drying out between waterings. In pine bark, 6 to 12 waterings per day are usually adequate. The important point is that plants should receive enough water so they do not wilt. A wilting plant is not growing. If the permanent wilting point is reached because of a prolonged period without water, the growing point may be killed. To be certain that plants receive enough water, allow enough so some drainage from the bags (10-20 percent) is apparent after each watering. Be aware that a prolonged cloudy period followed by bright sun may cause severe wilting. Be ready to increase the amount of water in this situation. pHIt is a good idea to check the nutrient solution pH daily with a pH meter. At a minimum, check pH every time you prepare fertilizer solution. A “pocket” pH meter is a tool every grower should have. The optimum pH range for the nutrient solution is 5.6 to 5.8. If the pH of the solution is too high, caused by alka- line water, add an acid in small quantities to lower the pH to within this range. The choices of materials to use are sulfuric acid (H2SO4), nitric acid (HNO3), or phosphoric acid (H3PO4). Sulfuric acid, the least expensive, can be purchased from an auto supply store as battery acid. However, there is an advantage to using phosphoric or nitric acid, since they supply nutrients in addition to lowering the pH. (Phosphoric acid supplies phosphorus and nitric acid supplies nitrogen.) Although they are more expensive than sulfuric acid, they may be preferred for this reason. Phosphoric acid and nitric acid are inexpensive sources of these elements when compared to other fertilizers. Handle all acids very carefully because they can cause injury. Nitric acid can be very hazardous. Phosphoric acid lowers pH more than the other two for a given volume. It is advisable to use these acids directly from the drum or container in which they are purchased so there is no risk involved while pouring them. Never use vinegar (acetic acid) or swimming pool acid (sodium bisulfate) to lower the pH. To determine how much acid to add to a bulk or concentrate tank of nutrient solution, take 1 gallon of solution and add 1 ml of acid at a time until the pH of the nutrient solution is within the range stated. Then, multiply the amount added to 1 gallon times the number of gallons in the tank. If you are using a bulk tank, this is the amount of acid to add. However, if you are using an injector system, multiply this amount by (times) the ratio. Be aware that the pH scale is not linear; it is logarithmic. For example, if 10 drops lower the pH from 8.0 to 7.5, there is no reason to believe that 20 drops will lower the pH to 7.0. You may reach the point where one more drop lowers the pH to 5.0 or lower. So, be careful! A pH that is too low can be very damaging to the crop. Check and double check the pH after adjusting it with acid. If the pH is too low (below 5.5), you can bring it up by using materials such as sodium carbonate, caustic soda, potassium bicarbonate, potassium hydroxide, or caustic potash. The recommended material is potassium carbonate, which has a pH of 8.2. This will also supply potassium. Avoid the sodium sources since plants do not need sodium. If the irrigation system, or emitters, become clogged during the production of a crop, they can be cleaned out after the cropping season is over. Use an acid mixed with water to adjust to a pH of 4.5. While this acidic mixture is running through the system, tap emitters to break up any crust that has formed. Do not do this while tomato plants are growing in the greenhouse&emdash;a pH this low may kill plants! FertilityThe subject of fertility is probably among the most confusing for growers of greenhouse tomatoes; however, it is important to production. The keys to a successful nutrition program include the following:
Fertilizer Measurement and VocabularySeveral units are used to express the fertility level of nutrient solutions (fertilizer dissolved in water). This causes confusion among growers since the use of different units makes it difficult to understand different readings among growers. This section explains the different units. Electrical conductivity (EC) is a measure of the ability of a solution to conduct electricity&emdash;the more concentrated the fertilizer solution, the more electricity it will conduct and the higher the reading will be. The general unit is mho (pronounced MO) with the plural being mhos (pronounced MOZE). You will notice that mho spelled backwards is ohm (pronounced OM), the unit of resistance in electrical jargon. Mhos, the reverse of ohms, is a measure of conductivity rather than resistance. There are two units of mhos commonly used: micro-mhos (µmhos) (pronounced micro-MOZE) and millimhos (mmhos) (pronounced milli-MOZE). A micromho is one millionth of a mho and a millimho is one thousandth of a mho. Therefore, there are 1,000 micromhos in a millimho. Another way of looking at it is that a millimho is 1,000 times bigger than a micromho. Either scale can be used. Convert from micromhos to millimhos by sliding the decimal point 3 places to the left, and vice versa. Typical readings of millimhos are 0.30 to 2.50, while typical readings of micromhos are 300 to 2,500. Millimhos are more commonly used than micromhos on most meters today. Some portable EC meters measure in microsemens (µs). These are equivalent to micromhos (µmhos), and are more commonly used in European countries. The best way to understand the nutrient status of a fertilizer solution and to communicate with other people is to know how many parts per million of each element you are applying. Parts per million (ppm) is the unit used to measure nitrogen concentration, or any other specific nutrient in solution. These units are usually within the range of 50 to 300 ppm for nitrogen. For mature, producing plants, 125 to 200 ppm nitrogen (N) is needed, depending on the particular circumstances. This is not directly related to or convertible to an exact measure of EC or total dissolved solids (TDS) in a nutrient solution (see below). This is because both EC and total dissolved solids are measurements of everything dissolved in the solution, not just nitrogen. Another way of measuring the amount of fertilizer in solution is by measuring dissolved solids. This is also referred to as total dissolved solids or TDS. The units commonly used for TDS are also parts per million (ppm). If you knew the ppm of each element dissolved in the solution, and added them up, along with the ppm of the water, you would get the ppm TDS. This is a measure of all salts in solution, not just nitrogen, so it is not the same as measuring ppm of nitrogen. Some of these salts may have been in the water supply before any fertilizer was added. For this reason, this form of measurement is not recommended. If a reading is 1,500 ppm TDS, how do you know if this is due to nitrogen or some other nutrient? You don’t. You even may have water very high in sodium (salt) with no nitrogen. TDS is not a reliable measurement for this reason. The readings of ppm dissolved solids are not directly convertible to millimhos or micromhos for a fertilizer; however, the conversions can be calculated for specific fertilizers. A rule of thumb (very crude conversion) is if your millimho reading is in the range 0.9 to 1.9, then mmhos x 680 = ppm total dissolved solids. If your millimho reading is in the range 2.0 to 2.8, then mmhos x 700 = ppm total dissolved solids. Remember&endash;this is only a rule of thumb and does not give an exact conversion. An important point: Any time dissolved solids or EC are measured in a solution, it is very important to know the dissolved solids or EC of the water source used to make the solution (it cannot be assumed to be 0). There may be sodium or some other dissolved element in your tap water that can lead to false readings when you measure your nutrient solution. Subtract the water source EC or dissolved solids measurement from that of the nutrient solution to find the true value caused by fertilizer. This is the number to compare to charts to decide if the correct amount of fertilizer is in solution. Table 2. General guidelines for amount of fertilizer to use
Methods of Mixing FertilizersThere are two principal systems for mixing fertilizers: the bulk tank system and the injector or proportioner system. Both methods are acceptable and can produce high yields and excellent quality tomatoes. Bulk TanksThe bulk tank system consists of a tank (plastic, concrete, steel, PVC, etc.) of appropriate size depending on the square footage of the greenhouse(s). A 100-gallon tank is fine for one greenhouse, whereas a 1,000 to 2,000 gallon tank may be preferable for several greenhouses. The larger the tank, the less frequently it will have to be filled. But if the tank is too large, you will have to wait too long until it is empty to mix up a new batch of fertilizer of a stronger concentration or with some other change in the formula. For a new grower, or a grower with only one or two bays, the bulk tank system is easier to understand and probably causes fewer mistakes, providing that the directions that come with the fertilizer are followed. Mixing fertilizer is a matter of adding so many ounces (or pounds) of dry fertilizer per 100 gallons of water. The fertilizer must be completely dissolved in the water. Any precipitate (settled out fertilizer) will not reach the plants. Therefore, it may be necessary to stir the solution, by hand with a “paddle” or with an electric mixer, or use a circulation pump. Be sure to check the pH and EC of the solution each time you mix a new batch. InjectorsWith the injector system, a concentrated mixture of fertilizer solution is diluted with the injector (proportioner) to the final concentration required by the plants. The simplest and least expensive type of injector is a hozon proportioner, often used for fertilizing lawns and garden plants, but it is not appropriate for commercial production. The most complicated and expensive is the Anderson Injector; there are many intermediate models in cost and complexity. Generally, the more you spend, the more accurate the injector is. Inexpensive models will vary their injection ratio depending on water pressure, which is often variable. This is because the injection is timed rather than depending on volume of water. Better models are dose specific, meaning that the concentrate injected depends on a given volume of water passing through the device. Equally important, the higher priced injectors are adjustable. A knob or dial on the head can be turned to increase or decrease the dose of fertilizer concentrate injected into the water. The fertilizer solution goes from the concentrate tank to the injector, where it is diluted by being injected into the irrigation system. A water meter monitors the flow of water and then sends out a signal when enough water has passed through. The meter can be mechanical or electrical. This signal from the meter is a pulse of water, which is sent to the pilot valve (skinner valve). This small volume of water is discarded after it passes through the pilot valve. Concentrate is held in small containers (e.g., 10 to 50 gallons). Two heads and two concentrate tanks (at a minimum) are needed: one for the calcium nitrate (tank B) and the other for all other nutrients (tank A). This is necessary so calcium does not combine chemically with phosphates or sulfates when the elements are in high concentration, especially under a high pH. The resulting compounds, calcium hydrogen phosphate or calcium sulfate, are hard precipitates and can clog the injector and irrigation system. However, once these elements are diluted, there is no problem. If the pH is higher than 5.8, it is advisable to use a third head to inject acid. This is needed to keep the pH in the 5.6 - 5.8 range (see pH section above). With an injector system, there is no cost involved for the large tank needed for the bulk tank system. However, a high quality injector can be expensive. You can control the fertility level with an injector system better than with a bulk tank system since you can adjust the dose by simply turning a knob or dial. Furthermore, as better precision in the fertility program is desired (as money permits), you can add more injector heads. Ultimately, a head can be used for each fertilizer element. Individual adjustments can be made based on regular tissue analyses. Injector CalibrationIt is important to know the injection ratio so you can calculate how much fertilizer to mix in the concentrate tanks. Some come with tables that designate this ratio, i.e., 1:9, 1:16, 1:100, 1:200. On certain brands, the ratio is designated as a percent, i.e., 1 percent rather than 1:100. Some injectors let you adjust this ratio by turning a knob or a dial, or by adding rings, while others are fixed at one setting. If you do not know the ratio, it is necessary to calibrate the injector to learn this important number. Using a beaker or graduated cylinder, measure how much water is sucked up by the injector in one minute. Then, using several beakers, one at each of several emitters (that is, ten locations), measure how much water is distributed to plants in one minute. Take an average of the number of beakers in which water is collected in the greenhouse. Multiply this average amount emitted per plant in one minute by the number of emitters in the greenhouse. The injection ratio is the ratio of the output to the input. Divide the total amount emitted in the greenhouse in one minute by the total amount sucked up in one minute. State the ratio as 1:X, where X is the number you get after dividing. Your concentrated solution is diluted X times with water. (There are X parts water for each one part of concentrated fertilizer solution.) Plant ResponseHow do you know what is the right amount of fertilizer? In addition to following the directions on the bag and taking regular foliar analyses, the plant also gives an indication. If tops of plants "ball up" with dense, curling-under growth, the nutrient solution is a little high in nitrogen. Another sign of having nitrogen too high is when the clusters of flowers end in leaves or shoot growth (these should be pruned off). This condition will not necessarily decrease yield unless nitrogen is excessively high. If stem diameter is extremely small and plants are spindly, fertilizer concentration is too low. Other signs include faded or yellowed foliage, decreased vigor, blossoms that don't set fruit, and yield reduction. Other fertilizer deficiency symptoms are discussed in the section Nutrient Deficiency Symptoms. Modified Steiner SolutionOne of the most recognized nutrient solutions is the Modified Steiner Solution. The original Steiner Solution was published by Dr. Abram A. Steiner in an article called "Soilless Culture," in the Proceedings of the 6th Colloquium of the International Potash Institute at Florence, Italy, in 1968. Dr. John Larsen (Professor Emeritus, retired, of Texas A&M University) modified this formula according to his research involving plant requirements and tissue analysis. According to Dr. Larsen, there is no need to exceed a level of 200 ppm nitrogen. The Modified Steiner Solution will supply nutrients as follows:
The suggested use of the Modified Steiner Solution is as follows:
As you can see, the Modified Steiner Solution never calls for more than 171 ppm nitrogen during the life of the crop. Most of the time, the fertility level is much lower. There are many disagreements about the proper level of fertilizer solution to use. Some people feel that the levels in the Modified Steiner Solution are too low for optimum production; however, the majority of researchers agree that these levels and ratios are appropriate to grow greenhouse tomatoes, and, therefore, they are the basis of many of the commercial fertilizers on the market today. Commercial FertilizersVarious commercial fertilizer mixes are available for use in bulk tank systems or with injectors. Some mixes are "complete," while others require the purchase of additional calcium nitrate, potassium nitrate, magnesium sulfate (same as epsom salts), sequestered iron (Fe 330), or some other fertilizer. Follow the recommendations on the bag exactly as stated if you use these commercial blends. Adjust as needed after gaining experience. The most common mistake new growers make is not following the directions concerning amount of fertilizer. The choice of a commercial fertilizer is up to the grower. However, be certain that the mix has been formulated specifically for greenhouse tomatoes. An all-purpose fertilizer such as 20-20-20 is not appropriate for this crop, although it is fine for bedding plants and other general use in the greenhouse. For educational purposes, three examples showing how to use some of the commercial mixes are provided. The mention of a brand name is not an endorsement and is not meant to exclude other brands on the market. TotalGro Bag Culture Tomato Special (3-13-29)Source: TotalGro, P.O. Box 805, Winnsboro, LA 71295 (1- 800-433-3055). For the southern part of the United States, this is a low nitrogen-type fertilizer designed especially for growing greenhouse tomatoes using pine bark. It contains 3 percent nitrogen (N), 13 percent phosphate (P2O5), and 29 percent potash (K2O). In addition, it provides all of the magnesium (Mg) required, as well as S, B, Cu, Fe, Mn, Mo, and Zn. This fertilizer does not contain calcium, so use in conjunction with calcium nitrate. When used at the recommended rates (Table 3) and combined with calcium nitrate as shown, it will supply 100 ppm nitrogen and all of the calcium and magnesium needed, in addition to the other nutrients (Table 4). Submit tissue samples for laboratory analysis to determine when fertilizer rates need to be adjusted. to use per 100 gallons of water for 110 ppm N
Table 3 shows how much will be applied at the rates indicated. With this table, you can check the concentration with an EC meter. The EC of the solution with both fertilizers should be 1.86 when mixed as shown. Be sure to subtract the ppm or TDS of the source water from the reading of the nutrient solution before comparing the reading to the table. The relationship between TDS, EC, and ppm N is unique for each fertilizer. Therefore, Table 3 does not apply to other fertilizers. Make it a practice to check the EC of the solution each time that you mix a new batch. 3-13-29 plus calcium nitrate at rates in Table 3
Hydro-Gardens Chem-Gro Tomato Formula (4-18-38)Source: Hydro-Gardens, Inc., P.O. Box 25845, Colorado Springs, CO 80936-5845 (1-800-634-6362). This is a low nitrogen-type fertilizer, supplying 4 percent nitrogen (N), 18 percent phosphate (P2O5), and 38 percent potash (K2O). In addition, it provides Mg, B, Cu, Fe, Mn, Mo, and Zn. This fertilizer does not supply calcium, so calcium nitrate must be used in conjunction with the 4-18-38. The recommendations for mixing Chem-Gro are shown in Table 5. In addition, if the source water has less than 50 ppm calcium, then add calcium chloride in an amount equal to 25 percent of the calcium nitrate used per 100 gallons water. This can be added to the calcium nitrate concentrate tank, if an injector system is being used. to use per 100 gallons of water for each stage of plant growth
Peters Peat-Lite Special (15-11-29)Source: Peters Fertilizer Products, The Scotts Company, 14111 Scotts Lawn Rd., Marysville, OH 34041 (1-800-492-8255). This supplies 15 percent, 11 percent, and 29 percent of nitrogen (N), phosphate (P2O5), and potash (K2O), respectively, in addition to small amounts of Mg, Fe, Mn, B, Zn, Cu, and Mo. According to the directions, 9 ounces of fertilizer per gallon of concentrate will provide 100 ppm N if a 1:100 injector is used. For 50 ppm N with a 1:100 injector, use half this amount. Likewise, for 200 ppm N with a 1:100 injector, use 18 ounces per gallon of concentrate. Adjust the amount of fertilizer up or down, depending on the maturity stage of the crop. The same amount would be used for a bulk tank system per 100 gallons of water (since 1 gallon of concentrate used with a 1:100 injector is the same as using 100 gallons of mixed nutrient solution in bulk). Table 6 coordinates
the EC reading with the ppm of nitrogen for this fertilizer. Be sure to
subtract the EC of the water from the reading of the nutrient solution
before comparing the reading to Table 6. This fertilizer does not contain
adequate calcium or magnesium, so these must be added from other sources.
To accomplish this, use only half 15-11-29 with the other half of the
nitrogen source coming from calcium nitrate. This way, you are able to
supply calcium as well. To supply 100 ppm nitrogen, use 4.5 ounces 15-11-29
(instead of 9 ounces) and 4.5 ounces calcium nitrate (calcium nitrate
is 15.5% N; about the same N content as 15-11-29) per gallon. In addition,
add 1.8 ounces magnesium sulfate (epsom salt) and 0.07 ounces (2 grams)
iron chelate per gallon to supply magnesium and iron. As you need to go
up or down in feed, maintain the same ratios of these fertilizers. For
example, for 150 ppm N, use 6.75 ounces 15-11-29, 6.75 ounces calcium
nitrate, 2.7 ounces magnesium sulfate, and 0.105 ounces (3 grams) iron
chelate per gallon. For source water with a high alkalinity (total carbonates), the manufacturer suggests using Peters Excel K-Cell 14-5-38. EC readings using Peters Peat-Lite Special (15-11-29)
How To Calculate Element Level in a FertilizerThere are many times you need to know how much of a fertilizer element (such as nitrogen) is contained in the fertilizer solution. The concentration is usually measured in units of parts per million (ppm). This simply means the number of parts of a fertilizer element per million parts of water, on a weight basis. For example, 1 part nitrogen per 1 million parts of water is 1 ppm; or, 1 pound of nitrogen per 1 million pounds of water is 1 ppm. The following formulas are ways to calculate the concentration of any fertilizer element in water. They are easy to use, and a couple of examples are shown. By using these equations, you will be able to know exactly how much of each fertilizer element you are feeding your plants. You can use the following formulas to calculate the amount of any fertilizer element, not just nitrogen. There is essentially only one formula; however, if you are using an injector system, there is another factor to use, namely the injection ratio. With a bulk tank system, there is no injector, and, therefore, no injection ratio, so this number is left out. #1. Injector System #2. Bulk Tank
System Examples Example 1. You use 25 pounds of a 15-11-29 fertilizer mix in a 30-gallon concentrate tank, then use a 1:100 Anderson Injector. How much nitrogen are your plants getting? Since you have an injector system, use formula #1. ppm = (15) x (25) x (16) x (.75) x (100/30) x (1/100)ppm = 150 ppm nitrogen Example 2. You use a bulk tank system with 15 pounds of 8-5-16 fertilizer mix. Your bulk tank holds 600 gallons of fertilizer. You also add 10 pounds of potassium nitrate (13.75% N) to be sure the plants get plenty of nitrogen. How much nitrogen are the plants getting? Since you have a bulk tank system, use formula #2. ppm from 8-5-16 = (8) x (15) x (16) x (.75) x (100/600)= 240 ppm N ppm from KNO3 = (13.75) x (10) x (16) x (.75) x(100/600) = 275 ppm N Total ppm N = 240 + 275 = 515 This is much too high for greenhouse tomatoes. Nutrient Composition of FertilizersFor your handy reference, the list shows the amount of each element in the following fertilizers:
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