Growing Tomatoes Hydroponically (part 3)
By Dr. Merle Jensen
Growing Media
Various growing media can be used in hydroponic systems. However, any system must have the following four qualities:
- sufficient support for the plants
- appropriate distribution of air, since roots need oxygen and respire other gasses, such as carbon dioxide
- maximum water availability for the plant roots
- accessible nutrient solution with consistent chemical characteristics
Deep Flow Hydroponics
The classic hydroponic system, where plants are supported so that their roots hang into a nutrient solution, is generally called “deep flow hydroponics.”
This system is appropriate for hobbyists and large scale production of leafy vegetable crops. The system consists of horizontal, rectangular-shaped tanks lined with plastic. The nutrient solution is monitored, replenished, recalculated, and aerated. Commercial facilities are now quite popular in Japan. The rectangular pools act as frictionless conveyor belts where large, moveable floats of plants (lettuce) can be transported from transplant to harvest.
Nutrient Film Technique
A modification of the deep flow system is called “nutrient film technique”, where a thin film of nutrient solution flows through plastic lined channels, which contain the plant roots. The walls of the channels are flexible; this permits them to be drawn together around the base of each plant, excluding light and preventing evaporation. For lettuce production, the plants are planted through holes in a flexible plastic material that covers each trough. Nutrient solution is pumped to the higher end of each channel and flows by gravity past the plant roots to catchment pipes and a sump. The solution is monitored for replenishment of salts and water before it is recycled.
Capillary material in the channel prevents young plants from drying out, and the roots soon grow into a tangled mat. This method is mainly used for tomatoes.
Aeroponics
Aeroponics is another technique, where nutrient solution is sprayed as a fine mist in sealed root chambers. The plants are grown in holes in panels of expanded polystyrene or other material. The plant roots are suspended in midair beneath the panel and enclosed in a spraying box. The box is sealed so that the roots are in darkness (to inhibit algae growth) and in saturation humidity. A misting system sprays the nutrient solution over the roots periodically. The system is normally turned on for only a few seconds every two to three minutes. This is sufficient to keep roots moist and the nutrient solution aerated. Systems were developed by Dr. Merle Jensen at the University of Arizona, for lettuce, spinach, and even tomatoes, although the latter was judged not to be economically viable. In fact, there are no known large-scale commercial aeroponic operations in the United States, although several small companies market systems for home use.
Aggregate Hydroponics
In aggregate hydroponic systems, a solid, inert medium provides support for the plants. As in liquid systems, the nutrient solution is delivered directly to the plant roots. Aggregate systems may be either open or closed, depending on whether surplus amounts of the solution are to be recovered and reused. Open systems do not recycle the nutrient solutions; closed systems do.
In most open hydroponic systems, excess nutrient solution is recovered; however the surplus is not recycled to the plants, but is disposed of in evaporation ponds or used to irrigate adjacent landscape plantings or wind breaks. Because the nutrient solutions are not recycled, such open systems are less sensitive to the composition of the medium used or to the salinity of the water. These factors have generated experiments with a wide range of growing media and the development of more cost-efficient designs for containing them.
There are numerous types of media used in aggregate hydroponic systems. They include peat, vermiculite, or a combination of both, to which may be added polystyrene beads, small waste pieces of polystyrene beads, or perlite to reduce the total cost. Other media such as coconut coir, sand or sawdust, are also common in some regions of the world.
For growing row crops such as tomato, cucumber, and pepper, the two most popular artificial growing media are rockwool and perlite. Both of these media can be used in either closed or open systems (gravel is not recommended as an aggregate in either system). Both media are lightweight when dry, easily handled and easier to steam-sterilize than many other types of aggregate materials. Both can be incorporated as a soil amendment after crops have been grown in it.
Rockwool, or stonewool, is produced from basalt rock, and can come as spun wool, resembling fiberglass, or it can be granulated, offering an alternative to perlite and vermiculite in terms of water holding capacity and aeration. Stonewool has a high pH, generally greater than 8.0, however, it has essentially no buffering capacity, meaning it will not affect the pH of the nutrient solution nor will it affect any other media it is mixed with, such as peat moss (which has a pH of 3.8 to 4.5). Stonewool can be purchased in prepackaged “slabs”(commonly 15 x 7.5 x 100 cm long), ready to use, or as bulk granules for those growers who wish to mix their own soilless media.
Perlite is usually bagged in opaque white bags with drip irrigation tubes at each plant and drainage slits in the bags. Perlite is an inert media providing excellent aeration and water holding capacity. As in rockwool, it can be steam sterilized, rebagged and reused several times.
When both perlite and rockwool are used as closed systems, great care must be taken to avoid the buildup of toxic salts and to keep the system free of nematodes and soilborn diseases. Once certain diseases are introduced, the infested nutrient solution will contaminate the entire planting. In addition to the common practice of sterilizing the recirculating solution, there is current research exploring the use of surfactants to control certain root diseases. Such systems can be capital intensive because they require leak proof growing beds as well as subgrade mechanical systems and nutrient storage tanks.
Plant Nutrition
Water Quality
Good, consistent water quality is essential for hydroponics. Fresh water free from pesticide runoff, microbial contamination, algae, or high levels of salts must be available throughout the year. The levels of pH and alkalinity (measured as carbonates and bicarbonates) of the raw water affects the absorption of certain nutrients by the roots. Water pH levels above the desirable range (5.0 to 7.0) may hinder absorption of some plant nutrients; pH levels below this range permit excessive absorption of some nutrients, which may lead to toxic levels of those elements.
In arid areas, or areas near salt water, the concentration of sodium chloride (NaCl) may be too high for optimal plant growth (greater than 50 parts per million or 1.5 mmol/liter). The hardness of the incoming water will also have an effect on the nutrient solution. Hardness is a measure of the concentrations of calcium and magnesium carbonates, which are often quite high in areas of limestone rock. The naturally occurring concentrations of these minerals in hard water must be taken into consideration when calculating the amount of nutrient salts to add to the nutrient solution, and may interfere with the availability of other essential nutrients, such as iron. Similarly, concentrations of other essential elements may be found in very high levels in poor quality water. For example, water may carry high levels of iron, selenium, boron, or sulfur; and municipal water may have undesirably high levels of chlorine.
The electrical conductivity of good quality raw water should be below 0.5 mS/cm or mmhos/cm. It is advisable to invest in a complete analysis of the water quality, including all major and minor elements, microbial contamination and pesticide residues before any further work is done.
For more information on desirable ranges for specific elements in irrigation water, see Jensen and Malter, 1995, referenced in the Links and References section of this website.
Nutrient Solution Recipes
There are sixteen elements which are generally considered to be essential for good plant growth. The macro elements are those required in “high” concentrations: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), Sulfur (S), and Magnesium (Mg). Carbon must be supplied to the plant as carbon dioxide gas (CO2). In a small operation or one with large amounts of fresh air movement, additional CO2 may not be required. Larger operations, or ones with high density plantings will need a CO2 generator (See CO2 enrichment, detailed below). Hydrogen is available in sufficient quantities from the atmosphere and oxygen is supplied from well-aerated nutrient solutions. Nitrogen, phosphorus, potassium, calcium, sulfur and magnesium must all be supplied by the nutrient solution.
The micro elements are also essential for growth, but required in smaller concentrations. There is still some disagreement, but generally the micro elements are thought to be: Iron (Fe), Chlorine (Cl), Manganese (Mn), Boron (B), Zinc (Zn), Copper (Cu), and Molybdenum (Mo). Certain plant species may need others for good growth: Silica (Si), Aluminum (Al), Cobalt (Co), Vanadium (V), and Selenium (Se).
Small greenhouse operations often buy ready-made nutrient formulations; only water need be added to prepare the nutrient solution. Larger facilities prepare their own solutions. The commonly used salts and the required amounts to make 1000 liters of 1 ppm solution are given in Table 1. Multiplying the value for a salt by the number of ppm desired in the formula will yield the number of grams to be used per 1000 liters.
Nutrient solutions need to be adjusted during the growing cycle of the crop and are different for each crop grown. Leaf crops generally need higher N, root crops need higher K, and fruit crops such as tomatoes or cucumbers should maintain relatively low N levels.
The nutrient solution for tomatoes is generally made in two or three levels for the various stages of growth (see Table 2, below). Only the macro nutrients change, becoming progressively more concentrated as the crop matures. The micronutrients remain the same throughout the growth cycle. The first stage of growth (Level A formula) is for seedlings from the first true leaf until the plants are 24 inches (62 cm) tall, when initial fruit is 1/4 - 1/2 inches (1 to 1.5 cm) in diameter. After that, Level B formula is used. While the formula in Table 2 has been standard for many years, some new tomato varieties may require much higher nitrogen and potassium. It is advisable for commercial growers to consult their seed company for the recommended nutrient formulas for the tomato variety grown. Optimizing the N:K ratio is important as the crop matures and as the available light and day length changes. Under high light conditions, plants use more N. High K during the fall and early winter months improves fruit quality. It is common practice to double the ratio of K:N during winter months when plants receive less light. The optimum pH of the nutrient solution should be 5.5-6.0. The pH of the nutrient solution can be lowered with phosphoric acid.
The micronutrients should remain at the same concentration throughout the life of the crop. Optium concentrations for tomatoes are: Boron 0.44, Copper 0.05, Chlorine 0.85, Manganese 0.62, Molybdenum 0.06, Zinc 0.09, Iron 2.5 ppm (mg/L).
If a concentrated stock solution is used for the macronutrients, then the calcium salts should be kept apart from the other salts in a separate solution. Nitric or phosphoric acid can be used to lower the pH if necessary; concentrated acid should always be carefully diluted before it is added to the stock solutions.
Symptoms of Nutrient Deficiencies and Toxicities
Nutritional disorders can be very complex, involving temperature, humidity, day length and disease as well as nutrient levels. Multiple disorders can produce a syndrome which does not resemble any single disorder. Some growers feel that relying on plant disorder symptoms is a reactive, not a pro-active approach, since by the time symptoms appear, the yields will already have been adversely affected.
Symptoms of nutritional disorders should never be ignored, however, and excellent sources of information are available to key out specific problems.
Professional growers should keep such sources and horticultural experts near at hand, and have their nutrient solutions analyzed routinely. Table 4 outlines some common nutrient disorder symptoms in tomatoes.
As soon as any deficiency is confirmed, the nutrient solution should be changed with the concentration of the deficient element increased 25 to 30%. After the deficiency is rectified, the concentration should be lowered back down to slightly higher than normal levels. Foliar sprays can be applied for a faster response, however burning of the plants may result. It is best to test a foliar spray on a few plants and wait several days to observe the effects before spraying a whole crop.
Sampling (Nutrient Solution and Plant Tissue)
Nutrient solution analysis is absolutely necessary in a closed system, where the solution is re-used, and recommended in an open system to verify concentrations of macro and microelements. Plants take up nutrients in varying amounts depending on their needs. Although monitoring pH and EC will give an indication of changes in the nutrient solution, it cannot indicate changes in preferential uptake of particular ions. In a closed system, if no analysis is possible, then the nutrient solution should be completely changed every two weeks.
Plant tissue analysis can provide other information about the growing system. That is, tissue analysis can indicate any problems the plants may be having in absorbing nutrients which are present in the solution. For example, fluctuating pH levels, high cation exchange capacity of the media, high humidity, or diseases and nematodes can prevent nutrient uptake by a plant.
On a commercial scale, nutrient solution and plant tissue analysis is absolutely required. Plant tissue analysis allows the grower to detect a problem in the uptake/assimilation of nutrients which may not be apparent in a nutrient solution analysis. Consult with the testing laboratory for information on sampling and sample prep. For more information on expected levels of individual elements in tomato tissue analysis, see Hydroponic Food Production by Howard Resh, 1995, (cited in the Links and References page of this website).
Electrical Conductivity (EC) is a convenient estimation of Total Dissolved Solutes or Total Dissolved Salts (TDS) in the solution. However, although EC is a function of the salts in the solution, it does not indicate the relative concentration of the major nutrients, or the quantitiy of trace elements (micro nutrients) present. For example, high levels of calcium can give a lower EC reading than the equivalent concentration of sodium ions. A grower would not be able to detect these changes by monitoring EC alone. Although changes in TDS and EC can indicate a change in the nutrient solution, they should not be relied on exclusively.
Carbon Dioxide Enrichment
Carbon dioxide is necessary for growth, and optimal levels for tomatoes may be two to five times the normal atmospheric levels (1000 to 1500 ppm CO2 versus ambient levels of 350 ppm). Plants can deplete the CO2 in a closed greenhouse in a matter of hours, significantly reducing growth rates. Growers using CO2 enrichment have claimed to see a 20 to 30% increase in tomato yields, and accelerating flowering and fruiting by as much as 10 days.
Specially designed CO2 generators are natural gas or propane burners hooked up to sensors. Large commercial growers often use the flue gases from a hot water boiler burning natural gas as a source of CO2, or they will use bottled CO2. It is important that the CO2 be free of contaminate gases, as tomatoes are extremely sensitive to many gases, especially ethylene. Plants enjoying elevated levels of CO2 can be expected to increase fertilizer and water requirements.
Be sure to read the conclusion of this four part article, Growing Tomatoes Hydroponically by Merle Jensen, in the next issue of Maximum Yield.