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.
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.
Table 1. Fertilizer salts (adapted from Jensen and Malter, 1995)
| Fertilizer Salts | element supplied | grams of fertilizer needed per 1000 liters of water to provide 1 mg/l (ppm) of the nutrient specified |
| Boric Acid [H3BO3] | B | 5.64 |
| Calcium nitrate [Ca(NO3)2·4H2O] (15.5-0-0) | N | 6.45 |
| Ca | 4.70 | |
| Cupric chloride [CuCl2·2H2O] | Cu | 2.68 |
| Copper sulfate [Cu(SO4)·5H2O] | Cu | 3.91 |
| Chelated iron (9%) | Fe | 11.10 |
| Ferrous sulfate [FeSO4] | Fe | 5.54 |
| Magnesium sulfate [MgSO4·7H2O] (Epsom salts) | Mg | 10.75 |
| Manganese chloride [MnCl2·4H2O] | Mn | 3.60 |
| Manganese sulfate [MnSO4·4H2O] | Mn | 4.05 |
| Molybdenum trioxide [MoO3] | Mo | 1.50 |
| Monopotassium phosphate [KH2PO4] (0-22.5-28) | K | 3.53 |
| P | 4.45 | |
| Potassium chloride [KCl] (0-0-49.8) | K | 2.05 |
| Potassium nitrate [KNO3] (13.75-0-36.9) | N | 7.30 |
| K | 2.70 | |
| Potassium sulfate [K2SO4] (0-0-43.3) | K | 2.50 |
| Zinc sulfate [ZnSO4·7H2O] | Zn | 4.42 |
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.
Table 2. Preparation of macronutrient and iron solutions for tomato (adapted from Jensen and Malter, 1995)
| Chemical compound (fertilizer grade) | Level A seedlings to first fruit set (g/1000 liters) | Level B Fruit set to harvest (g/1000 liters) | Nutrient | Level A (ppm or mg/L) |
Level B (ppm or mg/L) |
|||
| Magnesium sulfate (Epsom salts) | 500 | 500 | Mg | 50 | 50 | |||
| Monopotassium phosphate (0-22.5-28) | 270 | 270 | K | 199 | 199 | |||
| Potassium nitrate (13.75-0-36.9) | 200 | 200 | P | 62 | 62 | |||
| Potassium sulfate (0-0-43.3) | 100 | 100 | N | 113 | 144 | |||
| Calcium nitrate (15.5-0-0) | 500 | 680 | Ca | 122 | 165 | |||
| Chelated iron | 25 | 25 | Fe | 2.5 | 2.5 |
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).
Table 3. Preparation of micronutrient stock solution for tomatoes. Use 250 mL of this micronutrient stock in each 1000 liters of nutrient solution from Table 2, above. (adapted from Jensen and Malter, 1995)
| Fertilizer Salt | grams of chemical in 450 mL stock solution |
| Boric acid | 7.50 |
| Manganous chloride | 6.75 |
| Cupric chloride | 0.37 |
| Molybdenum trioxide | 0.15 |
| Zinc sulfate | 1.18 |
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.
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 (see section on Links and References in this website). 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.
Table 4. Common Nutrient Disorders in Tomatoes (adapted from Resh, 1995)
| Element | Deficiency | Toxicity |
| Nitrogen | older leaves are
chlorotic (yellow), spindly plants, small fruit remedy: use foliar spray of 0.25% to 0.5% solution of urea |
plants dark green with abundant foliage but little root growth or fruit production, flower drop |
| Phosphorus | plants stunted, maturity delayed, purplish color under younger leaves | no recognizable symptoms, however Cu and Zn deficiencies may occur in presence of excess P |
| Potassium | older leaves
chlorotic, with scattered dead spots. uneven ripening in
fruit (blotchy) remedy: use foliar spray of 2% potassium sulfate |
not usually absorbed by plants in excessive amounts, but high levels may lead to deficiencies in Mg, Mn, Zn, or Fe |
| Sulfur | S deficiency is very rare, some yellowing in young leaves, upper leaves become stiff and curl downward. stems, veins, and petioles turn purple | stunted growth, may see interveinal yellowing or leaf burning |
| Magnesium | interveinal chlorosis
on older leaves remedy: use foliar spray with 10% magnesium sulfate |
no visual symptoms |
| Calcium | Blossom end rot on
fruit, yellowing on margins of young leaves, undersides
turning purple, curling of leaves. growing tip and root
tip death, thick woody stems. can be caused by boron
deficiency remedy: foliar spray of 0.75 to 1.0% calcium nitrate solution or 0.4% calcium chloride |
no visual symptoms |
| Iron | pronounced interveinal
chlorosis on young leaves, starting at margins and
spreading through entire leaf. stunted growth and aborted
flowers. High pH can lead to iron deficiency, low pH can
lead to preferential uptake of aluminum, restricting iron
absorption remedy: foliar spray with 0.2 to 0.5% iron chelate every 3 to 4 days |
not usually a problem |
| Chlorine | very rarely a problem, but manifests as wilted leaves, chlorotic with a bronze color. stunted root growth | burning of leaf tips, bronzing or yellowing, leaf drop and stunted growth |
| Manganese | interveinal chlorosis
on older leaves, light green leaves with dead patches
ringed in yellow, few flowers or fruit remedy: foliar spray using 1% solution of manganese sulfate |
chlorosis, stunted growth |
| Boron | growing points wither
and die, interveinal chlorosis of upper leaves, brittle
leaves. boron deficiency can lead to calcium deficiency remedy: use a foliar spray of 0.1 to 0.25% borax |
yellowing of leaf tip, leading to browning |
| Zinc | reduction of internode
length, puckered margins on leaves, brown spots on
petioles, small leaves, sometimes long and narrow remedy: foliar spray with 0.1 to 0.5% solution of zinc sulfate |
commonly accompanied by Fe chlorosis |
| Copper | young leaves dark
green and misshapen, curling into a tube, petioles bent
downward, few or no flowers remedy: use foliar spray with 0.1 to 0.2% solution of copper sulfate to which 0.5% hydrated lime has been added. |
reduced growth, symptoms of Fe chlorosis |
| Molybdenum | interveinal chlorosis
on older leaves, margins of leaves curl up upward remedy: foliar spray with 0.07 to 0.1% solution of ammonium or sodium molybdate |
tomato leaves turn golden yellow |
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.
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 is necessary for growth, and optimal levels for tomatoes
may be 2 to 5 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.