When growing plants 'traditionally' considerable effort is expended in trying to maintain a 'fertile' soil such as adding mulch and other additives over time, and alternating types of crops such as legumes versus non-legumes for example. The hydroponic approach removes some of this complexity (i.e knowing your soil type in relation to nutrient status and availability and consequent effect on plant growth and how this changes with time/season).
However, it is then essential to know which nutrients and of what concentration are necessary in the hydroponic solution, and how these change with time and plant growth. Notionally, once this information is known, it is relatively easy to mix the required chemicals and then add this to the mix circulating within the hydroponic system. As usual, in practice 'things' are not quite so simple.
The simplest approach would perhaps be to just use commerically available hydroponic nutrient solutions. However, perusal of hydroponic literature shows that there is a wide range of recommenedations for the concentration of each individual plant nutrient, which depends upon individual plant/crop and also the stage of plant growth (vegetative, flowering etc). So 'just' choicing a commerical solution, aimed at being a 'one size fits all' is likely not to be optimal.
Further, the commerical solutions are relatively expensive when compared to buying the basic chemicals and preparing DIY nutrient solutions. This is particularly the case considering that nutrient solutions need to discarded/replenished frequently (2-4 weeks depending upon circumstances) and unless dealing with small 'window box' setups, even relatively modest hydroponic systems required considerable quantities of nutrient solution to be in circulation. Finally, experimenting with the concentrations and consitutents of the hydroponic nutrient solution is one of the "fun" aspects of DIY hydroponics.
The starting point for a DIY nutrient solution is recognising that plants have concentration limits for each chemical element which can be classified as 'deficient' if too low, 'just adequate' and if the concentration is to high, can become 'toxic'. However, the available information for toxicity concentration ranges appears to be in terms of mg/kg in soil (particularly in terms of 'metals'). Whereas, 'deficiency' and 'adequacy' concentration ranges appear to derived from mg/kg in dried plant material. That is, the plant material has been sampled, dried and analysed to determine what concentrations of chemical elements are present. These concentrations are then compared to physical observations of the plant growth which shown if the plant was healthy, low/high productivity etc. Therefore, such information is not directly applicable to determining concentrations of nutrients in the hydroponic solution.
The alternative is to use the experience of other practitioners, who have dealt with this issue over time. Hydroponics is a widely and well know subject and 'mature' endeavour (over a hundred years of practical application) - particularly with information widely available on the Internet). Consequently, there is an huge variety of recommended nutrient solution compositions which reflect the myriad types of hydroponic approach, crop type, local environmental conditions and other various factors.
The following table, sourced from (4), (5), (6) shows the generally accepted concentrations for essential elements for plant growth, and the concentrations recommended by a variety of authors for a number of plant crops:
|Conc. in mg/L
|"General" Range in Nutrient Solutions|
|Typical Ranges mg/L||200-300||100-200||100-200||30-175||70-150||30-80|
|Recommendation by Crop|
|"Commerical Greenhouse Vegetable Production Solutions|
|Uni/Gov. Recommendations by Crop|
|UA CEAC - leafy greens||89||106||210||31||32||24|
|UA CEAC - tomato etc||169||115||341||39||119||48|
|DPI NSW Asian Leafy||70||116||201||22||26||20|
|"Common" Recommended Nutrient Solution Formulations|
|Average of all Above References|
While this information is obviously helpful, it doesn't really help with a 'recipe' as such. The elements/components listed in the above table are available from a variety of chemicals, and more over, different plants will require potentially different concentrations, which also potentially changes depending upon growth stage (vegetative, flowering, fruiting etc).
A number of books give potentially useful hydroponic 'recipes', however, an excellent online source is an article by Mattson and Peters (5). In addition to providing 'recipes' for hydroponic solutions, this reference points out the importance of testing the quality of the water with which the hydroponic solutions are to made. If your water quality has high levels of hardness, alkalinity or other consitutents, this will need to be taken into account when calculating nutrient solution makeup. My local water supply is high quality (electrical conductivity <100 uS/cm, pH ~7) so 'background' chemical concentration within the actual water was not a concern.
For vegetative crops (e.g. lettuce, herbs), most nutrient-solution recipes are not adjusted to change nutrient concentrations while they grow; whereas, in fruiting crops the ratio may be adjusted to alter when plant growth changes from vegetative, to flowering and then fruiting. As pointed out in the article by Mattson and Peters "for a new grower, a good starting point is to simply develop one recipe that works decently well for a range of crop growth stages and conditions. Later, you can work on honing the recipe, optimizing it for different growth stages or based on your current growing conditions".
That was the approach that I took, focusing on growing pechay (Brassica rapa L. cv group Pak Choi) as an easy initial crop/plant (see the Photographs Section). We grow a lot of pechay normally within our vegetable gardens, and it is easy to grow, forgiving on conditions and provides tasty leaves useful in soups and stir-fry's through to just salad.
Nutrient Solution Formulation/Calculation - Pechay (Pak Choi)
Following recommendations cited in the previous section for 'green leafy crops' gives an approximate agreement for phosphorus (22-50 mg/L), sulfur (26-32 mg/L) and magnesium (20-65 mg/L). Whereas, for calcium, nitrogen and potassium two likely authoritative references list 70-89 mg/L Ca, 106-116 mg/L N, and 201-210 mg/L K, compared to that listed in (4) of 200 mg/L for calcium and nitrogen with 300 mg/L potassium. Opting fof the higher concentrations for Ca, N and K enables the solution to approximate concentrations recommended for tomato crops. Therefore, these higher concentrations for Ca, N and K (over that strictly recommended for green leafy crops) would also allow some earlier experimentation with tomatoes.
Once the concentrations of the various elements in the nutrient solution have been chosen, the next issue is to select which base chemicals will be used in the actual formulation, and how to calculate actual quantities of individual chemicals required. During this initial testing phase, I have choosen to ignore 'micro nutrient' constituents (iron, copper, manganese etc) as the commerical chemicals used for the macro-nutrients (Ca, Mg, N, K etc) have sufficient impurities that addition of specific micro-nutrients is likely not necessary. Particularly considering the small scale, DIY nature of the current exercise.
The 'base' chemicals from which to produce the nutrient solution that are available from local agricultural and horticultural suppliers include the following in my local area:
- calcium nitrate
- sodium nitrate
- potassium sulphate
- magnesium sulphate
- ammonium dihydrogen orthophosphate
- ammonium sulphate
- mono-potassium phosphate
- potassium nitrate
Note that calcium can be included in the nutrient solution by using an amount of calcium nitrate (which then also introduces some nitrogen via the nitrate counter-ion in the compound). Whereas, an element such as phosphate could be sourced from a number of the compounds available (and consequently also then introduce a range of elements from the counter-ions, e.g. if using potassium phosphate as the source of phosphorus in the nutrient solution, this would also unavoidably introduce an amount of potassium. Consequently, there is some 'mixing and matching' required in order to select which actual chemicals (and physical amounts) will be used to supply the concentrations of individual elements needed in the nutrient solution.
An Excel spreadsheet was developed to help with this chemical selection and the necessary calculations (see screen-shot below as an example).
The total volume of nutrient solution desired is entered into cell D1 (in litres). The desired concentration (mg/L) of each element (Ca, N, P, K and Mg) in the nutrient solution is entered in column B, rows 4 to 8.
I only had calcium nitrate available, so all the calcium in the nutrient solution will come from this compound. The cell D4 calculates the necessary amount of calcium nitrate (in gms) to give the desired concentration of calcium (in mg/L) in the nutrient solution, which in this case is 50.1 gm calcium nitrate to give 170 mg/L. However, 50.1 gm of calcium nitrate will also contribute nitrogen to the nutrient solution (from the nitrate counter-ion to calcium in the compound). Cell D11 calculates the amount of nitrogen added in gms, and cell B11 calculates this amount as mg/L in the nutrient solution. In this case this equates to a total of 137 mg/L nitrogen, compared to the desired range of 70-89 mg/L for green leafy crops recommended in the references above. While this is greater than necessary for green leafy crops, the additional nitrogen allows this same nutrient solution (in terms of nitrogen) to be also used for tomatoes.
The next element selected is magnesium, as this element again can only be sourced from a single available compound, magnesium sulfate. Cell H8 calculates the necessary amount of magnesium sulfate in gms to produce the desired concentration of 39.5 mg/L in the nutrient solution. Similarily, since magnesium sulfate is a compound, sulfate will also be added to the nutrient solution. Cell H12 calculates the amount of sulfate added in gms (as a consequence of adding the required amount of magnesium) and cell B12 gives this value in mg/L in the nutrient solution.
Similar calculation is performed for potassium, using potassium sulphate as the source.
Phosphorous could be added from a variety of compounds available, with ammonium dihydrogen phosphate being selected. This is because adding the necessary amount of this compound to give the desired concentration of phosphate will also contribute ammonium to the nutrient solution. This ammonium adds to the total nitrogen concentration, but, importantly, gives nitrogen in the form of ammonium, in conjunction with the other nitrogen in solution in the form of nitrate (from the calcium nitrate added for the calcium component).
This example shows that a nutrient solution with a set of desired concentrations for individual macro-nutrients can be formulated from a variety of actual chemical compounds. Further, an iterative approach is required to the calculations as each compound is formed from a cation (positive charged part) and a anion (negative charge part). Chlorides are usually not used as chloride is only generally required by plants in small quantities, all nitrate compounds are readily soluble (and add nitrogen), sulfates used appropriately (e.g usually only source of magnesium).
Once the calculations have been performed, the necessary amount of each individual chemical is weighed and dissolved in a seperate one litre quantity of water. The nutrient reservior of the hydroponic system (total volume of 50L in my case) is then filled with approximately 40L of water. Starting with the 'nitrate' containing compounds (except those that contain calcium), the prior dissolved chemicals (1L quantities) are added seperately to the nutrient reservior, mixing well between each addition. Then the other dissolved compounds are added individual, with stirring between each addition, but calcium is added last. The prior dissolution, and adding of the dissolved compounds individually to the bulk solution is to avoid precipitation within the nutrient solution.
An alternative approach is to form concentrated 'two part' solutions, which can be diluted as necessary. However, I favoured weighing seperate quantities as and when required, which avoids having to store the concentrated solutions, and possible adverse reactions wasting or deteriorating the solutions over time.
The final stage before the nutrient solution is ready for application is to check the pH and then adjust to the range of pH 6 - 6.5 using caustic soda (sodium hydroxide).
||Microsoft Excel spreadsheet to calculate nutrient solution formulation
Nutrient Solution Replenishment/Testing
The nutrient solution concentration will change over time due to uptake by the growing plants, or other processes that generally decrease the concencentration of individual nutrients. This will require the nutrient solution to be periodically replaced.
Without having the equipment to perform analytical testing or the scale of hydroponic production to warrant testing by a laboratory, I simply discard the hydroponic solution every 2-3 weeks during the early growth stage, and then weekly during flowering/fruiting.
This is an advantage of the DIY nutrient solution approach, as the amount of chemical required (a few ten's of grams per 50L batch) is relatively inexpensive, and discarding the solution is much cheaper than having chemical analysis performed. Although, DIY chemical analysis is a further topic that can be explored when time permits.