Nutrients

Like any other organism also plants need nutrients in order to keep their metabolism working. But on contrary to us humans, they do not desire "a good tasting meal" but rather the main ingredients they depend on [55]. This is due to the mechanism how plants acquire their nutrients, which is in dissolved and specific ionic form [59]. Hence, plants are a piece a natures great cycle and depend on the insects, bacteria and co (called decomposer) to put organic matter apart and into their basic structure. Only as such, it will be soluble in the soil's surface water and therefore becoming available for plants, if additional parameters like temperature, concentration or acidity are in the right range as well [31, 59]. And even if they are, the plants' roots have to be at the right spot to the right time to fulfill their task; fascinatingly, they mostly do! Nonetheless, what if we would provide these nutrients directly to the roots – would the plants notice?

That they will develop and grow faster if applied to the plants' preferences, you can see for yourself at your own garden. Your plants will not be able to distinguish between organic fertilizers or traditional ones [55]. The reason behind this is as mentioned: only as very specific and simple-structured ion, the plants will be able to pick them under certain conditions. Ensuring these ions and conditions apply to all time for our plant's nutrients just lead to idealistic nutrient uptake.

Nonetheless, this was just the introduction into an overall very complex topic. Get to know what kind of nutrients are needed in which quantities and for what; how the ideal conditions differ between species and how they even change over the stages of the very same plant's development; and how we can find the right fit to provide these nutrients under the right conditions, you will learn in this section.

MaCro-Nutrients

List of Macro Nutrients [31, 55, 59]:

Hydrogen (H; uptake as H₂0 or H⁺; part of all organic components and necessary for all kind of ion exchange mechanisms)
Carbon (C; uptake as CO₂; part of all organic components)
Oxygen (O; uptake as H₂O or O₂; part of all polarized organic components)
Nitrogen (N; uptake as NH₄⁺ or NO₃^-; part of Proteins and and needed for Chlorophyll)
Potassium (K; uptake as K⁺; needed for many processes and reactions)
Calcium (Ca; uptake as Ca²⁺; regulator of many processes and part of membranes)
Magnesium (Mg; uptake as Mg²⁺; central part of chlorophyll and regulator of many enzymes)
Phosphorous (P; uptake as H₂PO₄^2-; essential for energy transport and conversion and part of nucleotides)
Sulfur (S; uptake as SO₄^2-; essential for proteins and some coenzymes)

The macro nutrients are essential to keep the plants' metabolism running and hence the plants alive [55]. Most of them are required in huge quantities and some have to be available all the time, while the others are at least required during certain metabolic processes during the day [59]. Whereas the first three elements listed on the left hand side are the most important components of plants, they are by nature also available through the air and soil. The quantity and availability of all the other macro nutrient however varies significantly in soil: The quantity depends on the decayed organic material, which has to be decomposed by bacteria. The availability depends on the molecular form they are in, as plants are only able to adsorb dissolved nutrients. Therefore, the availability of these nutrients depends mainly on the water content and the overall mixture of the medium they are growing in, which allows certain nutrients to become available in their dissolved state [55, 59]. Additionally, the active and passive adsorption are affected by other environmental parameters like temperature, acidity and interdependencies between different nutrients themselves [59]. It is therefore very important to provide optimal conditions, without which plants may suffer to death in the worst case [31].

Hydroponic systems therefore directly use dissolved nutrients as fertilizers in their solution to feed the plants, which are chemically spoken identical to the nutrients available in soil [55]. Additionally, the exclusion of soil makes it easier to control and regulate the important parameters [31].

Micro-Nutrients

List of Micro Nutrients [31, 55, 59]:

Chlorine (Cl; uptake as Cl^-; required as enzyme activator)
Iron (Fe; uptake as Fe²⁺ or Fe³⁺; part of certain enzymes and acts as electron carrier)
Manganese (Mn; uptake as Mn²⁺; activator of some enzymes)
Boron (B; uptake as BO₃^2- or B₄O₇^2-; may be part of transport molecules)
Zinc (Zn; uptake as Zn²⁺; activator of some enzymes)
Copper (Cu; uptake as Cu²⁺ or Cu³⁺; part of certain enzymes and acts as electron carrier)
Molybdenum (Mo; uptake as MoO₄^2-; essential for nitrogen fixation)

The micro nutrients are also required for various functions throughout the plants, as you can see in the list aside. It is again sorted according to the decreasing average concentration found in leaf tissues, having chlorine with the highest and molybdenum with the lowest [31]. Nonetheless, the concentration and hence their absorption quantities are up to 1000 times lower than those of the macro nutrients. This is due to the fact, that these elements are only needed within certain molecules fulfilling very specific tasks, but not for major structural purposes of the plant [59]. As some of these micro-nutrients are well studied in the realm of plant physiology, others may still fulfill purposes we don't know yet [55]. Nevertheless, we must never forget to provide our plants also with all micro-nutrients to guaranty a healthy development.

Trace Elements

List of some Trace Elements [55]:

Silicon (Si)
Nickel (Ni)
Aluminium (Al)
Cobalt (Co)
Vanadium (V)
Selenium (Se)
Platinum (Pt)
...

Trace elements are found in minimal concentrations in some plants. Still, their positive effect has yet to be proven or is even completely unknown [55]. It is arguable, if trace elements should be specifically included in fertilizers or not, as they are also sometimes not available in soil, appearance may cause negative effects on the plants' health and their reciprocal effects on macro- and micro-nutrients are difficult to foresee [31, 59]. Additionally, some of them will be found in fertilizers anyway due to impurities [55]. So to summarize: there are also other nutrients out there which may have an effect on the plant's health, but there occurrence in small amounts doesn't seem to be significantly impactful according to research now [55].

divers Nutrient requirements for Different Plant

Like comparing an elephant with a leopard, also plant species desire their specific diet. In hydroponic that translates to the concentration of dissolved macro- and micro-nutrients in the solution provided to the plants' roots. But let us dive into the standard solution proposition to allow generally a healthy growth first. Like in soil, an ideal hydroponic solution should be maintained at a pH between 5,5 and 6,5 [5, 9, 27, 31]. That is due to the different availability of dissolved nutrients for plants, as they can only be absorbed in a certain range [57]:

Nutrient availability for root uptake depending on acidity [57].

While there may be some which prefer a more acidic environment (like blueberries), others will desire higher concentrations on all nutrients to satisfy their hunger (e.g., tomatoes and peppers).

With an hydroponic solution, all nutrients are provided. That implies, that every plant should be able to grow based on the nutrients available, and certainly they can. So why should we adapt the solution to the plants? Well, let us examine different cases to get to the answer. Rosemary, e.g., is not the fastest grower out there. Additionally, it doesn't require a lot of water, but likes hot and dry climates. Hence, it is used to have only little amounts (leading to small concentrations) of nutrients available for uptake – rosemary may therefore be able to grow within a nutrient rich solution but is not used to do so and could get health issues [31]. On the contrary, let us look at tomatoes: a fast growing, light loving crop that requires a lot of nutrients to build its stem, leafs and fruits within only a few months. Additionally, it depends on high oxygen levels aerating their roots. For the growing phase, it will also need high amounts of nitrogen, whereas during the fruiting phase a lot of potassium is absorbed [31]. Nonetheless, cultivation has lead to very divers species, able to grow in all kinds of environments. To sum it up, your tomato plants will profit from higher nutrient concentrations and as much light as possible. And then there is the last example of the blueberry, which in nature is found on very acidic soils in shady places [27]. It may therefore have hard times to be grown under the same conditions as the examples before, as it requires a very acidic solution to be able absorbing the nutrients needed [27].

On purpose, the examples chosen are very different from each other and you may be ok just using a standard solution for all of your plants. Nonetheless it may be helpful to know in which environments your varieties normally grow and what they are used to, in order to make small changes in your solution to help them out. Find out more about the specific crops here.

Nutrient requirements during plant development

Since most herbs and greens are harvested before getting into the flowering stage, the solution in the hydroponic system only has to be aimed for delivering perfect growing conditions. Hence, mainly nitrogen has to be provided, which is irreplaceable for building stem and leaves [31, 59]. On the contrary, fruiting crops must pass through the flowering and fruiting phases, which concentrate more on other nutrients compared to the initial growing stage. Therefore, more potassium and other nutrients (depending on the crop in question) have to be provided [59], which are essential in building the flowers and fruits. Therefore, you will find many multiple-part fertilizers, which separate their solutions according to these growing stages into growth and flowering, with sometimes additional concentrates for root enhancement (containing partly essential molecules and others [55]) or pH adjustments (contain an acid or base mixture, respectively) to allow adjustments to the solution over time.

Finding the right fit

There are many different fertilizers on the market, all trying to convince you being the best. If you already have your company you are happy with, then you are good to go. For all the others, it may take time to find the fertilizers that satisfy your expectations, but let us get into this topic in more detail.

First, it should be mentioned that just small changes in molecular concentrations will lead to different short- and longterm results regarding the plants' development and the acidity of the nutrient solution. As these dependencies are not only very complicated but also not completely understood into all detail [55], there are significant differences between products and companies, even though their descriptions may seem very similar! This doesn't necessarily mean, that there are many bad fertilizers out there. But you may have to do more maintenance work with one solution, keeping it in the desired ranges of acidity and nutrient concentration, than with another.

You will find dry fertilizers and solutions. The latter are just nutrients already dissolved in water, that is it. Therefore it doesn't make a difference in the end, as you will have to dilute either of them in order to get the right concentrations. Then there are also one-part and multiple-part fertilizers. While the all-in-one solutions are easy to handle and may work for your crops [5], you will be limited to provide small adaptations according to the varieties' desires or the plants' growing stage they are currently in. On the contrary, do not let fool you in believing you have to buy each variety a costumed fertilizer mixture – there are no perfect fertilizers for specific varieties, as the plants' health also depend on other environmental factors, which by the way also alter the very nutrient solution in question [31, 59]. Hence, it is more important to know your varieties' needs and be able to make slight changes if necessary. For that reason, there exist multiple-part fertilizers, which often distinguish between growing stages and give you advice on how much you should add to your solution at which point. In addition, some of these products even provide you with so-called pH-down and pH-up components that allow you to alter the acidity or alkalinity, respectively.

To sum it up, you have to find out for yourself, whether you like to have it as easy as possible or eager to try out and improve your results.

Link to References