
In this blog, we’re still in the third wedge of the Aquaponics Cycle: the plants.
Our plant has absorbed water and nutrients through its roots and transported them to the rest of the plant. We talked about what happens when nutrient absorption works well, but it is not likely that all the nutrients a plant needs to grow will be ready and available at the root surface waiting to be absorbed. What does the plant do when it can’t get enough of a certain nutrient it needs? The three methods include changing itself, its environment, and networking with soil microbes.
1. Self-improvement. The plant will put more energy into growing roots with increased surface area, increasing what is called the “root-to-shoot” ratio. This improves the chance that the roots will connect with a stash of the nutrient the plant needs. Looking back at the blog on nutrient absorption, we know that nutrients enter the roots through different carriers that are shaped to bind/absorb specific nutrients. In the event of a nutrient deficiency, the plant will:
1) Make stronger versions of the carrier, that attracts the deficient nutrient even more effectively; and
2) Build and install more of those carriers on the roots.
With more root surface area, covered in more carriers that are built to better absorb the needed nutrient, if the available form of the nutrient is out there, the plant roots will find and absorb it.
2. Home improvement. The plant modifies the environment around its roots. Nutrients need to be in a certain ionic form for them to be absorbed. Absorbability is based on pH (see graph), with certain nutrients being more easily absorbed at acidic/low pHs (e.g. iron, copper) and some at basic/high pHs (e.g. potassium, sulphur). The plant will release acid into the area around its roots to decrease pH, depending on the nutrient it needs. This will change the nutrient into the required form, so the roots can absorb it. The positively charged hydrogen ions which are released from the roots to decrease pH can also free other positively charged nutrient ions (e.g. Potassium, Magnesium, Calcium, etc) which are stuck to negatively charged surfaces like soil particles, making these positively charged nutrients available for root uptake. This process is called Cation Exchange.

Nutrient availability to plants changes depending on the pH around the plant root. Thicker bars represent more nutrient availability. Image source: Michigan State University Extension.
3. How to win friends and influence microbes. Plants have co-evolved with soil microbes over millions of years, and have settled into a beneficial, symbiotic relationship. When the plant needs a certain nutrient, it releases a chemical calling card (flavonoid) that attracts specific microbes that excel at obtaining that nutrient. Some microbes are manufacturers, like nitrogen-fixing bacteria which convert nitrogen gas from the air into ammonia the plant can absorb. Some microbes are labourers who modify the nutrient into a form the plant can absorb, like phosphorus-solubilizing bacteria which break up solid phosphorus compounds to release dissolved phosphorus for plant absorption. Some microbes are procurement specialists, like beneficial fungi which use their vast, root-like networks (hyphae) to search out the needed nutrient and deliver it back to the plant. You read that right: Door Dash for plants.
The plant pays for these services using the sugars made by photosynthesis in the leaves. By supporting healthy plant growth, the microbes get sugars they can use to grow/reproduce. By giving up sugar to the microbes, the plant can secure the nutrient supply it needs to grow. Everybody wins.
These methods can look a little different in water-based growing systems like aquaponics and hydroponics. Method 1, self-improvement, works well for both aquaponics and hydroponics. Method 2, environment modification around the roots, does not work as well for either, since water flows past the roots so quickly, the pH can’t be changed by the plant as efficiently as it can in a low-flow system like soil growing. One of the benefits of aquaponics over traditional hydroponic growing happens with Method 3, recruiting microbes. Many hydroponic growers completely sterilize their systems in between crops – this eliminates disease-causing microbes, but it also means there are no beneficial bacteria around to be paid. It is on the hydroponic farmer to provide exactly what the plant needs, at the correct pH, when the plant needs it. Meanwhile, aquaponics systems are rarely sterilized, and have a complex population of microbes (microbiome), which can aid in supplying the nutrients the plants need. It is believed that this microbial activity is why aquaponics systems perform as well as hydroponics systems and grow the same weight in plants, despite having some nutrient concentrations up to 100x lower compared to traditional hydroponics.
What else can these wonderful microbes do in aquaponics? In my next blog, we’ll discuss cannabis and how/why we believe aquaponics grows it really well.
