Low pH in aquaponics.
pH effect on nutrient absorption
Plants use carbon, hydrogen, and oxygen from the air, soil, (or in our
case grow bed media) and water to make simple foods that they process
through photosynthesis.
Remember, the movement of hydrogen ions affects our pH. The
chemical interaction of the plants contributes to the hydrogen ion
movement process.
16 Basic minerals are considered to be essential elements for the proper
growth of plants. These minerals also interact with water pH in order
for the plant to absorb them.
In order to make our aquaponics system thrive we need a basic
understanding of the plant nutrient uptake process, so let’s look at how
plants absorb minerals in the first place.
Most mineral elements are absorbed through the plants roots, some can
be absorbed through the leaves but most are absorbed from water near
the roots.
In order for a plant to be able to absorb nutrients it must first break
them down into chemical compounds that the roots can readily absorb.
Plants, just like all organisms must respire (breathe) in one way or
another. Plants breathe in carbon dioxide from the air and then use that
carbon dioxide to form a special enzyme around the root system. This
enzyme creates a weak carbonic acid that breaks down nutrients in the
soil and makes them available for plant uptake.
The carbonic acid breaks down organic matter into organic salts (or ions) that can be more readily exchanged.
In an aquaponics system this action will contribute to the water pH by
gradually adding carbonic acid to the water. In effect driving down the
pH of the entire systems water. The larger the plant roots are, the more
this action will occur.
Plants absorb minerals in the form of ions, they do this through a
chemical process of dissociation. Dissociation is the process by which
basic ions are dis-lodged or dissociated from the base element.
The process is controlled by what is called the cation exchange capacity or
(CEC) of the media the plant grows in. Nutrient rich water in our grow
beds greatly improve this process over conventional soil media.
This is because hydrogen ions are released when the carbonic acid
(H2CO3) is formed from the water and the carbon dioxide during root
respiration. The hydrogen ions will then exchange positions with those
of available nutrients, thereby making the nutrient available for plant
uptake.
The pH of the growing medium affects the availability of nutrients,
particularly the trace elements. At high and low pH values, the nutrient
elements combine with other chemicals in the medium and are not
available for plant growth.
The overall result is that the absorption process adds to the acidity of
the water in our system. When the pH changes to a certain point
however, nutrient absorption will cease.
Another issue with high acid levels is root damage. As acid levels climb a
tipping point is reached where the acid begins to breakdown organic
material in the grow bed. This includes the plant roots AND our
beneficial bacteria that we have worked so hard and long to culture.
Which if left unchecked will lead to septic or anaerobic conditions,
because now instead of fish waste solids in our grow bed we have plant
waste solids as the roots die off and begin to decay.
The bacteria that break down the ammonia and ammonium also create
more hydrogen ions, as do the bacteria that create anaerobic conditions.
Both of which will cause the acid level to increase.
So we have a situation where the bio-filter itself also contributes to the
water dropping in pH value toward acid. When added to the plants
carbonic acid a runaway pH drop can occur. What we need is to buffer
the pH so it won’t do this.
Solving this problem is simple though; just add some Calcium Carbonate
or Calcium Bicarbonate into the sump tank to raise the systems pH. This
stuff is sold at the local farmers co-op at about $10 for a 50lb bag. Or you
can use crushed eggshells in a stocking to do the same thing.
This is called a buffer. Unlike powdered lime, The acid in the water from
the plants and bio-filter slowly dissolve it away and keeps the water pH
stabilized during the process. Not only does it keep our pH in check but
the acid dissolving away the mineral adds essential calcium to the
water. Normally this buffering action would take place in our soil, but as
we have no soil, these buffers or minerals, are not present.