growers know
LEGUMES AND
NITROGEN FIXATION
Nitrogen fixation by legume plants
occurs when symbiotic bacteria
known as Rhizobia colonize the root
system and take up residence within
visible root nodules. These bacteria
can absorb nitrogen gas (N 2 ) from the
air and convert it into ammonia (NH 3 )
to be used by the plant. In return, the
bacteria receive a carbohydrate food
supply from the plant.
The bacteria’s contribution is
particularly useful for crops where
nitrogen may be limiting in the soil
or growing medium as it provides a
source of free nitrogen fertilizer. In a
well-fed hydroponic crop where we can
supply plentiful nitrogen in a highly
soluble and immediately available
form, the use of nitrogen fixation may
not seem to be that beneficial. However,
with the increasing interest in organic
nutrients and hydro-organic systems,
nitrogen-fixing crops do have a major
advantage. Organic nutrients and
systems often experience issues with
nitrogen nutrition, as plants have a
high requirement for this element and
can deplete it from organic solutions
and systems very rapidly under rapid
growth conditions. Nitrogen availability
is dependant on the rate of microbial
conversion of organic materials to plant-
available forms of nitrogen, and this
may not keep pace with high-density,
vigorous plant growth.
Even though there is no soil present to
act as a plentiful source of inoculum in
hydroponics, Rhizobia symbiosis does
often occur naturally. To speed up the
process, plants can be inoculated with
the correct species of nitrogen-fixing
bacteria. These are widely available as
microbial products or incorporated as
seed coatings.
Also, keep in mind that the root
nodules that form on hydroponic legume
crops are visible and appear as knobbly,
rounded growths that can reach the
size of a pea, firmly attached to the root
system. These have sometimes been
mistaken for root pathogens or other
diseases by inexperienced growers, but
they are a perfectly natural occurrence.
GOURMET LEGUMES
The legume crops commonly grown in
hydroponic systems tend to exclude
commodity crops such as pulses and
instead focus on the higher-value fresh
vegetable, garnish, and gourmet baby
produce types. The development of
new, dwarf, or “container” varieties
of peas and beans are particularly
advantageous to indoor gardens and
small hydroponic systems, as they
124
grow cycle
Peanut plant seedlings.
produce high yields on relatively
small and compact plants that don’t
require trellises or supports like the
tall climbing types do. Both peas and
beans are also used as micro greens
and sprouts, taking even less space
than mature plants and requiring only
a few days from seeding to harvest.
Hydroponic systems suitable for
growing peas and beans include
substrate and solution culture, but
large, deep growing containers are
recommended for taller varieties and
longer-term crops such as climbing
pole beans, fava beans, and soybeans
due to the extensive nature of the
root system. Bush/dwarf beans, dwarf
pea varieties, and snow peas can be
grown in larger-sized nutrient flow
technique (NFT) channels.
To speed up the process,
plants can be inoculated
with the correct species of
nitrogen-fixing bacteria.”