Soy Beans and Nitrogen Fixing Bacteria

Growing Soy Beans

Soy Beans are always worth growing in your garden. Like any bean, they are a legume and have that wonderful ability to attract a bacteria to live in nodules in its root system. It doesn’t just attract any old bacteria, but a specific bacteria that fixes nitrogen from the air. The bacteria and the soy bean create a symbiotic relationship that is based on “you scratch my back and I’ll scratch yours”. In this case the soybean provides the bacteria with carbohydrates, sugars and oxygen and the bacteria converts nitrogen from the air into  plant available amino acids that feed the plant.

Soybean “Edamame”

Edamame is a large-seeded type of soy bean that is eaten as a green vegetable. It is both highly nutritious and a delicious nutty taste treat. It is prepared by boiling or steaming the pods for 5 – 10 minutes, then cooling under running water. It is eaten by squeezing the seeds from the fibrous pods. It grows best in temperate and subtropical climates. The seeds can be used as a dried bean, to make tofu, miso, soy flour and soy milk.


Store the inoculant in the fridge until ready to sow seed. To use, slightly moisten seed, stir in inoculant then add a small quantity of agricultural lime to coat the seed and make it easier to sow evenly. Sow seed immediately.

Nitrogen fixation is critical for high soybean yields. For nitrogen fixation to occur, the nitrogen-fixing bacteria known as Bradyrhizobia japonicum must be established in the soil through seed inoculation. Soybean can obtain up to 50 to 75{cd07be7979728a86b172d4c3c193ee8254293b2598ced4c874c66a7b3dbba444} of its nitrogen requirements from the air when nitrogen-fixing bacteria have established functioning nodules on the roots.

The soybean-bacteria symbiosis is mutually beneficial

Edamame Soy Bean roots with nodules containing bacteria.

Edamame Soy Bean roots with nodules containing bacteria.

The soybean plant gets nitrogen from the bacteria, while providing the bacteria with carbohydrates. When soybean seed germinates, the bacteria invade the root hairs of the seedling and begin to multiply. Nodules, which house the bacteria, form on the roots. Under field conditions, the first nodules form within a week after seedling emergence and become visible as they increase in size.

There are some things I have learnt about growing soy beans that I haven’t seen in any text books or papers. The main one is that they improve their protein count when grown in calcium rich soil. Therefore, not only is it an advantage to coat the seeds in agricultural lime during the inoculation process, it is also a benefit to lime the soil that you are growing them in. In Queensland, Australia we have acid sulphate soils so lime not only increases the protein count, it also helps raise the pH of the soil towards neutral.

To find out if your soil is acidic, alkaline or neutral, get some litmus paper. Mix up a slurry of your soil and dip the litmus paper in and see what colour it goes. Use the colour chart to give you a rough idea of whether your soil is acidic or alkaline. If alkaline, use gypsum, or as it is chemically known, calcium sulphate to lower the alkalinity of the soil. From what I remember, hydrangeas will also indicate soil pH. If they are blue it is acidic and red if the soil is alkaline. Though it is also the amount of aluminium in the soil that has more influence, but it is more available to the plant in acidic conditions and less available in alkaline soil.