Q. I’ve been told I should inoculate my bean seeds. Why should I do this, and how do I do it? Does alfalfa need it? Any others?
A. Why Use Rhizobia Inoculants? Rhizobia bacteria are a group of soil-based microorganisms, which establish symbiotic relationships with beans and other legume plants, such as alfalfa and clover. They then form nodules on the roots of the legumes, where they store nitrogen and provide it to the plants. In return, the plants provide carbon and energy for the Rhizobia, which the plant produces by photosynthesis. Nitrogen is vital for plant growth. It is abundant in the atmosphere as N2, and in soil organic matter in other forms, but because of its volatility, it is not available in the NO3 form that plants can use.
Conventional methods of providing nitrogen to plants include 1) adding nitrogen fertilizers to the soil, or 2) inoculating with the Rhizobia nitrogen-fixing bacteria. Rhizobia then take N2 atmospheric nitrogen and convert it to the NO3 inorganic form that is useable by plants. In addition, the nitrogen they store in nodules on the plant roots, if the roots are left in the ground, can reduce nitrogen fertilizer requirements for the next growing season.
Therefore, before planting, some gardeners “inoculate” their bean seeds. This consists of coating the seeds with a small amount of powder containing the Rhizobia bacteria, thus effectively enabling the plants to draw nitrogen from the air and deposit it in nodules on their roots. This ability to “fix” nitrogen is unique to plants in the legume family, because of the symbiotic relationship with Rhizobia bacteria.
Inoculants only need to be used in poor soils that don’t have much soil nitrogen for plants to use. In fertile soil, the bacteria occur naturally, so inoculants are not needed. The inoculant comes in small packages, it is usually available where you buy your seeds, and it takes very little to do the job. Immediately before planting, put the bean seeds in a pan and add a little water to moisten them. Then add a small amount of inoculant, and stir the beans with a stick until they have a little powder on their seed coats.
To gain the maximum benefit from inoculating your been seeds, spade or till disease-free bean plants into the earth immediately after the last harvest. This adds organic matter to the soil and releases the nitrogen from the root nodules as well. Always destroy all diseased plants immediately.
Some people mistakenly believe planting beans and corn together is good, because the beans can climb the corn stalks, and the corn can get nitrogen from the beans. This is not a good idea for several reasons. The two plants are competing for the same water, food (13 nutrients are needed!), and light, and both will suffer for having to share. In addition, the nitrogen in the bean root nodules is “fixed” and unavailable to the corn. It is only released to the soil after the bean plant has completed its life cycle.