Legumes must be inoculated


Q. I am new to vegetable gardening and would like to plant peas. The seed catalogue suggests inoculating pea seed before planting. What is inoculate, and is it necessary?

Pat from Lisbon

A. Yes, it is necessary. What it is will take a little more explaining.

Nitrogen is an essential element for plant growth, a component of proteins, nucleic acids and chlorophyll. Plants receiving sufficient nitrogen will exhibit vigorous growth and develop healthy dark green leaves.

Although nitrogen is the most abundant gas in the atmosphere, plants are unable to use it in that form. They can only absorb it from the soil in the form of ammonium or nitrate. Animal waste and decayed organic matter (humus) are natural sources of nitrogen. Through a process called lightning fixation, a small amount of atmospheric nitrogen is delivered to the ground with precipitation. Commercial fertilizers can be used as well.

Legumes (peas, beans, clover, alfalfa and vetch) are unique in that they have a symbiotic relationship with a soil bacterium called rhizobium that live in small tumor-like structures called nodules on their roots. The bacteria convert nitrogen gas from the air into ammonium, which can then be used by the plant. It is called a symbiotic relationship because each organism benefits from the relationship. The legume receives nitrogen that stimulates growth, and the bacterium receives carbohydrates produced during photosynthesis.

Although rhizobium bacteria may be present in the soil, the amount may not be adequate to inoculate a crop of legumes. There are also several strains of rhizobium and each legume requires a specific strain.

The inoculate listed in the seed catalogue is a mixture of ground peat and rhizobium bacteria packaged in a small plastic bag. It has a certain shelf life (see the expiration date) and must be stored in a cool, dark location. Acidic soils will reduce the life of rhizobium so a soil test is recommended before planting to determine soil pH.

To inoculate, moisten seeds with just enough water in a shallow bowl to coat seeds, pour on the inoculate and mix well to coat seeds. Plant seeds immediately after inoculation. Exposure to UV light and heat can kill the bacteria.

As plants become established, pull one to see the nodules on the roots. Nodules that are actively fixing nitrogen should be pink to reddish when cut open.

For information on pea varieties and how to grow them, go to: http://go.osu.edu/peasplease

Today’s answer provided by Merabeth Steffen, OSU Extension master gardener volunteer. Regular hours for the clinic resume April 2. Submit questions to the clinic at 330-533-5538 or drop samples off to the Extension office in Canfield.

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