March 28, 2008 

One reason growers plant legumes like clovers and alfalfa is because the plantsproduce their own nitrogen — at least usually they do.

Nitrogen fertilizer is expensive and using other means to add nitrogen to the soil can be cost effective.

One way is to use more legumes - like alfalfa for hay and clovers or alfalfa in grass pastures. Legumes make their own nitrogen by forming root nodules with bacteria call Rhizobium. If you dig up a legume plant, these nodules are the small, pinkish clusters of lumpy growths on the roots.

Many soils do not normally have enough of these Rhizobium to form nodules naturally. To be sure your legumes produce nitrogen, add Rhizobium bacteria to your seed.

Many top quality legume seeds are pre-inoculated with a seed coating that contains these bacteria; however, sometimes a little more inoculant needs to be added to ensure good nodule formation.

That's because Rhizobium are living bacteria. Pre-inoculation attaches them to seeds during winter for spring planting. If this seed is held over for planting later, many of these bacteria may die due to heat during storage or the length of time between attachment and planting. As a result, older pre-inoculated seed may contain too few live bacteria to form many nodules. And, of course, any seed that has not been pre-inoculated will have no bacteria for producing nitrogen.

To avoid this problem, buy inoculant prepared specifically for your legume and add it to the seed as directed just prior to planting. Each type of legume has its own inoculant so be sure to get the right one. This nitrogen insurance will cost less than a dollar per acre, a fairly inexpensive option for ensuring nitrogen production.

Bruce Anderson
Extension Forage Specialist