Scientifically Speaking

By Dr. Wiley Johnson

I have written before about nitrogen fixation of clovers and other legume plants. Judging from the questions we receive, however, I think it is time to educate our newer readers and refresh the memory of everyone else. If this seems like old stuff to you, please turn the page.

All the clovers, alfalfa, peas, vetches, soybeans and many other species of the legume family of plants have a special symbiotic relationship with a group of bacteria called Rhizobium. A symbiotic relationship is one in which two different organisms live together in a mutually beneficial arraignment. In this case, the clover provides a home for the rhizobia on root structures called nodules. Additionally, the clover provides energy (food from photosynthesis) for the rhizobia to live, multiply and carry on nitrogen fixation. The rhizobia contribute large amounts of plant-useable nitrogen compounds to the host plant. These nitrogen compounds have been produced by the bacteria from nitrogen in the air.  

Our air is about 80 percent nitrogen. However, it is almost all in a form that is essentially inert. The rhizobia change this inert nitrogen to useable nitrogen compounds.

This useable nitrogen is converted in amounts to meet the maximum needs of the host legume plant. Some useable nitrogen compounds seep from the nodules. Nodules and other plant parts slough off and decay releasing available nitrogen. Legume plants die in part or in total releasing available nitrogen as they decay. When absorbed by other plants, this symbiotically fixed nitrogen is identical to manufactured fertilizer nitrogen.

It is important that we do everything we can to maximize this nitrogen fixing process. There is no out-of-pocket cost. The legume plant pays the bill by providing a home and energy to the rhizobia.

What can we do to insure and encourage this process? First, make sure the soil pH is right. A soil that is too acid hinders proper nodulation and activity. Also, a properly fertilized and vigorously growing plant is essential. Therefore, get a soil test and lime and fertilize as indicated.

Secondly, make sure the proper and most efficient rhizobia are present. Laboratories that produce and sell cultures of Rhizobium are continually selecting those strains that are most vigorous and efficient in nitrogen fixation. Also, strains of Rhizobium are legume species specific. That is, bacteria that are effective on white clover will not be effective on alfalfa or other legume species. Each legume species has its own effective strain of Rhizobium.

Once introduced into the soil, rhizobia can live on their own for a while – even years with ideal conditions. However, adversity such as severe drought, flooding or low fertility will drastically reduce their numbers. The safe thing to do is inoculate (add bacteria) seed with the proper bacteria each time you plant a leguminous crop.

Bacteria are purchased at seed stores in packages usually suitable for inoculating 50 pounds of seed. The packages contain a slightly moist medium with some nutrients and the bacteria. The packages will indicate the proper legume species and will also be dated. Do not use carry-over inoculum. These bacteria are living organisms. Protect the packages from extreme heat (which can occur in a closed vehicle in the summer). Store in a cool room until used. Storing in a refrigerator is ideal.

Often, when a legume is planted on a piece of newground or where that legume species has never before been grown, successful inoculation of the plants is spotty to some degree. What you see is areas of vigorously growing plants with other areas of less vigorous or even very weak growth. With a perennial like white clover, this condition will become less a problem as the rhizobia multiply within the inoculated plants, are released and spread to inoculate the other plants.

All Imperial Whitetail® Clover seed is coated with our new Impact Blue brand seed coating. This trademarked coating consists of very fine lime, a mixture of essential micronutrients, a bountiful supply of the correct Rhizobium, a nutrient additive to extend the life of the bacteria and adhesive polymers to stick everything to the seed. Impact Blue has been proven to enhance high seedling establishment and nodulation. The bacteria strains used on all Imperial Whitetail products lasts up to 18 months, if properly stored, before re-inoculation is necessary. The advantage over uncoated seed increases as the conditions for establishment become more severe. “Should carry-over pre-inoculated seed be inoculated again?” many customers ask. Yes. First, check the inoculation expiration date. Then, if the date has passed, get a package of the proper bacterial strain from your seed dealer and mix it with the seed immediately before planting. For additional questions, call 1-800-688-3030 to speak with one of our consultants.