Q. What are the benefits of planting legumes in wildlife food plots?
To enhance the quality of your wildlife food plots, consider incorporating legumes into your management strategy. Legumes belong to a family of plants that, with the assistance of Rhizobium bacteria, have the ability to remove nitrogen gas from the atmosphere and convert it into a form of nitrogen that plants can use. The legume host receives nitrogen in exchange for sugars and other compounds that the bacteria need to survive and fix nitrogen. Legumes provide benefits in addition to adding large amounts of nitrogen to the soil. They are highly digestible and have high nutritional value, with high levels of protein, calcium, phosphorus, and magnesium. Notably, phosphorus and calcium are particularly important during lactation in does and antler development in bucks.
Q. Why plant legumes for nitrogen?
The price of commercial nitrogen fertilizers has increased significantly in recent years. The time of cheap inputs has passed. The cost of natural gas-based nitrogen fertilizers (high energy requirements to produce synthetic fertilizers) will almost certainly continue to increase. When the cost of fossil fuels is high, the prices of commercial nitrogen fertilizers increase. So, if you can eliminate a portion of your nitrogen fertilizer cost by growing legumes in your food plots, why not do so?
Q. How does “biological nitrogen fixation” work?
The ability of legumes to obtain nitrogen is the result of a symbiotic (you scratch my back I’ll scratch yours”) relationship between the legume and the Rhizobium bacteria in the soil. When Rhizobium bacteria infect the roots of legume plants tiny gall-like structures called nodules are formed on the roots of the legume host plant which in turn receives nitrogen in exchange for sugars and other compounds the bacteria need to survive and fix nitrogen. This biologically “fixed” nitrogen supplies the legume plant with nitrogen and also becomes available to non-legume plants in food plot mixes as well as for post-season crops through decomposition of roots and above ground plant material (stems & leaves).
Q. Why do legume seed need to be inoculated?
Inoculating legume seed with nodule-forming bacteria has been a standard practice since the early 1900’s. Seed inoculation ensures good nitrogen fixation by legumes. Inoculation is the process of applying Rhizobium bacteria prior to planting to help the plant fix nitrogen. Inoculation is recommended if the legume to be planted has never been grown in the field or has not been grown for several years. Rhizobia bacteria are very common in the soil and there are many different species. Because the legume-rhizobia relationship is very specific, not all Rhizobia bacteria can form nodules on all legumes. That’s why its important to select the correct inoculant for the legume species you are planting, or nodules will not form. Be sure to check the inoculant package for the legume species you are growing.
Q. When does the “infection of roots by bacteria” happen and how?
With proper inoculation seedling legume roots will quickly become infected with Rhizobium bacteria, and nodules will begin to appear on legume roots usually 21 to 28 days after plant emergence. As plants become older, the number and size of nodules will indicate the extent of nitrogen fixation. Soil fertility and pH are directly related to the number of Rhizobium bacteria present in the soil. Make sure the soil pH is adequate for proper legume growth and calcium, phosphorus and potassium levels are not limited. Soil conditions such as moisture and temperature also influence bacteria’s survival and nitrogen fixation. Soil Rhizobium populations can be reduced by any stress (drought) that reduces plant activity.
Q. How can I tell if nitrogen is being fixed?
A simple way to determine if root nodules are actively fixing nitrogen is to dig up a few strong, healthy plants. Carefully brush away soil from the roots and root hairs. Look for the number and size of the nodules present (are there many or only a few?) Root nodules can vary in size, number, and shape depending on the legume species and sometimes among species between different varieties. Cut a few nodules in half. If the nodule is pink to dark red inside, then nitrogen fixation is occurring. The pink and red colors are due to a compound, leghemoglobin, that controls oxygen flow to the bacteria. If the insides of nodules are green to white, then no nitrogen is being fixed.
Q. When is the nitrogen available?
Biologically fixed nitrogen is available to the host legume once the roots become infected & well nodulated. Nitrogen transfer from legumes like clover to grasses such as small grain occurs mainly through the breakdown of decomposing plant material, such as roots and leaves, becoming available for non-legume plants to absorb. The majority of the excess nitrogen produced by legumes benefits crops planted in the next growing season.
Q. Should I apply nitrogen fertilizer to a clover/ small grain wildlife food plot at planting?
The amount of nitrogen fixed by legumes varies depending on legume species, soil conditions, proper inoculation, the amount of bacteria available, etc. It’s easier for the legume to take up commercial nitrogen fertilizer from the soil than to fix nitrogen from the atmosphere. The more commercial nitrogen applied to the soil, the less bacterial infection occurs, fewer nodules are formed and lower amounts of biological nitrogen will be fixed. However, keep in mind that it takes several weeks after emergence for properly inoculated legumes to begin fixing nitrogen. Therefore, a small amount of commercial nitrogen fertilizer applied as a starter may prove beneficial to legumes and grasses planted in a mixture. Typically, 20 to 30 pounds per acre of nitrogen is recommended at planting for legume-grass mixtures when establishing wildlife food plots. An additional 20 to 30 pounds per acre may be applied if more forage is needed during the growing season. Remember, over-application of nitrogen will make nodules “lazy”.
Resources
Ball, D. (n.d.). Inoculation of forage legume seed. Auburn University Extension Service.
Johnson, J. M., & McKee, R. W. (n.d.). Legumes and nitrogen, frequently asked questions: The what, when, where, & how of nitrogen fixation. Alabama Cooperative Extension System; Auburn University.
Author
Marion Barnes, Cooperative Extension, Agronomy and Natural Resources Agent
Clemson University Cooperative Extension Service offers its programs to people of all ages, regardless of race, color, gender, religion, national origin, disability, political beliefs, sexual orientation, gender identity, marital or family status and is an equal opportunity employer.
