Legume Crops Fix Nitrogen

Including legumes in crop rotation can contribute to the nitrogen budget.

Rhizobia nodules on bean roots fix nitrogen in the soil.

With the high price of nitrogen fertilizers, legume crops have become more valuable in crop rotations. Legume crops, such as alfalfa, field bean, and soybean have the unique ability to convert atmospheric nitrogen gas (N2) into a form usable by the plant through a process called nitrogen fixation. Because the air we breathe contains more than 78% N₂, there is an abundance of N2 that legumes can utilize. Legumes are able to utilize atmospheric N2 as a result of a mutually beneficial (symbiotic) relationship they form with either Rhizobium or Bradyrhizobium bacteria. Some legumes, such as alfalfa, can fix enough plant available nitrogen (N) to supply their entire N needs after the crop is established (Table 5). Consequently, supplemental N fertilization is usually not necessary. Other legume crops such as field bean and soybean only fix a portion of their N needs and require supplemental fertilizer N for high yield.

In the symbiotic relationship with the legumes, bacteria invade plant root hair tissue to stimulate the production of a visible structure called a nodule. The plant supplies energy to the bacteria from photosynthesis. For their part in the symbiotic relationship, the bacteria convert atmospheric N2 gas to ammonium (NH4+) in the nodules.

Rhizobium species are identified by their ability to form functional nodules on specific legume species. Each legume requires a specific compatible species and strain of Rhizobium. For example, the species of bacteria that forms nodules with alfalfa will not nodulate dry beans or soybeans. Commercial inoculants are labeled according to the plant species with which the bacteria are compatible.

If the legume crop was grown in the field previously, there is a good chance that the soil already contains the correct rhizobial species for nodulation. However, native rhizobial populations found in soil often are less effective than commercial inoculants composed of rhizobial strains selected for maximum N fixation. Inoculum can be applied directly to the seed prior to planting, or by metering the inoculum into the seed furrow during planting. Three basic forms of commercial inoculum are solid, liquid and freeze-dried. The most commonly used are solid, peat-based inoculants that can be purchased for seed or direct soil application. Liquid inoculants are available in broth culture or as frozen concentrate. Broth or frozen concentrates usually are mixed with water and sprayed into the seed furrow at planting. Because liquid inoculants must be kept frozen or refrigerated during shipment and storage, their availability through normal distribution channels is limited.

Be careful not to mix seed inoculation with chemical seed treatment. Most seed disinfectants, including fungicides, are toxic to rhizobia bacteria. Do not apply inoculum directly to seeds that are treated with a bactericide, such as streptomycin, unless you use a resistant strain of the rhizobia. Although some rhizobia species are slightly tolerant to certain chemical compounds, inoculating chemically treated legume seed requires special precautions. If the seed is pretreated with pesticide, it is best to apply the inoculum in the furrow.

When in doubt about a field’s rhizobial population, applying inoculum is a good practice, especially if the legume has not recently been grown there. However, even when more efficient strains are introduced into the soil, there is no guarantee these inoculated strains will compete well with native strains for entry into plant roots. Many studies have been conducted on the application of commercial inoculants into soils that already contain the correct rhizobial bacteria. In some studies, a significant yield increase has been observed, in other studies, no response occurred. The best method to evaluate the response of inoculants on your farm is to test several inoculants and an untreated control in fields using replicated strip tests. Be sure to dig plants during mid-season to count nodule number, nodule mass and whether they are effectively fixing N. An actively fixing nodule will be pink to reddish when cut open, rather than tan (ineffective) or green (dying).

Maintain proper soil fertility to ensure nodulation and N2fixation. However, do not apply large amounts of N fertilizer with legumes, because plants tend to stop fixing N2 gas when soil N is high. Phosphorous and potassium can also affect nodulation and N2 fixation. Research shows that additions of phosphorus (P2O5) and/or potassium (K20) increase the number of nodules formed, fresh weight of nodules, and amount of N fixed per nodule. However, in most Colorado soils potassium is usually adequate, while phosphorus is often limiting.

by Mark Brick
Professor

It’s A Good Time To Spread Manure!

Improve soil quality while applying this nitrogen source.

With nitrogen (N) fertilizer prices high, it’s a good time to re-consider having manure hauled in and applied to your farm ground. Different types of manure have varying amounts of nitrogen in them, and even within a manure type, there is a lot of variation depending on feeding and management practices. Table 6 shows average nutrient concentrations for different kinds of manure.

Based on the current value of the nitrogen and phosphorus in the manure alone, beef manure is worth $13.70 per ton. So if you can get manure for less than that, consider that a good deal! Sheep and poultry manures would be valued even higher due to their higher nutrient concentrations. But be careful to avoid spring applications of fresh poultry or swine manures due to their high ammonium levels that can burn plants.

Of course, manure’s value is actually greater than the numbers shown above since manure is also a good source of potassium and micronutrients such as zinc, iron, sulfur, and boron. Plus, manure is a terrific soil amendment. By increasing soil organic matter levels, manure can improve a soil’s water and nutrient holding capacity and also improve drainage and aeration. Manure also makes a good food source for the bacteria, fungi, and worms that recycle soil nutrients and improve it physical properties.

One important thing to remember about manure nitrogen is that it will mineralize or become available to crops more slowly than commercial fertilizers. In effect, manure acts as a slow release N source, releasing the N over about a three year period. So will applying manure now (in the spring) delay the availability of the manure N even more? Dr. Merle Vigil of the USDA Great Plains Research Center in Akron and Brad Jakubowski (CSU graduate student) studied N availability from fall and spring applied beef manure. Under irrigated conditions, N availability to corn was identical for either application time (measured at both V6 and tassle growth stages). On the other hand, under dryland conditions, fall applications had significantly more N available at V6 than spring applications. By the time corn was tasseling, there was no significant difference in N availability from fall and spring applications, even under dryland. This research demonstrates that under irrigated conditions, manuring now (in the spring) will not delay N release from manure.

When you are spreading manure, choose fields with the lowest soil NO3-N levels and the highest N need, rather than those with a long-term manuring history. Choosing crops that will give you a yield response for the added manure nutrients, makes the most out of the manure application, and saves you the most in fertilizer costs. If there are other yield limiting factors such as heavy weed populations, high water table, or poor irrigation uniformity, this will limit the impact of the manure nutrients on yields. Apply manure where nutrients are the greatest yield limiting factor to get the most bang out of your manure spreading dollar.

If you value manure for it’s N content, it is critical to conserve that N so that plants can use it. It’s important to minimize volatilization losses (losses of ammonia gas into the air). If manure is broadcast and not incorporated, up to 30% of the ammonium in the manure will be lost to the air within just four days of spreading. Incorporating immediately will reduce that loss to less than 5%. So to get the most N value from the manure, be sure to incorporate as soon as possible.

If you manage a feedlot, leaving manure in open lots results in the greatest N losses to the air. Storing manure in a manure pack reduces N loss by about a third. Scraping pens and hauling manure out on a daily basis reduces N losses even more. So, if you value manure for its N content, manage manure to minimize N volatilization and add value to the manure.

Lastly, you’ll get the most out of the manure application if it is applied as uniformly as possible with properly calibrated equipment. When you load the spreader trucks, be sure to even out the load in the truck, because uneven loading results in uneven application. Monitor the distribution of manure as it comes out the back, so you can get the proper overlap to even out the application rate. Otherwise, you may end up with N deficiencies in some spots and too much N in other spots, even though over the whole field, the application rate was correct.

You may have avoided manure application in the past because of high transportation costs. But check your figures again this year, since the fertilizer value of the manure may outweigh the transportation costs. And what other fertilizer increases soil organic matter and improves soil properties?!

by Jessica G. Davis
Extension Specialist
Soils