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Nitrogen Dilemma Q&A

By October 19, 2021October 21st, 2021No Comments
What can I plant to make nitrogen for next year’s corn?

With the recent spike in the cost of nitrogen fertilizer, and even higher prices expected going forward, people are asking if there is something they can plant to biologically produce nitrogen fertility prior to next year’s corn crop.  Most people are aware that there are bacteria that live on legume roots that can produce available nitrogen fertility, but most people are unsure just how effective biological nitrogen fixation can be to reduce their needs for purchased nitrogen fertilizer. SO here are some important questions to be answered:

“How much nitrogen can be made available through biological means? Can we produce enough to reduce or eliminate the need for purchased nitrogen fertilizer?” 

The amount of nitrogen that can be produced by a legume cover crop is directly related to the amount of biomass the legume can grow. The amount of biomass the legume can produce depends on an infinite number of factors: soil moisture, temperature, mineral nutrient availability, soil compaction, and so on. So it is impossible to predict in advance exactly how much nitrogen a legume can produce. However, we can rely on the past performance of legume cover crops to get an idea of the potential. Summer annual legumes fix more nitrogen per day of growth than winter annual legumes because there is more sunlight and more favorable temperatures; but winter annual legumes can have more days of active growth due to their ability to grow over fall, winter and spring combined. The more days of active growth, the more biomass that can be produced, and the more biomass, the more nitrogen.  In general, legume biomass runs about 3% nitrogen as an average. Thus, a ton of legume biomass will contain about 60 pounds of nitrogen. It is common for a summer annual legume to produce about 70 pounds of biomass per day of active growth, or about a ton per month if there is sufficient moisture. Obviously, then a summer annual legume planted in May and allowed to grow until frost will make more nitrogen than a summer annual legume planted in July and allowed to grow until frost.   Winter annual legumes, if planted in fall, usually make most of their growth (and thus nitrogen) in spring, and the longer they are allowed to grow in spring, the more biomass and more nitrogen they can produce. A winter annual legume terminated in early April will not produce as much nitrogen as one allowed to grow until May, and one terminated in May will not produce as much as one allowed to grow until the first of June, although there is seldom any point in allowing a cool-season legume to grow any longer than that as growth slows tremendously once summer temperatures begin to climb above 80F.  In our plots at Bladen last year, winter annual legumes planted Oct 1 and terminated June 1 produced as much as 4.2 tons of biomass containing 240 total pounds of nitrogen. Obviously, if we had terminated on May 1, the total N would have been less than this amount. 

“How available is this nitrogen to the next crop?”

The most important factor to realize when answering this question is that the nitrogen in terminated legume biomass is completely unavailable to plants until it is decayed by microbes.  In other words, the microbes sit at the table first, and the plants get the table scraps. So how much do the plants get? It depends on the decay rate, which depends on the microbial activity. Microbial activity increases with warm temperatures and requires an optimum balance of soil moisture and soil oxygen.  Also, the amount of nitrogen tied up or released upon the decay of the residue depends upon the carbon to nitrogen ratio, which is another way of expressing protein content. The carbon content of residue remains constant through the life of a plant, while the nitrogen content drops with maturity. As nitrogen content (and protein content) drops, the ratio of carbon to nitrogen increases. Residue with a high carbon-nitrogen ratio (or low protein) will decay very slowly, and during the decay of low protein material the microbes are starving for nitrogen and will tie up any available nitrogen, leaving little for plants to use; this is why using a low protein-containing cover crop like mature rye or sorghum-sudan prior to a nitrogen hungry crop like corn can result in a nitrogen deficient crop. Legume residue, on the other hand, tends to maintain high protein content even into maturity, and of course, the nitrogen fixation increases the total amount of nitrogen in the residue.  With legume residue, typically half the total nitrogen contained in the residue becomes available to the following crop. What happens to the other half? It becomes part of soil organic matter, and ordinarily, half the remainder becomes available each succeeding year. So for example, if a residue contains 240 total pounds of nitrogen, then the first year will have 120 available pounds, the second year will have about 60 pounds available, the third year will have about 30, and so on. 

“I am in a corn-soybean rotation, can I plant something after soybean harvest that will make all the nitrogen needs for 200-bushel corn before I plant corn as soon as soil temperatures reach 50 degrees Fahrenheit?”

No. Next question. 

“Ok, since the answer to the last question was “no”, let’s be a bit more realistic. Here is a better question: How can I take advantage of biological nitrogen fixation to reduce my fertilizer needs?”

Ok, this is a better question than the last one. First, let me explain just why the answer to the previous question was “no”.  After soybean harvest and prior to planting corn early, there just aren’t enough warm days to grow enough legume to fix much nitrogen. We need more days of plant growth to accomplish the task. How can we do that? To begin with, let us play with the corn planting date a bit. Do you HAVE to plant the first week of April? Or can you delay that planting date until later, say in early May? If that is the case, then we can use the warmer temperatures and longer daylight hours of April (compared to earlier in the spring) to grow more legume biomass than we can by killing the cover in early April. This can dramatically increase the amount of nitrogen fixed by a winter annual legume, such as winter peas, hairy vetch, crimson clover, or balansa clover.  If nitrogen production is particularly valued, such as with organic farming where alternate sources of nitrogen fertility might be very expensive and hard to acquire, then allowing the winter annual legume to grow as late as June might be desired. In this case, it is possible to produce over 200 pounds of total nitrogen, which equates to around 100 pounds per acre of available nitrogen. While considerable, this isn’t enough in most cases to maximize corn yield, and the corn will usually require some supplemental nitrogen. But it definitely helps. BUT WAIT! If let’s suppose, you have been cover cropping for a few years, you not only have nitrogen being released by the cover crop you just terminated, you also are getting nitrogen released from last years cover crop, plus nitrogen from the cover two years ago, three years ago, and so on. Add all these up, and you might just get very, very close to having enough nitrogen to meet the needs of high-yielding corn. This is a compelling reason to incorporate cover crops as a routine practice in your cropping system; the nitrogen fertility benefits of cover crops, like many other cover crop benefits, accrue over time. The longer you cover crop, the more benefit is achieved. 

Another strategy to increase the number of days which are occupied by legumes is to begin the growth of legumes earlier than prior to soybean harvest. Of course, aerial seeding into soybeans prior to harvest will gain an additional two or three weeks of growth, but in order to really make an impact, it might be worth considering (for a multitude of reasons) getting out of the corn-soybean rotation and including a year of winter cereal to create a three-year rotation. This allows the planting of a summer annual legume (such as cowpeas, lablab, sunn hemp, or mung beans) right after cereal harvest in the summer and which can still be followed by a later planting of the cover crop including winter legumes in the fall to create a relay race of nitrogen fixation. Such a sequence can realistically provide enough nitrogen to completely meet the needs of a high-yielding corn crop, as well as bring other benefits to the soil and following crops. Research and farmer experience both demonstrate that extending the length of a crop rotation from two years to three years increases the yield and profitability of all the crops in the rotation by a wide margin. Though the cereal grain itself may not be as profitable as a crop of corn or soybeans, by allowing the insertion of a warm-season cover crop and increasing the yield of the other crops in the rotation the profitability of a three-year corn-soybean-winter cereal rotation including cover crops is far better than a two-year corn-soybean rotation. Another three-year rotation that is worth exploring where cereal grains do not perform well is the corn-soybean-cattle rotation which includes a full year of pastured cover crops in between soybeans and corn. (See our article on this rotation in the 7th edition of the Green Cover Seed Soil Health Resource Guide, available at www.greencover.com)

However, the most effective way to provide biologically derived nitrogen fertility in a crop rotation is to incorporate a perennial pasture sod with a generous dose of legumes in the pasture mix. It is well documented that corn following a terminated sod of pastured grasses, forbs, and legumes will produce maximum yield without the need for nitrogen fertilizer. The inclusion of a pastured sod in a crop rotation historically has been referred to as ley farming and was once considered essential for the maintenance of high crop yields. If someone is interested in a crop rotation that can achieve maximum crop yields without purchased nitrogen fertilizer, this is worth learning about. 

“Can I graze my legume cover crop and still assume a nitrogen credit?”

Most of the nitrogen ingested by a grazing animal will return to the field in manure and urine, but it will not be uniformly distributed, it will be concentrated in little circles across the field and will likely not benefit every plant in the field. The degree of defoliation is also important to answer this question. Proper grazing management should dictate that no more than about half the biomass of a cover crop is removed by grazing; as you begin to exceed 50% removal, the cover crop will no longer be performing the essential functions of a cover crop, such as protection of soil from raindrop impact and solar baking, and production of root exudates to feed microbes. Additionally, animals forced to graze plant stems instead of leaves will perform poorly and will often lose weight. A properly grazed cover crop may still have a decent nitrogen credit, while the same cover crop grazed to the ground will offer little nitrogen credit. It is best to assume a grazed cover crop to have less nitrogen credit than an ungrazed one (maybe 50-75% depending on the degree of grazing pressure and the uniformity of manure and urine deposition), but the long term soil benefits of grazing are going to be better than not grazing due to the magical properties of manure, and the positive impact of grazing on cash flow is undeniable. 

“Is there any other way to fix nitrogen other than with legumes?”

Yes. The nitrogen fixation that occurs through symbiotic bacteria that live on the roots of legumes is the best known natural nitrogen fixation process, but it isn’t the only one. There are nitrogen-fixing organisms that live freely in the soil unassociated with plants, and yet others that live in the rhizosphere of many plants (most prominently warm-season grasses), and others that live inside the vascular system of plants. There is almost always a small population of these organisms in our cropland soils, but since they are suppressed by the use of water-soluble nitrogen fertilizers, they are usually in very low abundance. Our Bi-Azo inoculant includes two of these nitrogen-fixing bacteria (Azotobacter and Azospirillum) and this inoculant can be used to rapidly restore a working population of these nitrogen-fixing organisms at very low cost  These bacteria do not fix nearly as much nitrogen as the Rhizobium bacteria on legume roots, but they do add to the total nitrogen fixed by a diverse cover crop mixture by enabling the nonlegume cover crops to make some additional nitrogen. Bi-Azo does not fit in every cropping situation; in a field heavily fertilized with synthetic nitrogen, these organisms will produce little if any nitrogen. However, Bi-Azo can be quite valuable to increase the amount of total nitrogen produced by a diverse cover crop that includes grasses, or for inoculating a nitrogen dependent crop like corn or sorghum that is receiving most of its nitrogen needs from carbon-based sources such as decaying cover crop residue, compost or animal manure. 

In summary, it is possible to dramatically reduce our dependence on purchased nitrogen fertilizer, but it may take some modification of our planting dates or crop rotations to completely sever our addiction to synthetic nitrogen. Also, although we realize nitrogen production is important, don’t get too focused on legumes and forget to include grasses and forbs into your mixture as well, because a diverse mixture will bring far more benefits in both the short and long term than a legume monoculture. 

 

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