Nitrogen is one of the more influential inputs that will determine yields of most crops. Under conventional production practices, there is the option to purchase. 78% of the of atmosphere is nitrogen. Legumes have adapted to fix atmospheric nitrogen through nodules that converts to plant available forms. By including legumes that are have the proper inoculants, producers can help increase the soil nitrogen levels. Organic producers have been utilizing legumes as their nitrogen source for many years.
Historically, both organic and conventional producers also have been fallowing the land, which allows decomposition of organic matter that will also increase soil nitrogen levels. The cost is loss of organic matter, and fuel burned by equipment. With the loss of organic matter, there is also a loss of microbes in the soil. With cultivation, the residues are worked into the soil, creating new food sources for the microbes, especially bacteria. The aeration of the soil plus the new food source will allow the bacterial population to explode. Cultivation is like a tornado ripping through a trailer park. It destroys the homes of microbes, killing many, especially fungi. With the loss of cover, as the daytime temperatures rise, the soil temperature rises. As the soil passes the optimal range for the microbes, they start to die, releasing nutrients which can volatilize or leach away. Similarly, as the soil temperature starts to drop, soil microbial activity drops, so more nutrients are not being tied up and are subject to loss.
By utilizing legumes, the soil can get a source of nitrogen without sacrificing soil microbes. Legumes will actually stimulate and support mycorrhizal populations, along with bacteria in the soil.
The process of nitrogen fixation is quite energy expensive for the plant. So if there is free nitrogen in the soil, the plant will utilize the free nitrogen. When the plant runs out of the free nitrogen, then it will look for the symbiotic relationship with the rhizobium. For an annual crop, this could be devastating, causing late maturing crops, low yields, and a poorly nodulated plant that will fix less nitrogen.
The energy expense to the plant is due to the high requirement to split atmospheric nitrogen. Phosphate is one of the keys for energy transfer. Soils low in available phosphate and iron will have legumes with low nitrogen fixation. Boron is also important in transferring sugars throughout the plant. Molybdenum is another micronutrient that can cause nodulation issues.
When growing legumes and the nitrogen fixation is important, the nodules need to be examined throughout the year. The nodules need to be examined to see if they are actually active. Nodules can die or be sloughed off throughout the growing season if the plant does not require them. To see if the nodules are active, cut one open. If they are grey, they are not active. Active nodules should be salmon coloured.
Once the legume has fixed the nitrogen, it will accumulate protein within the plant. Most legumes will fix most of the nitrogen when the plant starts to flower. This is the highest requirement for nitrogen for the plant. It requires proper nodulation prior to flowering, allowing the plant to be prepared for the large need.
Now the nitrogen is fixed by the plant, as the plant rots, nitrogen is released. The important thing now is there anything growing to capture the free nitrogen. This will be influenced by soil moisture, soil temperature, soil microbiology levels, and carbon:nitrogen level in the plant. Adequate soil moisture combined with warm temperatures with an active microbial population working on a tight C:N ratio residue will rot quickly, releasing significant nitrogen. Drier, cooler conditions with a lower microbial population working on more lignified residue will rot slower.
A quickly rotting residue will require either a crop to be growing quickly after legume termination or have another cover crop to capture this flux of free nitrogen. Species like Brassicas and grasses are high nitrogen scavenging plants. If they are grown late into the year and allowed to be terminated by frost, they will rot quickly in the spring. If no capture crop is used, the nitrogen is subject to leaching and volatilization losses.
A slower rotting residue may actually require extra nitrogen added to break down the residue. It will be determined by the carbon:nitrogen ratio. To maintain a 24:1 ratio will maintain soil residue levels. A tighter ration will break down more residues quicker, where a wider ratio will allow more residue build up.
A slower breakdown of residue will be required when the nitrogen will not be required until later in the growing season. Ideally, a combination of quick and slower release nitrogen source will allow nitrogen feeding throughout the growing season. Some of the slow release nitrogen will come from the decomposition of the organic matter in the soil, along with the more lignified humus in the soil.
Combinations of different legume species is a wise option when attempting to increase total soil nitrogen. Utilizing an annual with a biennial would increase the time of nitrogen fixation throughout the year. Including a early and later maturing species would also help extend fixation. The next step is to examine when the plant material will start to decompose, releasing the nitrogen and develop a plan on capturing it for later release.
