Nitrogen, Nitrogen, Nitrogen
Nitrogen in the Garden
If you are an organic grower and are new to using feathermeal or seed meals you are in for a pleasant surprise. If your garden is small and needs to be very productive you will be delighted with the response. In this photo, you can see how a generous application of nitrogen (along with some boron and gypsum – calcium sulfate, for the sulfur) can change a garden:
Both Veronica Broccoli plants were grown in mineral balanced soil, but the plant on the left received a fall application of feathermeal, gypsum, and boron. ALL the minerals need to be present for great results.
In our own garden we want to grow the best quality (nutrient dense) food we can. Our secondary goal is the production of fresh and preserved vegetables, year-round. So, our gardening is skewed toward quality and efficiency, without too many other constraints. Our limitation is labor — we do all the vegetable gardening ourselves. For us, it makes sense to run the garden with the accelerator pretty much wide open. By adding nitrogen, we can plant closer together and get a larger harvest from a smaller area. This makes mulching and weeding easier. Honestly, we like harvesting quantities of large, great tasting vegetables. But, this strategy is not for all circumstances. If you space your plants further apart you will get fewer but larger vegetables. You won’t need as much nitrogen. Your plants will be more drought tolerant. Your plants can maybe live on less applied water, as they will be drawing moisture from a larger area. Determining nitrogen application rates requires understanding your garden’s limits to growth, and some real thought about what you are trying to accomplish in your garden.
Nitrogen, Life, Fixation, and the Atmosphere
Nitrogen is an essential component of all plant life; it is needed for building proteins, DNA and chlorophyll. Plants take it up from the soil, not from the air. In many soils nitrogen can be the limiting nutrient for plant growth.
A plant’s need for available nitrogen is not always convenient for the garden-farmer. There is a delay between planting and peak nitrogen uptake which makes it difficult to know when and whether to fertilize, and how much to fertilize with. To optimize growth, it is important to understand as much as possible about how nitrogen works in the soil and in the plant. (Figure at right is from this reference.)
Even though nitrogen is the most common gas in air, plants are not capable of utilizing it directly. Even leguminous plants which are commonly known as nitrogen fixers rely on a symbiotic relationship with the diazotrophic bacteria that do the actual N fixation.
In the atmosphere nitrogen exists almost entirely in the form of N2 – two nitrogen atoms bound together by three covalent bonds. These bonds are quite strong, and it is a good thing that a natural process exists that can break these bonds; our lives depend on it! The process relies in part on a reaction involving an iron molybdenum cofactor. Molybdenum and iron (or -vanadium nitrogenase) must be present in the soil for nitrogen fixation to occur.
Besides nitrogen fixation, nitrogen is naturally available in the soil as a product of the decomposition of once-living beings. Nitrogen cycles from living being to living being via the soil.
According to many university agricultural advisors, the main plant-available forms of nitrogen are nitrate (NO3-), ammonium (NH4+) and urea ( CO(NH2)2 ). These are the plant-available nitrogen sources supplied by non-organic, chemical-industrial fertilizer, and they have been studied extensively. Other plant-available forms of nitrogen exist, such as amino acids, and even more complex forms of nitrogen such as proteins have been shown to be utilized by plants (see this ref). Recently it has been found that roots “eat” entire microbes (see this ref.), strip their nutrients, and send the remains out to gather more! There is more to nitrogen supply than the agro-chem industry would have us think!
Carbon, Nitrogen and Microbes
In the soil, the transformation of once-living materials into plant-available nitrogen happens due to microbial activity. Besides nitrogen, microbes need carbon in large quantities as an energy source and to build their bodies. When we “feed the soil” with cover crops and fertilizers, it’s dinner time for the microbes. Given ample food, moisture and warm conditions, their populations explode.
According to the USDA, microbes need a diet of about 24 parts carbon to 1 part nitrogen. Of that, they use 16 parts carbon for energy and 8 parts to maintain their bodies. Any excess carbon will remain in the soil. Any excess nitrogen will be available to plants.
Since microbes are responsible for converting once-living materials into plant-available nitrogen sources, the carbon-to-nitrogen ratio of the amendment source determines the rate and amount of nitrogen made plant-available during decomposition (see here and here.) If the carbon-to-nitrogen (C:N) ratio of the source is too high and is approaching 20:1, the microbes will use up the plant available nitrogen in the soil for themselves, leaving none for plant growth. It’s the amendments and composts with a C:N ratio less than 15:1 that leave provide excess nitrogen in the soil for plant growth.
Stable soil organic matter has a C:N ratio between 11:1 and 9:1 (see this ref.) and a large amount of the nitrogen is in the form of amino acids, a plant accessible form of N. These forms are stable in the soil if organic matter levels are maintained or increased.
Nitrogen release rates
Few composts and fertilizers release their entire nitrogen component in the first season. Part of the remaining unreleased fraction may be held over from season to season to provide an increasing pool of available nitrogen. The amount retained from year to year depends on several factors. Nitrate nitrogen, the most common source of plant-available nitrogen in the soil, is easily leached, so more is lost in rainy locations. In light soils water moves through the soil more rapidly, so more nitrogen can be lost to leaching than in clay soils. In hot climates, microbial activity is higher and both carbon and nitrogen are released at higher rates.
Climate plays a big part in how much organic matter can be built up in the soil. Soils have a natural soil organic matter percentage that they tend toward. It is very difficult to maintain high soil organic matter in an irrigated soil in a hot desert climate. And it is relatively easy to build up organic matter in a cool climate with a long winter. In areas where nitrogen is easily leached or lost to the atmosphere, it’s like the meter is running. If nothing is planted, the nitrogen may just go away.
How much Nitrogen shall I add?
Farmers love nitrogen because they love to see green and vigorous plants, and beginning gardeners do too. However, too much nitrogen results in weak plants susceptible to insect and disease attack. And, if you want to grow nutrient dense plants it is important to hit the “sweet spot”. Too much nitrogen will decrease nutrient density. Garden-farmers may have to go through a stage where they enjoy the vigorous growth and large sized plants high nitrogen applications bring in order to appreciate smaller, more robust vegetables.
OrganiCalc uses 150 lbs/acre of N at the default target, however this may be changed by utilizing the pull-down menu or by entering a specific amount.
Nitrogen management decisions are best made in the field. Many growers rely on observation to determine when and how much nitrogen to add. While plants don’t require many nutrients when they are small, it is easier to incorporate N pre-plant. But, this comes at a cost in materials. While sidedressing during the growing season is more efficient of materials, it adds labor.
When considering “how much nitrogen shall I add?”, it is important to consider all the factors.
What am I growing?
- Different crops have different nitrogen demands. For vegetables, rates vary from less than 65 up to almost 400 lbs/acre N. For precision, it is best to do an internet search for nitrogen demand of your crop in your area. Accessing this information can be confusing. It is fairly easy to find crop removal rates, but these are less than the total demand for the crop. The following table is for Plant Utilization – what it takes to grow a crop. These figures are not really what we need for organic growing as much of our nitrogen can be already in the soil, but it is a good starting point.
Partial listing from: A&L Laboratories Agronomy Handbook. Highly Recommended. A&L credits The Potash and Phosphorus Institute, The Fertilizer Institute, California Fertility Association.
How much N will be released from my soil organic matter?
- This is a difficult question to answer. The OM% value from the lab tells us nothing about the quality of the organic matter.
- N release depends on the percentage of organic matter in the soil, it’s state of decomposition, soil texture, temperature and moisture levels. Higher oxygen levels (for example, following tillage) will cause a spike in the decomposition rate and flush of available nitrogen, at the expense of soil carbon.
- If you have been routinely adding good quality compost or manure, chances are you will have a higher level of nitrogen available for release.
- If you live in a rainy climate, the anion forms of nitrogen, nitrate and nitrite, may have been lost due to leaching.
- According to the NRCS/USDA, each percent of soil organic matter in the 6” of a medium textured soil releases 10-20 lbs/acre N per year. We have more about testing for available nitrogen below.
- Not all SOM is created equal! Raised bed “soil” mixes may include unfinished high carbon composts that will not supply nitrogen. They may have high levels of organic matter but little plant available nitrogen. These soils need nitrogen amendments.
How much N will be available from decomposing crop residue and cover crops?
- Vegetable residues contain anywhere from 45-240 lbs/ac N depending on the biomass and type of crop. High throughput organic garden-farms may choose to compost these rather than incorporate them into the field.
- When cover crops or crop residue are incorporated into the soil, decomposition can release substances harmful to plants for the next 3 weeks or so depending on soil temperature and moisture.
- Cover crops may release 0 – 85 lbs/ac N after incorporation into the soil. The amount of N they release depends on the proportion of legumes, overall biomass, and the stage at which it is incorporated into the soil.
- It is entirely possible that cover crops will require nitrogen from the soil in order to decompose. Non-leguminous cover crops past the bud stage are especially likely.
- The figure above is from this source. PAN is Plant Available Nitrogen.
Can I test for the nitrogen content of my soil?
- Yes and no. The Soil, Paste and More test includes a test for ammonium and nitrate. For organic growers, there is little to no ammonium in the soil because it is quickly converted to nitrate by microbes. Nitrate is easily leached and so is not a stable figure if you are in a rainy or over-irrigated situation.
- Horiba makes a nitrate tester for in-field testing.
- Soil bioactivity tests (like the Haney test from Logan) are popular and can be an indication of available nitrogen.
Which nitrogen fertilizers are available to me?
- Composted manures are good nitrogen sources but may build up potassium and phosphorus to unacceptably high levels with excess use. Only about a third of the nitrogen in composted manure will be available in the 12 weeks after it is applied, even under good conditions. However, if you consistently apply compost, nitrogen levels may build up but phosphorus and potassium levels will build up more.
- Yard trimming compost, such as from a city green waste recycling program, is a great mulch but will tie up nitrogen if incorporated into the soil.
|Typical %N||Typical C:N Ratio||N available after 12 weeks||Releases in:||General use|
|Municipal yard trimming compost||0.5 - 2||13 - 20||0 - 4%||years||building carbon reserves|
|Poultry manure compost||2 - 5||6 - 8||30 - 35%||weeks - months||building carbon reserves
|Granular fertilizers (except guano)||2 - 7||5 - 7||38 - 60%||days - weeks||steady N supply|
|Blood meal / powdered feathers||13 - 15||3 - 4||65 - 70%||days||sidedress N supply|
|Liquid fertilizers||2 - 4||4 - 6||50 - 100%||days||sidedress N supply|
|Guano||12 - 13||3 - 4||80 - 90%||days||immediate N supply|
- Granular fertilizers such as pelleted feather meal or seed meals are good nitrogen sources to incorporate at planting. They tend to release slowly over the season. Be aware that soybean meal needs to be incorporated 2 weeks before planting. See this study.
- If you know the protein content (of soybean meal, for example) you can multiply protein by 0.16 and convert protein percent to nitrogen percent.
- Blood meal, powdered feather meal and guanos release quickly and are good for side dressing during times the plants are growing quickly.
- Liquid fertilizers such as hydrolyzed fish products or soluble amino acids have readily available N and are good to apply when the plants are actively growing.
- Recently, amino acid soy hydrolysates, about 16-0-0 have become available. These are readily available to the plant, saving 20-some percent of their energy as they don’t need to convert nitrate to amino acids.
- If you get your soil ecosystem right, free-living nitrogen-fixing microbes may be able to supply all your nitrogen needs (please let us know if you have been able to set up the conditions for these to flourish).
To determine how much nitrogen to apply using amendments:
- Find the total amount the crop will use. For example, 120 lbs/ac for lettuce.
- OrganiCalc assumes that all the nitrogen listed on the label of the amendment will be available. This works if you have been consistently applying high quality composts and nitrogen amendments, since extra N will be available for release from your soil organic matter.
- If your soil is depleted or you are on a new plot, increase the amount of nitrogen to amend.
- Subtract the amount of N released from a cover crop. For example, if we had a 20% legume cover crop turned in at bud, it might release 20 lbs/ac N. 120 – 20 = 100 #/ac N remaining to be supplied by amendments.
Consider when and how you will be applying amendments.
- A high dose of highly soluble nitrogen at planting is a recipe for aphids and other sucking insects. Blood meal and guanos should only be applied to rapidly growing plants.
- Likewise, it is tempting to force overwintered plants with a nitrogen application. Resist this temptation!
- A handful of slow-release fertilizer and vermicompost under transplants will get them off to a good start.
- Broadcast amendments will encourage roots to spread out.
- Plant-based compost is a great mulch. Don’t rely on it for nitrogen.
You are the best judge
Nitrogen applications are very powerful. They are like stepping on the garden growth accelerator. You want to be sure everything is in tune, and all the other minerals are present and available before ramping up nitrogen applications. In an ecosystem, it is the scarcest resource that limits growth. If your plants lack water, or other minerals, don’t apply lots of nitrogen.
You, the garden-farmer, are the best judge of when to apply nitrogen. You can factor in your knowledge about your growing conditions, your observations and your intuition. Decisions about nitrogen are best made in the field.
An interesting reference: University of California – Nitrogen Planning and Management in Organic Production of Annual Crops